A
Height of sea level relative to the centre of the Earth. See also relative sea level.
The process of supercooled water droplets freezing on impact with snowflakes, ice particles or other cold objects including aircraft.
Cooling of a parcel of gas by expansion, with no heat exchange between the parcel and the surrounding air.
Warming of a parcel of gas by compression, with no heat exchange between the parcel and the surrounding air.
A process where a parcel of air cools or warms due to a change in pressure and volume (expansion or compression), with no heat exchange between the parcel and the surrounding air.
Fog transported by local winds from one locality to another.
Horizontal movement of air in the lower atmosphere due to wind.
Fog resulting from the movement of moist air over a colder surface and the consequent cooling of the air to saturation.
The delay in time between the transit of the moon and the highest spring tide. Normally one or two days, but it varies widely. In many places the maximum tidal range is one or two days after the new or full moon, and the minimum range is a day or two after first and third quarter.
The mass of air per unit volume.
Large body of air with approximately uniform temperature and water vapour throughout.
Any deposit of ice forming on an aircraft.
Instrument that determines altitude using atmospheric pressure.
The vertical distance from mean sea level to an object aloft.
A principal cloud type, forming in the middle levels of the troposphere, and appearing as a white and/or grey layer or patch with a waved aspect. In aviation forecasts and reports it is coded as AC.
A principal cloud type, forming in the middle levels of the troposphere, and appearing as a grey or bluish sheet. In aviation forecasts and reports it is coded as AS.
A point in the sea which is a node with little or no tidal amplitude around which the tide rotates, for a particular tidal constituent.
An uphill wind generated by the heating of a sloping surface.
Instrument used to measure wind speed and, in some cases, wind direction.
The non-standard propagation of a beam of energy (radio or radar) under certain atmospheric conditions, which results in false echoes (i.e. non-precipitation) on a radar image. Usually caused by unusual rates of refraction in the atmosphere.
Difference from the average value.
For example, if the maximum temperature for June in Melbourne was 1 °C higher than the long-term average for this month, the anomaly would be +1 °C.
The international standard is to use the 30-year average from 1961 to 1990 as the long-term average. However, more recent periods (for example 1981 to 2010) will sometimes be used to calculate an anomaly that is more descriptive of the current climate. Anomalies calculated from periods of less than 30 years may not accurately describe all the variation that can be expected.
Aircraft equipment, such as heating elements and flexible rubber strips, used to prevent or clear structural icing.
Also called high pressure systems, anticyclones are circulations of high air pressure that rotate anticlockwise in the southern hemisphere. They generally bring lighter winds and fine, settled conditions.
The upper portion of a cumulonimbus cloud that spreads out under the tropopause, often in the shape of a blacksmith’s anvil, sometimes for hundreds of kilometres downstream from the parent cloud. It indicates the mature or decaying stage of a thunderstorm.
The point in the elliptical orbit of Earth or other planet when it is furthest from the Sun.
The point in the Moon’s elliptical orbit when it is furthest from Earth. The tidal range tends to be reduced at this time – this is referred to as an apogean tide.
Apparent (‘feels like’) temperature measures how warm or cool you might feel, which can be different from the measured temperature. Our instruments measure ‘ambient temperature’ – how warm the air is in the shade and sheltered from the wind. However, apparent temperature also takes into account the effect of wind and humidity. It is based on a mathematical model of an adult walking outdoors in the shade.
An angle, which is a function of the longitudes of the Moon, Sun, lunar perigee, lunar node and solar perihelion, that is used in the determination of the tide-generating potential of a particular constituent at a particular time.
The gaseous portion of the physical environment that encircles the earth. The divisions of the atmosphere are the troposphere, the stratosphere, the mesosphere, the ionosphere, and the exosphere.
The total weight of the atmosphere above the point of measurement.
Periodic variations of the atmosphere caused by gravitational and radiational effects, analogous to ocean tides. See also radiational tides.
Mapping system that sets mean sea level (measured from 1966 to 1968) as zero elevation. Australian mapping measures the height of features from this level.
A perpetual or ongoing entitlement, by or under a law of a State, to exclusive access to a share of the water resources of a water resource plan area or other water management area.
The specific volume of water allocated to an Australian water access entitlement in a given water accounting period.
Weatherproof enclosure that usually includes a thermometer, barometer and humidity probe. It may also have a rain gauge and anemometer attached, and communications equipment to transmit the data it collects in real time. An AWS takes automated measurements of weather elements, including some or all of these:
- air temperature
- humidity
- wind speed and direction
- atmospheric pressure
- precipitation
- soil temperature at various levels below ground
- sea level
- cloud base height
- horizontal visibility.
One of the 4 seasons in a year. In the southern hemisphere it’s March, April and May.
A horizontal angle calculated clockwise from the meridian, especially the horizontal direction of a celestial point from a terrestrial point, expressed as the angular distance from a reference direction (hence often designated as true, magnetic, compass or relative, depending on the reference).
When applied to current or stream, it is a direction toward which the current or stream is flowing, and usually calculated from the north point. See also celestial sphere.
B
An anti-clockwise shift in the wind direction.
Instrument that measures air pressure.
An aneroid barometer doesn't contain liquid.
Area of country, generally a larger catchment area, drained by a river and its tributaries.
Measurement of water depth in oceans, seas or lakes.
The mathematical representation of the spring–neap cycle is in terms of the two signals (in this example the semi-diurnal tides M2 and S2) going in and out of phase over the course of a fortnight.
This phenomenon is known as beats or beating and in the context of tides is not confined to M2 and S2. When any pair of slightly different frequencies (of similar amplitude) add together, their sum undergoes a regular cycle between near-zero magnitude (neaps), and a magnitude equal to the sum of the pair (springs).
The period between successive neaps is equal to the inverse of the absolute value of the difference between the two frequencies. The frequencies of M2 and S2 are 1.9322 cpd and 2.0 cpd respectively. Their difference is the beat frequency 0.0678 cpd, the inverse of which, 14.75 days, is the beat period – also known as the fortnightly or spring–neap cycle.
Scale that estimates wind speed based on the effects we feel, see and hear (human observations).
A fixed physical object or mark used as a reference for a vertical point. A tidal benchmark is a point near a tidal station used as a reference for a tide gauge and tide measurements.
Very strong and cold wind loaded with snow, some lifted off the ground.
Dust that is raised locally by the wind to a height of at least two metres above the ground. In aviation forecasts and reports it is coded as BLDU.
Snow that is raised locally by the wind to a height of at least two metres above the ground. In aviation forecasts and reports it is coded as BLSN.
The temperature at which the vapour pressure of a liquid is equal to that of the surrounding atmosphere. The boiling point of pure water at the standard sea-level pressure of 1013.25 hPa is 100°C (373°K).
A solitary tidal wave with a steep leading edge that travels up a river, estuary or bay against the direction of the river current. Also known as Tidal bore.
Used to describe an amount of cloud covering the sky of between five and seven oktas (eighths). In aviation forecasts and reports it is coded as BKN.
Upward force which lifts a parcel of air. This happens when the air parcel is less dense than the surrounding environment.
Describes the relationship of the wind direction to the pressure distribution. In the southern hemisphere, if one stands with one’s back to the wind, lower pressure is to the right. Lower pressure will be to the left in the northern hemisphere.
C
Occurs when air (including warm air) with a high water vapour content is drawn into an aircraft’s carburettor (which is chilled by the fuel evaporation process coupled with the pressure reduction in the carburettor’s venturi) causing the water vapour to condense and then, if the temperature has dropped below 0°C, form ice on the surfaces of the carburettor. The ice may gradually block the venturi or cause jamming of the mechanical parts of the carburettor.
Land where water, especially rainfall, drains into storage, such as a lake, pond or basin.
Instrument for estimating cloud amount and height.
Temperature scale (range of values) where water freezes at 0 °C and boils at 100 °C at sea level.
Describes the likelihood of receiving a measurable amount of rain (>0.2 mm) during the day. For example, if the chance of rain for Mildura is 30%, rain will be measured in the Mildura rain gauge on 3 out of 10 days with similar weather conditions. Where there is a 30% chance of any rain, there is a 70% chance of no rain. By similar weather conditions we mean similar levels of moisture, temperatures, air movement and so on.
Change in weather indicated by a transition between 2 air masses over a short time. This is usually when a cooler air mass replaces a warmer one. A change may bring a rapid change in wind direction and sometimes rain. This is usually from a warmer north to northwesterly wind to a cooler southeast to southwesterly one.
A change in the form of water, e.g. liquid to vapour, ice to liquid, ice to vapour.
Artificial or constructed waterway designed to carry water.
The point or plane of reference to which all charted depths and drying heights are related (but not other heights and elevations). It is usually defined in terms of a low-water level, such as lowest astronomical tide.
In some ports a local port datum might be adopted as chart datum, for instance where sea levels are strongly affected by meteorological conditions and may fall significantly below the predicted astronomical tide.
A principal cloud type, forming in the high levels of the troposphere, composed of ice crystals which appear from the ground as very small elements in the form of grains or small ripples. In aviation forecasts and reports it is coded as CC.
A principal cloud type, forming in the high levels of the troposphere, composed of ice crystals which appear from the ground as a transparent sheet or veil, often creating a halo phenomenon around the sun or moon. In aviation forecasts and reports it is coded as CS.
Wispy, feathery-looking cloud formed high in the atmosphere and made of ice crystals.
Free from cloud, fog, mist or haze.
A form of turbulence occurring in regions of marked wind shear, particularly at the boundaries of jet streams, but may also be found in strong lee waves downstream from elevated typography. It occurs in the absence of any visual clues (i.e. clouds).
A sheet of transparent ice formed by the relatively slow freezing of large supercooled water droplets, i.e. rain. Does not seriously distort aerofoil shape but does add appreciably to aircraft weight and drag.
Weather or atmospheric conditions experienced or forecast in an area, averaged over a long period. This can be a few weeks to millions of years.
Scientific study of climate. Also known as climate science.
Mass of water droplets or ice crystals, caused by water vapour in the atmosphere condensing or freezing.
Degree to which cloud covers the sky. Forecasting terms relating to cloud cover:
- Clear
- Sunny
- Partly cloudy
- Cloudy
- Overcast.
Forecasts of cloud cover normally give the main state if no significant variations are expected. A clear day, for example, may see a few cloud patches.
Forecasters expecting significant variations in cloud amount may use such terms as sunny periods, sunny breaks, cloudy periods, cloudy at times, mostly/mainly sunny, mostly/mainly cloudy.
If expecting a major change in cloud cover, the forecast will usually indicate a distinct trend, for example, becoming sunny or cloud increasing.
More cloud than clear sky.
Cloud expected to cover about 65–100% of the sky for most the day. For example, during the day the sun would be obscured by cloud for lengthy periods.
See co-tidal line.
Lines on a co-tidal chart joining places which have the same tidal range or amplitude. Usually drawn for a particular constituent or tidal condition, such as spring tides.
Lines on a co-tidal chart joining places where tide has the same phase, for example where high water occurs at the same time. Usually drawn for a particular constituent or tidal condition.
The horizontal movement of colder air into a location of warmer air.
Leading edge of a relatively cold air mass moving into a region of warmer air.
Combined height of the sea and the swell that mariners experience on open waters. Refers to the average wave height of the highest third of the waves.
Also known as significant wave height and total wave height.
Part of the tidal stream velocity which, after resolution into orthogonal vectors, acts in a specified direction. For example North or East. Also known as harmonic constituent. See constituent.
A harmonic constituent of a tide with a speed equal to the sum or difference of the speeds of two or more elementary constituents. Compound tides usually occur in shallow water and result from the interaction of its two parent waves.
The height at which an adiabatically lifted air parcel will become saturated, whereupon condensation occurs. Corresponds to cloud base level.
Tiny particles upon which water vapour condenses.
Change from a gas to a liquid. This process is also called condensation – a term that is also used to describe the liquid droplets formed.
Stable unsaturated air that will become unstable if saturated.
The transfer of heat in response to a temperature gradient within an object or between objects that are in physical contact with one another. Transfer is from warmer to colder regions.
A weather chart representing conditions on a constant pressure surface, e.g. 500 hPa.
Elements that do not vary with time.
Harmonic tidal analysis represents the sea level record as the sum of cosine waves. Each wave (constituent) has a unique frequency (speed). For a given location, each frequency has an amplitude and phase which do not vary with time – these are called constants.
Tidal currents may be harmonically represented by tidal constants by converting them into north/south (along-stream) and east/west (across-stream) components.
One of the elements in a mathematical expression of the forces that produce a tide or tidal current. See constants.
The process whereby heat is conducted away from warmer air to a colder surface.
An extensive body of air, with a more or less uniform temperature and moisture profile, which has originated over a large land mass.
Line joining points of equal value on a surface.
A condensation trail that forms behind an aircraft when hot humid air from a jet exhaust mixes with environmental air of low water vapour and low temperature. If the increased vapour leads to saturation, the vapour will condense into water droplets and/or deposit into ice.
Process generally linked with warm rising air and the formation of cloud. For example, on hot days when the land and air above heats up, that air begins to rise. As the air rises it cools. If it cools enough, the water vapour in it condenses and forms cloud (sometimes called convective cloud). If a lot of air warms and rises, a thunderstorm may develop.
A cloud that owes its vertical development, and possibly its origination, to convection.
The lowest height at which condensation will occur as a result of convection due to surface heating.
When air flows into a region.
Main standard by which the world regulates clocks and time. It's also known as Greenwich Mean Time. In some forecasts and reports it is signified by the letter Z.
Describes how the Earth's rotation deflects large moving masses of air or water. In the southern hemisphere, these masses appear to turn to the left (and to the right in the northern hemisphere). The Coriolis effect influences large-scale oceanic and atmospheric circulations, including weather patterns, cyclone formation, tidal direction and currents. It's greatest at the poles and non-existent at the equator.
A moving body on the surface of the Earth experiences a tendency to turn to the left in the southern hemisphere (right in the northern hemisphere) due to Earth's rotation. This tendency is a result of the rotating reference frame rather than an actual force.
Known as the Coriolis force or acceleration, it's only noticeable with larger scale motions such as ocean currents and winds.
The Coriolis force affects the direction with which the tide propagates around an amphidrome.
It can also affect the propagation of the tide as it moves up a broad channel. For example, by tilting the water surface to the left or right of the direction of propagation.
Abbreviation of the Commonwealth Scientific and Industrial Research Organisation, Australia’s science agency.
Very tall, puffy, heaped, dark cloud that often has a dark base and brings rain. Some have a distinctive anvil-shaped top.
Also known as thunderstorm cloud, producing thunder and lightning.
Cloud that usually has a flat base and clumpy, cauliflower-shaped upper surface. It forms in the low levels of the atmosphere.
Rapid development or increasing intensity of a low pressure system.
Often called low pressure systems, cyclones are atmospheric circulations that rotate clockwise in the southern hemisphere, and anticlockwise in the northern hemisphere. They are areas of lower air pressure that may bring stronger winds, unsettled conditions, cloudiness and rain. See also tropical cyclone.
D
Barrier to obstruct water flow.
Practice of turning clocks ahead by 1 hour during the warmer months. Also known as 'summer time'. In Australia, daylight saving is observed in New South Wales, Victoria, South Australia, Tasmania, the Australian Capital Territory and Norfolk Island. Australia's Daylight Saving Time (DST) regions:
- Australian Eastern Daylight Time (AEDT) covers New South Wales except Broken Hill, Victoria, Tasmania, Australian Capital Territory
- Australian Central Daylight Time (ACDT) covers South Australia and Broken Hill
- Australian Western Daylight Time (AWDT) covers Western Australia
- Norfolk Island Daylight Time (NFDT) covers Norfolk Island.
Dividing any dataset into 10 equal parts creates deciles – each part is a decile. We use deciles to show where an element like rainfall is above or below average, compared to the long-term average. To create deciles:
- sort all values in the long-term record from lowest to highest
- divide the data into 10 equal-sized parts
- number the parts 1–10.
The lowest 10% of observations are in decile range 1 and the highest are in decile range 10. So, a monthly rainfall total in decile range 10 is in the top 10% of observed totals. It's higher than (at least) 90% of monthly observations over the long term.
The angular distance of an astronomical body north or south of the celestial equator, taken as positive when north of the equator and negative when south.
Indicator of moisture in the atmosphere used by the agricultural industry to help with decisions about when to spray crops.
The weight of air per unit volume.
The altitude in the International Standard Atmosphere at which a given air density is found.
A process in which a gas transforms into a solid, e.g. the process by which water vapour, in sub-freezing air, changes directly to ice without first becoming a liquid.
Tiny water droplets that form on objects at or near the ground. Dew is produced by condensation of water vapour from the surrounding air.
Measure of moisture in the air and the temperature to which air must cool for dew to form.
Where the sea or swell travels from and where the wind blows from. For example, a northerly swell comes from the north. A northerly wind blows from the north.
Shower or two: Not many, less than a few. Used to describe infrequent showers.
Few: Not many, amounting to a small number. Used to describe a small number of showers over a certain period.
The condition whereby daily high waters or low waters are of significantly different level. See also species.
A tide with only one high water and one low water occurring during a tidal day. Also known as single day tide.
Changes within the course of a day – for example, temperature changes.
We use this term to describe systematic changes that happen during the average day. For example, temperature normally has a diurnal variation. Usually it's warmer during the day and cooler at night.
When air flows out of a region.
Local South Australian term for a neap tide with minimal rise and fall over the course of a day or so.
A nautical term for the equatorial trough, an area which typically has calm or light and variable winds.
Strong and sometimes damaging gust of wind that falls heavily from a thunderstorm. It then spreads out and can be felt some distance away from the storm.
A descent of cool air associated with convective cloud.
A wind directed down a slope, often used to describe winds produced by processes larger in scale than the slope.
The total amount of terrestrial electromagnetic energy that falls on a specified unit horizontal surface.
The longwave irradiance emitted from the sky that falls on a specified unit horizontal surface.
Snow blown from the ground by the wind to a height of less than two metres.
Slow-falling and uniformly distributed precipitation in the form of tiny water droplets (diameters less than 0.5 millimeters), usually from stratus or stratocumulus clouds. In aviation forecasts and reports it is coded as DZ.
Light rain of very small droplets, less than 0.5 mm in diameter, very close to each other.
For terms describing drizzle in forecasts, see Intensity of drizzle.
Prolonged, unusually dry period when there is not enough water to meet normal use needs.
A line of constant potential temperature on a thermodynamic diagram. Meteorologically, the dry adiabat represents the meteorological processes that occur with the adiabatic ascent or descent of a parcel of dry air (where no liquid water is present and no condensation of water vapour occurs). As an air parcel ascends adiabatically, its pressure decreases and its temperature falls due to the expansion of the air parcel; as it descends, its temperature will rise due to compression of the air parcel.
The rate of change in temperature for dry air ascending or descending adiabatically. The rate is approximately 3°C per 1000 feet.
The boundary between dry and moist air masses.
Annual season in the tropics that brings heat, sunshine and dry weather. In northern Australia, the dry season is between May and September.
Air temperature measured by a thermometer exposed to the air but shielded from sunlight and rain. It’s taken in an instrument enclosure known as a Stevenson screen (white louvred box) raised 1.2 m above the ground.
Brief: Short duration
Intermittent: Stops at times
Occasional: Infrequent but recurrent
Frequent: Regularly and often
Continuous: Doesn't stop, or stops only briefly
Periods of rain: Rain is expected to fall most of the time, but there will be breaks
Small particles of earth or other matter suspended in the air. In aviation forecasts and reports it is coded as DU.
Small and rapidly rotating column of wind made visible by dust, dirt or debris within it. It usually occurs in arid or semi-arid areas. It is most likely to develop on clear, dry, hot afternoons when the sun warms the ground, heating the air above it and causing it to rise. Also called a willy-willy.
When strong wind picks up dust and dirt from the ground, raises it into the atmosphere and carries it over an extensive area. Dust storms can trigger health problems for some people and disrupt transport by limiting visibility.
E
A deformation of the solid earth in response to the gravitational tidal forces of the Sun and Moon. The largest effect is the semi-diurnal deformation nearly in phase with the transit of the Moon – its amplitude is less than 20 cm.
Intense low-pressure systems that happen on average several times a year off the east coast of Australia – in particular, southern Queensland, New South Wales and eastern Victoria.
The tidal phase during which the water level is falling, or the tidal current is flowing seaward (ebb current). Opposite to flood.
Describes the radio signal that appears on a radar display when it's reflected by precipitation such as rain or hail.
Dynamic biological community of interacting plant, animal and micro-organisms and their physical environment.
Small disturbance in the wind that can cause turbulence.
Movement of water in wind-driven currents. This process brings colder water to the coastline.
Refers to the extensive warming of the central and eastern Pacific Ocean that leads to a major shift in weather patterns across the Pacific Ocean and neighbouring countries. In Australia (particularly eastern Australia), drier and warmer-than-average conditions are more likely in El Niño events.
Climate driver in the Pacific Ocean that swings between 3 phases: El Niño, neutral and La Niña. Southern Oscillation is the changes in atmospheric pressure (and hence in climate) linked to the El Niño warming.
In relation to water, electrical conductivity is the capacity of the water to transmit a flow of electricity. It's a common measure of water salinity.
The distance between mean sea level and a point on the earth’s surface.
Used in at least two different ways in tidal work – as a synonym for phase lag, and for a period of time (usually a nodal cycle) over which a mean is calculated as the basis for a tidal datum.
The geographic circle at 0° latitude on the earth’s surface.
Zone of relatively low pressure along the equator, including across northern Australia.
The height at which a rising parcel of air will become equal in temperature to that of the environment, at which point it is no longer buoyant and thus will cease to rise in the atmosphere without forcing.
Theoretical model under which waters, uniformly covering the face of the Earth, respond instantly to the tide-producing forces of the Moon and Sun and form an equilibrium under action of these forces.
Friction, inertia, irregular distribution of the land masses are all disregarded under this theory, but the model provides an important theoretical basis for tidal science.
The hypothetical tide which would be produced by the lunar and solar gravitational forces under the equilibrium theory in the absence of any other ocean dynamics or terrestrial constraints.
The time when the sun crosses the equator, when night and day are of equal length. This is around 21 March (autumn equinox) and 22 September (spring equinox).
Global changes of sea level taking place over many years. Sometimes associated with ocean volume changes or with globally synchronous changes.
However, such ocean surface displacements are now understood to be spatially irregular.
Changing from liquid to vapour.
When assessing and accounting for water resources, evaporation refers to the movement of water from the land surface (mainly liquid) to the atmosphere (water vapour). Land surface water available for evaporation includes surface water, soil water, water within vegetation, and water on vegetation and paved surfaces.
Total amount of water moved from the land surface to the atmosphere through evaporation and plant transpiration.
Plant transpiration describes how water moves through a plant and evaporates from exposed parts such as leaves and stems.
One of many deviations in the Moon's orbit caused by changes in the solar gravitational potential during the course of orbit, which produces evectional constituents. See also variation.
The measure of energy received per unit area.
Area of clear skies that develops in the centre of a tropical cyclone. It usually comes with light winds and no rain.
Organised band of cumulonimbus clouds that surrounds the eye of a tropical cyclone.
F
Temperature scale where water freezes at 32°F and boils at 212°F.
The temperature scale, developed by the German physicist Daniel Gabriel Fahrenheit in 1714, where water at the standard sea-level pressure of 1013.25 hPa has a freezing point of +32°F and a boiling point of +212°F.
The lines or bands of cumulonimbus clouds that spiral into and around the centre of a tropical cyclone.
Used to describe an amount of cloud covering the sky of one or two oktas (eighths). In aviation forecasts and reports it is coded as FEW.
Flooding that happens within about 6 hours of rain. It's usually the result of intense local rain and water levels rise rapidly.
When water covers land that is normally dry.
The tidal phase during which the water level is rising, or the tidal current is flowing inland (flood current). Opposite to ebb. See also streams.
Flat or nearly flat land subject to flooding that is next to a stream or river.
A warm and dry downslope wind descending the lee side of a mountain range.
Warm, dry wind generally blowing downslope on the lee side (sheltered from the wind) of a mountain.
A suspension in the air, at or near the earth’s surface, of microscopic water droplets, or wet hygroscopic particles, reducing horizontal visibility to less than 1000 metres. In aviation forecasts and reports it is coded as FG.
A factor used to characterise the tides in an area as being mainly diurnal, semi-diurnal, or mixed.
A two-week tide cycle. In most parts of the world, the tides go through a fortnightly spring–neap cycle. These are beat phenomena rather than actual harmonics.
There are also some harmonics arising directly from the tide-generating potential which have a period of a fortnight.
Supercooled drizzle (i.e. drizzle existing at a temperature below 0°C) that freezes upon impact with surfaces to form glaze. In aviation forecasts and reports it is coded as FZDZ.
The temperature at which a liquid solidifies under any given set of conditions. Pure water under the standard sea-level pressure of 1013.25 hPa freezes at 0°C (32°F).
Any form of supercooled precipitation that freezes upon impact with surfaces to form glaze.
Supercooled rain that falls as liquid but freezes upon impact with surfaces to form glaze. In aviation forecasts and reports it is coded as FZRA.
The mechanical resistive force offered by one medium or body to the relative motion of another medium or body in contact with the first. In meteorology, it is the drag or resistance of the earth on the atmosphere.
The thin layer of the lower atmosphere within which wind direction and speed is subject to frictional effects caused by contact with the earth’s surface. Its depth is variable, ranging from tens of metres in a stable environment, to several kilometres in convective conditions over deserts.
The transition zone or interface between two air masses of different densities.
The boundary between air masses with different characteristics. See also:
The passage of a front over a specific point on the earth’s surface. Changes in temperature, dew point, wind and atmospheric pressure occur with a frontal passage.
Soft white ice crystals or frozen dewdrops on objects near the ground. It forms when the surface temperature falls below freezing point.
Violent, rotating column of air extending toward the ground from the base of a cumulus or cumulonimbus cloud.
A funnel cloud that reaches the ground is called a tornado if over land, and a waterspout if over water.
G
Wind blowing at speeds of 63–75 km/h.
The level surface that the ocean would take in the absence of tides, currents, water density variations and atmospheric effects. It is the surface of reference for astronomical observations and geodetic levelling. See also geopotential.
The earth’s gravitational potential is called the geopotential. A geopotential surface is one whose gravitational forces are equal everywhere. See also geoid.
This approximates the height of a pressure level above mean sea level. Meteorologists often use geopotential heights to analyse the state of the atmosphere at different levels above the ground.
A weather satellite, in a west to east orbit at an altitude of 35 786 km, that maintains the same position over the equator.
If hail larger than 5 cm across is expected, we use the words 'giant hail' in our forecast.
A coating of ice, generally clear and smooth, formed on surfaces by the freezing of supercooled rain, drizzle or fog.
Total short-wave (solar) energy received on a horizontal surface on the ground. It comes direct from the sun and indirectly from elsewhere in the sky, where it can be scattered downwards by clouds, dust particles and so on.
Total amount of electromagnetic energy emitted from the sun that falls on a specified unit horizontal surface.
Irradiance emitted from the sun that falls on a specified unit horizontal surface.
Organisations do not have to provide any water information in the grace period. During this time, an organisation can work out what data it will need to give. This includes historical and ongoing information.
The grace period is 3 months long. It starts when we add an organisation to certain categories of the regulations – person categories A–G and L.
The grace period doesn't apply to:
- water information in Category 5 or 7. Organisations required to give this information will always have longer than 3 months between being added to a person category and when they need to start giving data
- a subcategory of information that the organisation was required to give previously due to being named in another category immediately prior. In this way, the grace period applies only once for each type of information.
Organisations start giving data after the grace period expires. The actual start date depends on whether the data is required daily, weekly, monthly or yearly data. For example, if the grace period ends after the start of a week, the requirement to give weekly information doesn’t apply until the end of the next week.
Wind at the gradient level, about 1 km above the Earth's surface. It's the level most representative of air flow in the lower atmosphere, immediately above the layer affected by surface friction. The gradient level is free of local wind and topographic effects, such as sea breezes and downslope winds.
Natural warming process of the Earth. When the sun's energy reaches the Earth, some is reflected back to space and the rest is absorbed. The absorbed energy warms the Earth's surface, which sends heat energy back toward space as longwave radiation. This is partially trapped by greenhouse gases such as carbon dioxide, methane and water vapor. They radiate the energy in all directions, warming the Earth's surface and atmosphere.
Enhanced greenhouse effect
This term refers to the addition of more heat-trapping gases to the atmosphere. The extra gases are mainly due to the burning of fossil fuels (coal, oil and natural gas). Other sources, such as land clearing, make smaller contributions. Increased concentrations of these gases in the atmosphere are contributing to warming of Earth's surface.
Was used to refer to the primary time standard, but now replaced by Coordinated Universal Time (UTC).
Pattern of radar echoes from fixed objects on the ground, such as buildings or hills.
Sudden increase of wind for a short period, usually a few seconds.
The leading edge of cool air rushing down and out from a thunderstorm cloud. Is usually accompanied by a drop in temperature, a wind shift and a pressure jump.
H
Clumps of layered ice that form in updraughts (rising air) within thunderstorms.
For terms describing hail in forecasts see Intensity of hail, Giant hail and Large hail.
Extremely small dry particles suspended in the air that reduce horizontal visibility, like light mist. Haze is distinguished from mist by humidity. If the humidity is less than 90%, it's haze.
Three or more days in a row when both daytime and night-time temperatures are unusually high. Temperatures must be high compared to the local long-term climate and the recent past.
Atmospheric circulations of high air pressure that rotate anticlockwise in the southern hemisphere. High pressure systems generally bring lighter winds and fine, settled conditions.
The ocean areas for which the Bureau provides forecasts and warnings. These are part of the Global Maritime Distress Safety System.
See lunitidal interval.
The highest level of water which can be predicted to occur under average meteorological conditions and any combination of astronomical conditions.
Highest tide level predicted under average meteorological conditions and any possible combination of astronomical conditions. Often referred to as HAT.
The Water Regulations 2008 outline 2 types of water information – historical information, and new or ongoing information.
Historical information is information a person holds before the end of the grace period.
Regulation 7.04 lists certain subcategories of water information and person categories. An organisation listed in these person categories and holding these data during the grace period gives them as historical information. They give historical information within a fortnight after the end of the grace period.
A deposit of interlocking ice crystals formed by direct deposition on objects such as tree branches, leaf edges, wires and poles.
Measure of how much water vapour is in the air.
Large-scale severe storm with violent winds. This term is used in the USA. In Australia, these storms are called tropical cyclones and in Asia, typhoons.
Area of science concerned with the distribution, properties and movement of Earth's water.
Study of atmospheric processes that affect water resources. This includes the atmospheric and land phases of the hydrological cycle and the relationships involved.
I
Formation of ice or frost on an aircraft.
An elevation depressed below mean sea level by the amount equal to the sum of amplitudes of the four main harmonic constituents: M2, S2, K1 and O1. Also known as ISLW.
Aboriginal and Torres Strait Islander communities have seasonal calendars based on traditional weather, climate and environmental knowledge passed from generation to generation.
A method used to infer the amplitude and phase of constituents where a tidal data set has insufficient duration.
To separate a pair of constituents of similar frequency, you can infer the amplitude and phase of one member of the pair (generally the weaker) on the basis of an analysis of a longer data set from a nearby location or the equilibrium relationships.
The inference relationships between the two constituents must be accounted for in the analysis.
The Water Regulations 2008 separate water information into two types:
- historical – information a person holds before the end of the grace period
- new or ongoing – information that comes into a person's possession after the grace period has expired.
Regulation 7.04 lists certain subcategories of water information and person categories. An organisation listed in these person categories and holding these data during the grace period gives them as historical information. You give historical information within a fortnight after the end of the grace period.
New information is the information that comes into your possession after the grace period. You give new information at various specified timeframes on an ongoing/continuing basis.
Long-wave electromagnetic radiation which is emitted by all objects.
Coastal waters from the coast to 12 nautical miles out.
Solar radiation or heating received at the Earth’s surface. The name comes from INcoming SOLar radiATION.
State of the atmosphere where a parcel of air continues rising, after the lifting mechanism stops. Air parcels can begin to rise in many situations. These include:
- due to daytime heat
- ahead of an approaching cold front
- when airflows converge, as in a low pressure system.
Instability can lead to thunderstorms.
A set of regulations governing all aspects of civil aviation aircraft operations when a flight is conducted using instruments rather than outside visual reference.
Light
Up to 0.2 mm an hour. Can be felt on the face but is not visible. Produces little run-off from roads or roofs. Visibility is reduced, but not less than 1 km.
Moderate
0.2–0.4 mm an hour. Window and road surfaces streaming with moisture. Visibility generally between 400 m and 1 km.
Thick or heavy
Over 0.4 mm an hour. Visibility reduced to less than 400 m.
Light
Sparse hailstones, often mixed with rain.
Moderate
Enough hailstones that the ground looks white.
Heavy
Thick coverage of hail on the ground and it may pile up in drifts or against buildings.
Light
Up to 2 mm an hour. Individual drops easily identified, puddles form slowly, small streams may flow in gutters.
Moderate
2.2–6 mm an hour. Rapidly forming puddles, down-pipes flowing freely, some spray visible over hard surfaces.
Heavy
6.2–50 mm an hour. Falls in sheets, misty spray over hard surfaces, may cause roaring noise on roof.
Violent
More than 50 mm an hour. Gutters and downpipes overflowing, spray to a height of several centimetres over hard surfaces and may cause roaring noise on roof.
Light
Gives a water equivalent of up to 2 mm an hour.
Moderate
Gives a water equivalent of 2.2–6 mm an hour.
Heavy
Gives a water equivalent of more than 6 mm an hour.
Also called internal wave.
The ocean usually has a less dense upper layer overlying the much deeper, denser waters. Waves known as internal waves often occur on the interface between these two layers.
If the interface is gradual, the direction of wave propagation may have a vertical component, trapped by refraction within upper and lower limits.
Internal waves are usually caused by flow in the lower layer moving over an obstacle such as an undersea ridge. For example, semi-diurnal tidal flows into and out of a fjord. When this happens, a semi-diurnal internal wave, or internal tide, is produced.
Although these waves do not significantly affect the sea surface, they may be detected by satellite as bands of surface slicks due to the convergence of surface currents that are produced.
A hypothetical vertical distribution of atmospheric temperature, pressure and density that by international agreement is taken to be representative of the atmosphere for purposes of pressure altimeter calibrations, aircraft performance calculations, ballistic tables, etc.
The part on a beach or coastal zone that lies between high and low tidal levels. It is exposed or inundated depending on the tide.
The region where southeast and northeast trade winds meet, usually located between 10 degrees north and south of the equator. It is a broad area of low pressure, located in the southern hemisphere during our summer, and in the northern hemisphere during its summer.
Flood. Overflow of water onto land from rainfall, rivers or raised sea levels.
When air temperature increases with height. (Generally, air cools with height.) Low-level inversions can form on clear, calm nights as the ground radiates (loses) the heat it absorbed during the day. The warm air on the ground is replaced by cooler air. The inversion creates a boundary layer that restricts mixing between the air masses. Inversions act like a lid, trapping pollutants and causing smog over cities.
Measure of the rate of electromagnetic energy received per unit area.
Line on a weather map joining places with the same air pressure.
J
Powerful winds in the tropopause – the area of the atmosphere between the troposphere and the stratosphere. They are generated by strong differences in air pressure, which reflect the great temperature differences at high altitudes.
K
A symbol used in older texts on ocean tides, and modern texts on Earth tides, that refer to phase. This practice places the tidal phase lag and the reference signal in the local time zone.
Downslope wind caused by gravity when the air on the slope is denser than air at the same height away from the slope. The wind is linked to surface cooling of the slope.
A temperature scale, designed by Lord Kelvin of Scotland in 1848, where 0°K is defined as absolute zero (where all molecular movement stops). Water freezes at 273°K, and boils at 373°K. It is used primarily for scientific purposes.
Cloud that forms in Kelvin-Helmholtz waves. The clouds, sometimes referred to as billows, are in the shape of breaking waves. These clouds are often good indicators of instability and the presence of turbulence.
Occurs when velocity shear is present within a continuous fluid, or when there is sufficient velocity difference across the interface between two fluids.
A waveform disturbance that arises from Kelvin-Helmholtz instability. Named after Lord Kelvin and Hermann von Helmholtz.
While not a scientific term, 'king tide' is widely used to describe an exceptionally high tide. These tides are a natural and predictable part of the tidal cycle. The time of year they happen varies by location and between years.
Unit of speed equal to 1 nautical mile an hour.
Classifies climate based on annual and monthly average temperature and rainfall. It also takes into account the types of vegetation in each zone.
L
Refers to extensive cooling of the central and eastern tropical Pacific Ocean and neighbouring countries. In Australia (particularly eastern Australia), wet weather is more likely during La Niña events.
The delay in the time of occurrence of high and low water due to changes in relative position of the Moon and the Sun. See also lunitidal interval.
Breeze blowing from land towards the sea, due to the relative warmth of the sea.
The rate of change of temperature with height in the atmosphere.
If hail larger than 2 cm across is expected, we use the words ‘large hail' in our forecast.
The energy absorbed or released during a change of state. Evaporation, melting and sublimation (a change from solid to gas) absorb heat from the surrounding air as energy is needed to weaken the individual hydrogen bonds between the water molecules. Condensation, freezing and deposition (gas to solid) release the latent heat, thus adding heat to the surrounding air.
A geographic coordinate that specifies the north-south position of a point on the earth’s surface. It is measured as the angular distance, subtended at the earth’s centre, along a meridian from a point on the earth to the equator. The equator is designated as zero degrees and the poles as 90 degrees.
The side of an obstacle that is furthest away from the wind.
A more-or-less isolated cloud, downwind of a barrier, resembling a smooth lens with sharp outlines. They mostly occur in mountain waves, and thus indicate possible severe turbulence and icing.
The height at which a parcel of saturated air becomes warmer than the surrounding air and thus begins to rise freely until it reaches its equilibrium level.
The height at which a parcel of moist air becomes saturated when it is cooled by adiabatic lifting.
Wind speeds of 19 km/h or less.
The bright flash of light accompanying an electrical discharge:
- within a cloud
- between clouds
- from clouds to the ground
- from the cloud top into the surrounding atmosphere.
Possible', 'chance' and 'risk'
May be used interchangeably for location forecasts. Indicates a weather element is expected in the area but there is only a chance of it happening in that specific location. This is due to the random nature of showers and thunderstorms – they can be hit and miss. 'Risk' is generally used about thunderstorms.
'Slight', 'medium', 'high', 'very high' chance
Indicates whether a location is likely to receive a measurable amount of rainfall (>0.2 mm) during the day.
The globe is divided into standard time zones. Local standard time (LST) is the time specified for each zone. This is expressed in terms of hours ahead or behind coordinated universal time (UTC). The LST zones in Australia are:
- EST – Australian Eastern Standard Time (+10 UTC) in Queensland, New South Wales, Victoria and Tasmania
- CST – Australian Central Standard Time (+9.5 UTC) in Northern Territory and South Australia
- WST – Australian Western Standard Time (+8 UTC) in Western Australia.
Areas such as bays, harbours and inland waters on which frequent boating activity occurs.
A geographic coordinate that specifies the east-west position of a point on the earth’s surface. It is measured as an angle in reference to the Prime Meridian, which is designated as zero degrees longitude.
In the southern hemisphere, areas north of about 30° latitude.
For Australia, the low latitudes are north of a line from halfway between Perth and Geraldton in Western Australia to Bourke in New South Wales. This part of Australia generally has a subtropical to tropical climate.
A transient strong wind concentrated in relatively narrow bands near the earth’s surface.
Atmospheric circulations of low pressure that rotate clockwise in the southern hemisphere. Low pressure systems generally bring stronger winds, unsettled conditions, clouds and rain.
The lowest of the low waters of any specified tidal day due to the declinational effects of the Moon and Sun.
The lowest tide level which can be predicted to occur under average meteorological conditions and under any combination of astronomical conditions. Increasingly used as chart datum, for example, for all new Australian charts. Also referred to as LAT.
A tidal constituent whose origins are a combination of lunar and solar – that is, by coincidence, there are identical force frequencies stemming from both sources.
The average duration of time between the high point of the Moon to the next high tide at any given location. Also called high water interval.
M
Pulse of wind, cloud and rainfall that cycles eastwards around the Earth near the equator. It typically takes 30–60 days to make it all the way around the globe.
Any water storage that has a storage capacity of 1GL or more.
An air mass that has originated over an extensive water surface.
Region between the Indian and Pacific oceans, including the archipelagos of Indonesia, Borneo, New Guinea, the Philippine islands, the Malay Peninsula and surrounding seas. The region is made up of thousands of islands, mountainous terrain and many shallow seas.
The terms 'maritime' and 'continent' usually describe 2 opposite climate types. Used together, they describe the extensive interaction between ocean and land that happens across this region.
Average value. It's calculated by adding all the values in a dataset together and then dividing by the number of values.
For example, to calculate Hobart's average daily July rainfall, add together the daily rainfall totals and divide by 31 (the number of days in July).
A tidal level. The average of all high waters observed over a sufficiently long period.
Average level of bodies of water (the sea, lakes and so on) from which elevation can be measured.
A tidal datum. The arithmetic mean of hourly heights of the sea at the tidal station observed over a period of time (preferably 19 years). Also referred to as MSL.
Air pressure converted to equivalent pressure at mean sea level.
This is calculated from the air pressure recorded at various altitudes. As air pressure varies at different altitudes the readings must be converted to equivalent mean sea level pressures. This allows us to compare and analyse the pressures across an area. If they weren't converted, horizontal changes in pressure would be overridden by vertical variations, due to differences in altitude between observing stations.
A mean sea level pressure (MSLP) weather map therefore shows pressures affected by changing weather conditions, not distorted by the different altitudes where air pressure was recorded.
Disrupted air-flow resulting from wind flowing over or around terrain or manmade obstructions, whereby normal horizontal wind flow is disturbed and transformed into eddies and other irregular movements.
The middle value in a dataset when all values are ordered from lowest to highest. It exceeds half of all the values in the dataset. In the case of an even number, it's the mean of the 2 middle values.
For example, to find Hobart's median July rainfall:
- rank the rainfall amounts received each day in July from lowest to highest
- the middle value is the median.
Atmospheric circulation in which the north and south component of motion is pronounced.
Periodical or quasi-periodical changes in water level caused by the daily or seasonal variations in local meteorological conditions. See also radiational tides.
Science concerned with the processes and phenomena of the atmosphere, especially as a way to forecast the weather.
A severe downburst of wind, usually from a thunderstorm, over an area less than 4 km in diameter. If the area is larger, we use 'downburst' rather than 'microburst'.
In the southern hemisphere, areas between about 30° and 55° latitude.
For Australia, the mid-latitudes are south of a line from halfway between Geraldton and Perth in Western Australia to Bourke in New South Wales. This part of Australia generally has a temperate climate.
Any water storage in which water is stored for taking and that:
- is not a major storage
- has a storage capacity of 100 ML or more
- is not used primarily for the precipitation of ash
- is not a mine tailings dam
- is not managed by an urban infrastructure operator primarily for the purposes of flood management or pollution abatement.
Optical illusion caused by refraction (bending) of light in the atmosphere. This can happen when light passes through 2 air masses of different temperature.
For example, on a sunny day the ground can warm the air directly above it, while the next layer of air remains cold. In these conditions you may see the illusion of water over the road or in the desert.
Similar to fog, but you can see further than 1 km.
The type of tide characterised by large diurnal inequalities in heights and/or times of successive high and/or low waters. In general, a type of tide intermediate between predominantly semi-diurnal and predominantly diurnal. See also form factor.
The ratio of the mass of a variable atmospheric constituent to the mass of dry air. In meteorology, the term normally refers to water vapour.
Computer simulation of the atmosphere, ocean, land surface and sea ice. A model uses mathematical equations to predict future weather or climate. The predictions are based on the laws of physics and chemistry.
Wind blowing at speeds of 20–29 km/h.
Place where observations of the environment are made to a specific standard. This is typically a location where sensors measure properties of the environment (for example, river depth or air temperature).
Seasonal reversal of winds that happens in parts of the tropics.
In northern Australia, the prevailing wind is from the east or southeast for most of the year. During active monsoon periods (any time from November to April) the winds shift to northwesterly.
As the Australian summer approaches, the continent heats up. This creates low pressure, which draws the monsoon trough – a zone of low pressure and rising air – over northern Australia. The trough pulls in moist air from the warm oceans to our north and causes widespread cloud and rain. This is referred to as the monsoon.
Mostly more clear sky than cloud.
Cloud expected to cover about 15–30% of the sky for most the day. For example, during the day the sun is behind cloud for short periods of time.
An oscillation above and downwind of a mountain caused by a disturbance in the horizontal air flow due to the high ground. They usually occur in groups, and aircraft can expect to encounter moderate to severe turbulence, uncontrolled altitude loss and possible icing if flying through them. In aviation forecasts it is coded as MTW.
A storm system composed of two or more convective cells at various stages of their life-cycle.
Large, agriculturally important region in southeastern Australia. It is defined by the catchment areas of the Murray and Darling rivers.
N
The tides of decreased range, or tidal streams of decreased speed, occurring semi-monthly near the times of the first and third quarters of the moon when the moon is in quadrature. See also fortnightly tides.
Net global solar exposure plus the net longwave exposure.
Global solar exposure less the reflected global solar exposure.
Global solar irradiance less the reflected global solar irradiance.
Net global solar irradiance plus the net longwave irradiance.
Downward longwave exposure less the upward longwave exposure.
Downward longwave irradiance less the upward longwave exposure.
The Water Regulations 2008 separate water information into 2 types – historical information, and new or ongoing information.
New or ongoing information comes into a person's possession after the grace period has expired.
New information is the information that comes into your possession after the grace period. You give new information at various specified timeframes on an ongoing/continuing basis.
Low or middle-level thick dark cloud with more or less continuously falling rain, snow or sleet. In aviation forecasts and reports it is coded as NS.
Most abundant gas in air, comprising 78% by volume. It is colourless and odourless.
Small adjustments to the amplitudes and phases of harmonic constituents to allow for modulations over the 18.61-year nodal period. See regression of lunar nodes.
Tides produced in shallow water or by frictional effects in general, which have periods equivalent to 4, 6, 8 or more cycles per day.
Description of current weather conditions or those in the immediate future.
Forecasting the weather using a mathematical computer model, based on the physics and chemistry of the atmosphere.
O
Readings of the weather and wider environment. These are taken under the ocean, at ground level and at different heights in the atmosphere. They include air pressure, temperature, wind speed, rainfall, wave height, tides, currents, ozone levels and many more. Many observations are made by instruments, such as thermometers to record temperature. Some need human assessment, such as identifying cloud type.
When a cold front overtakes a warm front, joining up with the cool air ahead of it. This cuts off the warm air from the ground, pushing it upwards.
The coastal waters zone 12–60 nautical miles from the coast.
Wind blowing from the land out to sea.
Wind blowing from sea onto land.
Occurs where the flow of air is forced up and over physical barriers such as mountains.
Sky completely covered with cloud.
Cloud expected to cover 100% of the sky for most the day.
A shallow water constituent with a speed that is a multiple of the speed of one of the basic constituents of the tide-producing force.
One of the gases that make up the atmosphere. Each ozone molecule is made up of 3 oxygen atoms.
The ozone layer in the atmosphere absorbs most of the UV radiation from the sun before it reaches the Earth.
P
A theoretical small and self-contained volume of air responding to meteorological processes as a single entity.
Mostly more clear sky than cloud.
Cloud expected to cover about 30–65% of the sky for most the day. For example, during the day the sun is regularly behind cloud for short periods.
Duration in seconds between the most energetic waves at a specific point. The larger the period, the greater the amount of energy from the waves.
See perigee.
Tides of increased range as a result of the Moon being in perigee, particularly noticeable when perigee coincides with spring tides (perigean spring tides). The opposite situation is known as apogean.
The Moon's point of closest approach in its elliptical orbit around the Earth.
Over time, the orientation of the orbit within the orbital plane gradually rotates. As a consequence, the perigee circles the earth every 8.85 years. This period is known as the perigeal cycle (not to be confused with perigean tide) and designated 'p' in tidal literature.
This is distinct from the nodal cycle, in which the orbital plane itself rotates. The Moon is at perigee every 27.5546 days.
The point of closest approach in the Earth’s orbit around the Sun.
The Sun is at perihelion every 365.2596 days – currently this nearly coincides with the start of the year (coincidentally midsummer in the southern hemisphere).
The perihelion itself circles the Sun every 20,942 years in a rotation simlar to the perigeal cycle. The period is often designated 'p' or 'p1' in tidal literature.
In the Water Regulations 2008, persons refers to both private and public entities that must give water information to the Bureau. It includes individuals as well as trusts, organisations, companies, corporations and agencies of state, territory or Commonwealth governments.
The lag of the phase of the maximum of a tidal constituent behind the phase of a reference wave (usually the phase of the corresponding equilibrium constituent at Greenwich). Also called tidal epoch.
A report of in-flight weather by an aircraft pilot. Referred to as an AIREP.
An air mass that forms over a high latitude region. Continental polar air is formed over cold land surfaces and is typically very stable with low moisture.
A semi-continuous, semi-permanent boundary between polar and subtropical air masses.
A jet stream associated with the polar front, occurring at around 23-39,000 feet above the earth’s surface. It is depicted on analysis and forecast charts when the wind is 80 knots or more.
A satellite whose orbit passes over both of the earth’s poles.
Small tide of varying period (approximately 433 days, but varying) associated with changes in the Earth's axis of rotation known as the Chandler Wobble.
Considered a class of radiational tides, as the precession has been shown to be caused by oceanographic and meteorological variations, which redistribute water masses. This precession is independent of, and much smaller than, the precession of the equinoxes, which has a period of 26,000 years. See year.
The temperature that an unsaturated parcel of dry air would have if it is brought adiabatically from its initial state to a standard pressure, typically 1000 hPa.
A line of thunderstorms that may develop ahead of an advancing cold front, and having an orientation more or less parallel to the cold front.
An elongated area of relatively low pressure that may develop ahead of an advancing cold front.
Water particles that fall from clouds to the ground. This can be as a liquid (rain) or solid (hail, snow).
Also see Duration of precipitation/rain.
An aneroid barometer calibrated to indicate altitude by measuring atmospheric pressure and using the standard atmosphere pressure/height relations. The indicated altitude will only equal the actual altitude if the actual atmosphere is equivalent to the International Standard Atmosphere.
The altitude in the International Standard Atmosphere at which a given atmospheric pressure will be observed.
A wind that blows from one direction more frequently than any other during a given period.
See standard port.
Height and direction of the swell with the highest energy. This is sometimes called the dominant swell.
The meridian of 0° longitude, also known as the Greenwich Meridian.
The earlier arrival of the time of occurrence of high and low water due to changes in relative position of the Moon and the Sun. See also lunitidal interval.
Forecast that expresses possible outcomes as a probability or percentage.
High probabilities don't guarantee an outcome. They indicate only that it's highly likely.
For example, when there is a 70% chance of rain today, it means for every 10 times we have a day with similar weather conditions to today, 7 times it will rain and 3 times it won't. By similar weather conditions we mean similar levels of moisture, temperatures, air movement and so on.
Most severe flood likely to happen at a location. Such a flood would result from the most severe combination of weather and water conditions.
Theoretical greatest depth of precipitation possible for a location with a defined:
- storm area
- storm duration
- time of year.
A forecast weather chart. Commonly known as a prog chart.
Q
The condition whereby the angle formed by the Sun, Earth, and Moon is 90°. See also syzygy.
A front which is nearly stationary or moving very slowly.
R
Acronym for RAdio Detection And Ranging. An electronic instrument used to detect distant objects and measure their range by detecting scattered or reflected radio energy.
The process by which energy is propagated through any medium by virtue of the wave motion in that medium. Electromagnetic radiation, which emits heat and light, is one form.
Fog that forms when radiational cooling at the earth’s surface lowers the temperature of the air near the ground to, or below, its dew-point. RAIN Precipitation in the form of liquid water droplets greater than 0.5 mm diameter. In aviation forecasts and reports it is coded as RA.
The cooling of the earth’s surface and the adjacent air which occurs at night when the earth’s surface suffers a net loss of heat due to outgoing radiation being greater than incoming radiation.
A quasi-periodic rise and fall of sea level caused by meteorological variability. Also known as meteorological tides.
Precipitation in the form of liquid water droplets greater than 0.5 mm diameter. Rain precipitation is generally steady and widespread and normally falls from stratiform (layer) cloud. In aviation forecasts and reports it is coded as RA.
Condensed moisture from the atmosphere falling as liquid water drops greater than 0.5 mm in diameter.
Compared to showers, rain is steadier and normally falls from stratiform (layer) cloud.
For terms describing rain in forecasts see:
Day when rainfall of at least 0.2 mm is recorded.
Total liquid from precipitation or condensation from the atmosphere, measured in a rain gauge.
See Chance of any rain.
Likely rainfall is the amount of rain with a 50% chance of happening during the day.
This is used as the lower value of the possible rainfall range in town forecasts.
Possible higher rainfall is the amount of rain with a 25% chance of happening during the day. There is also a 75% chance there won't be this much rain.
This is used as the higher value of the possible rainfall range in town forecasts.
Possible rainfall range is part of town forecasts. It describes the expected rainfall range for the day.
The rainfall range highlights the variability of certain types of rain. On showery or thunderstorm days where rain is more 'hit and miss', the possible rainfall range may be quite large (for example, 5–30 mm). When steady rainfall is expected over a wide area, the range may be smaller (for example, 10–15 mm).
The difference between the maximum and minimum water levels during a typical tidal cycle.
A criterion used in tidal analysis, which requires that only constituents which are separated by at least one complete period from their neighbouring constituents over the length of data available should be included in the harmonic analysis of a given time series. See also synodic period.
A tidal stream which flows alternately in approximately opposite directions with slack water at each reversal of direction. Encountered mainly in straits and channels. Also known as a reversing stream. See also streams.
Total amount of electromagnetic energy emitted from the sun that is reflected from the ground and irradiates a specified unit horizontal surface from underneath.
Irradiance emitted from the sun that is reflected from the ground and irradiates a specified unit horizontal surface from underneath.
The clockwise or western rotational direction of the lunar nodes around the ecliptic. This is opposite to that of most other rotations and orbits of the solar system.
The ratio of the vapour pressure to the saturation vapour pressure with respect to water. Also known as the ratio of the existing amount of water vapour to that which could be held by a parcel of air. It is usually expressed as a percentage.
Amount of moisture in the air. It's a percentage of the maximum moisture the air could hold at that temperature.
Height of sea level above land at a particular location or point of reference. For example, a coastal benchmark.
The difference between the observed sea level and the tidal prediction for a given location.
Residuals are most commonly due to weather related effects, limitations of the harmonic model, harbour seiches and errors in measurement or data processing. All of which leave recognisable imprints in the data.
In relation to radar, it is the ability to read two distinct targets separately. The clearer the resolution, the nearer the two objects can be to each other and still be distinguishable.
A result of the large amplitudes. Occurs when the frequency of a force is equal to the natural oscillation frequency of a system.
Tidal resonance occurs when the natural period of an ocean or sea is close to the period of a tide-producing force.
The representation of observed tidal variations as frequency-dependent amplitude and phase responses to the forcing functions, usually the gravitational and radiational tide-producing forces.
Elongated area of high pressure. On a weather map it shows as rounded isobars extending outwards from a high pressure system and has a ridge line.
Ice formed by the rapid freezing of supercooled water droplets when they contact an exposed object, such as an aircraft frame, forming a white and opaque granular deposit of ice.
A narrow shearing current flowing offshore through the surf zone. Sometimes (misleadingly) called a rip tide. Rips are a part of a circulation cell forced by surface wave transport, and have little to do with tides.
A low-level, horizontal, tube-shaped cloud. Usually associated with a thunderstorm gust front, where the roll cloud is completely detached from the base of the cumulonimbus cloud. It will sometimes form with a cold front.
A tidal stream that flows continually with the direction of flow changing through all points of the compass during a tidal cycle. Usually found offshore where there are no restricting barriers.
This natural tendency of tidal flows (they become rectilinear only when restricted) is due to the Coriolis force. See also streams.
A cloud formation found in the lee of a mountain or similar barrier. The air rotates around a horizontal axis parallel to the barrier, causing a hazard to aircraft.
The maximum distance at which the runway, or lights or markers delineating it, can be seen from a position above a specified point on its centerline.
S
Remote sensing of the ocean surface height by satellite-mounted microwave radar.
Techniques have been developed for extracting the tidal constants for the larger constituents from the satellite data, despite its sampling interval being about twenty times longer than the semi-diurnal period.
It provides an accurate global ocean database of tidal constants, which was previously restricted to areas close to coastal tide gauges and to numerical models constrained over wide areas of the ocean surface.
To add something to the point where no more can be absorbed, dissolved, or retained. In meteorology, it is used when discussing the amount of water vapor in a volume of air.
The line on a thermodynamic diagram that depicts the change in temperature of a saturated air parcel as it rises or falls and undergoes cooling or heating due to adiabatic expansion or compression.
The rate of change in temperature of a saturated air parcel as it adiabatically ascends or descends through the atmosphere. The rate varies, from approximately 1.5 to 3°C per 1,000 feet, e.g., the more water vapour present in a rising parcel of air, the greater the condensation, and thus the greater is the latent heat released, reducing the rate of cooling.
Used to describe an amount of cloud covering the sky of three or four oktas (eighths). In aviation forecasts and reports it is coded as SCT.
Breeze blowing from the sea towards land, due to the relative warmth of the land.
A type of advection fog that forms when warm moist air advects over water with a cooler temperature, and the consequent cooling of that air to below its dewpoint by the underlying cooler water.
Water temperature close to the surface of the sea. It's usually measured 1–20 m below the sea surface.
See Wind waves.
The discontinuity in temperature and humidity that marks the leading edge of the intrusion of cool and moist maritime air associated with a sea-breeze.
In Australia, the seasons are:
- Spring – September, October, November
- Summer – December, January, February (the hottest months)
- Autumn – March, April, May
- Winter – June, July, August (the coldest months)
While some people and countries use the equinoxes and solstices to define the start of each season, it's a better fit to Australia's temperatures to use 1 March, 1 June, 1 September and 1 December. Defining each season as a set of three whole calendar months is also useful for presenting climate-based statistics.
In tide tables, a port for which predictions are required, but for which predictions from the nearest standard port must be used (with suitable corrections). Used when insufficient data for a reliable harmonic analysis is available.
Also called a subordinate port.
Height and direction of the swell with the second-highest energy.
A standing wave in an enclosed or semi-enclosed body of water set off by weather, seismicity, or incident ocean waves.
Seiches are primarily a resonance phenomenon, whose wavelength and period are determined by the geometry. A characteristic feature is the existence of nodes – points of minimum water level disturbance, but greatest horizontal currents – and antinodes, where the reverse occurs.
Relating to earthquakes.
The description of the tide-generating potential was simplified in several ways, two of which were by ignoring the continuous re-distribution of mass that occurs as the water moves in response to the tidal forces, and by ignoring the warping of the elastic solid earth surface as the water level varies. These effects are commonly combined into the term 'self-attraction and loading' (SAL).
A tide, having a period or cycle of approximately one-half of a tidal day.
The direction towards which the current or tidal stream is flowing.
The Bureau defines a severe thunderstorm as one with winds of 48 knots or greater, or hail of diameter 2 cm or larger, or tornadoes or flash floods.
Weather with the potential to affect safety and wellbeing or cause damage to property and infrastructure.
Precipitation, often short-lived (but may last up to an hour) and heavy, falling from convective clouds. They usually begin and end suddenly and occur over localised areas. In aviation forecasts and reports it is coded as SH.
Rain that usually begins and ends suddenly. Showers are relatively short-lived but may last half an hour – often, but not always, separated by blue sky. For terms describing showers in forecasts see:
Combined height of the sea and the swell that mariners experience on open waters. Refers to the average wave height of the highest third of the waves.
Also known as combined sea and swell, or total wave height.
Understanding what's going on in the environment – what it means now and in future for a particular task. Often used in relation to emergency management.
A thermodynamic diagram with a skewed temperature scale on the horizontal axis and a logarithmic scale of pressure on the vertical axis. It is used to plot radiosonde soundings which give a vertical profile of air temperature and dew point temperature through the atmosphere. Forecasters use it to evaluate and forecast air parcel behaviour. Some values that can be determined are the Convective Condensation Level, the Lifting Condensation Level, and the Level of Free Convection.
The state of a tidal stream when its speed is or nearly zero. Especially the moment when the reversing stream changes its direction. Also known as slack tide or slack. See also streams.
A mixture of rain and snow, or falling snow that is melting into rain.
Fog or haze thickened by smoke or other forms of atmospheric pollution. Can be hazardous to health.
Small particles produced by combustion that are suspended in the air (a transition to haze may occur when the smoke particles have travelled great distance and when the larger particles have settled out, the remaining haze particles become widely scattered through the atmosphere). In aviation observations and forecasts it is coded as FU.
Precipitation of ice crystals, most of which are branched (sometimes star-shaped).
Snow that falls for a short period, usually starting and ending suddenly. Typically, it falls from clouds formed by convection.
The water in the spaces between soil particles.
If all the spaces are occupied by air, the soil is completely dry. If they're all filled with water, the soil is saturated.
The time when the sun is at its furthest from the equator and the tilt in the Earth's axis lines up most with the direction of the sun. This means there is the greatest difference between daylight and night-time hours. These times mark the longest (around 22 December) and shortest (around 21 June) days in the southern hemisphere. They're known as the summer and winter solstices.
Tides occurring near the times of summer and winter solstice, when the sun is overhead at the Tropic of Cancer or Capricorn.
Calculated from the monthly or seasonal fluctuations in the air pressure difference between Tahiti and Darwin. It's used to gauge the strength of El Niño and La Niña events and their potential effect on the Australian region.
A classification of tidal constituents according to their period. The principle species are semidiurnal, diurnal, and long period.
The rate of change in the phase of a constituent expressed in degrees per hour. The speed is equal to 360° divided by the constituent period expressed in hours.
The rate at which a stream or current flows. Usually expressed in knots or metres per second.
One of the 4 seasons of the year. In the southern hemisphere it's September, October and November.
See tidal plane.
The tides of increased range, or tidal streams of increased speed, occurring semi-monthly near the times of full moon and new moon. See also fortnightly tides.
A sudden strong wind or storm that is usually intense but short-lived.
Extended narrow region (or line) of gusty winds. Often these come with thunderstorms and rain, but a squall line can also happen when skies are clear. It's generally part of a cold front or thunderstorm environment.
A luminous and sometimes audible electric discharge that occurs from objects, especially pointed ones, when the local electrical field (voltage) attains a strength of near 1000 volts per centimetre. It often occurs during the latter phases of a violent thunderstorm. It typically appears on the extremities of aircraft, lightning rods and steeples.
A state of the atmosphere in which a lifted air parcel will sink to its equilibrium level once the lifting mechanism ceases, due to the air parcel being denser (cooler) than the surrounding air.
The condition at high or low water when there is no perceptible change in the height of tide for a period of time. Sometimes called platform tide. See also streams.
A mathematical model of the atmosphere which is standardised so that predictable calculations can be made.
In the context of tide tables, a port for which sufficient data is available in order for a set of official predictions to be produced. Also known as a primary port, especially in North America.
An atmospheric wave that is stationary with respect to the earth’s surface.
Any water information in the aquifer framework used by a State.
Any water information in a State's National Groundwater Information System (NGIS) database.
Fog that forms when when cool air, passing over warm water, reaches its saturation point due to water evaporating from the warm water into the cooler air. Fog rising in the convection currents above the water give rise to a steaming appearance.
A well that is connected to the sea through a small opening which filters out any high frequency waves while admitting the long period tidal (and other) level variations. Used by some types of tide gauges to measure sea level, traditionally via a float or some other means.
See Thunderstorm.
Abnormal rise in sea level above the normal tide levels. These powerful ocean movements are caused by strong winds piling water up against the coast as a cyclone approaches.
An abnormal rise in sea level over and above the normal (astronomical) tide levels. It can be thought of as the change in the water level due to the presence of a storm.
These powerful ocean movements are caused by strong winds piling water up against the coast as a cyclone or other low pressure system approaches.
The water level that results from the combination of the storm surge and the normal (astronomical) tide.
Tide that combines storm surge with normal tide levels.
The high water mark on a shoreline, typified by the presence of flotsam and jetsam (types of marine debris associated with vessels).
Cloud that forms in a flattish layer rather than developing height.
Cloud that forms in the lower levels of the atmosphere in a relatively flat layer, with some taller, more clumpy areas. It can produce drizzle.
The boundary zone between the stratosphere and the mesosphere. In the stratosphere the temperature increases with height, with the stratopause being the point of maximum temperature, prior to a decrease in temperature in the mesosphere.
The layer of the atmosphere located between the troposphere and the mesosphere. It is characterized by an increase in temperature with height and an absence of convective clouds and associated turbulence.
Low cloud forming a uniform layer.
Flow of water in streams, rivers and other channels. Also called 'flow'.
Also known as tidal currents. Sometimes used to refer exclusively to the tidal currents along the main directions of ebb and flood.
The set of the current is the direction in which it flows. On the incoming tide, the streams are said to be in flood and the outgoing streams are in ebb. The stand of the tide occurs near high and low water when the water level is unchanging.
Tidal streams which flow back and forth along a line are rectilinear, whereas those that follow an elliptical circuit (due to the Coriolis force) are rotary. See also rotary flow.
Wind blowing at speeds of 40–50 km/h.
An area where 2 tectonic plates meet and move towards each other. One slides underneath the other and moves down into the Earth. The resulting undersea earthquakes are the main cause of tsunamis.
The process of ice changing directly into water vapour.
A descending motion of air in the atmosphere, usually with the implication that it extends over a broad area such as occurs with a high pressure system.
A band of relatively strong winds found between 20 and 40° latitude in the middle and upper troposphere. Jet streams form near boundaries of adjacent air masses with significant differences in temperature.
One of the 4 seasons of the year. In the southern hemisphere it's the 3 hottest months – December, January and February.
Times of day when sun protection is recommended (when the UV index is 3 and above).
Little chance of the sun being hidden by cloud.
Cloud expected to cover up to 15% of the sky for most the day.
A day can be called sunny though more than half the sky is covered in cirrus cloud. Cirrus clouds are often thin and wispy, allowing enough sunlight through to produce shadows.
Persistent, single, intense, rotating updraught and downdraught coexisting in a thunderstorm. It can bring violent winds (sometimes tornadoes), heavy rain and hail.
When the temperature of a liquid falls below its freezing point, without it becoming a solid.
Wind speed and direction measured at 10 m above the Earth's surface. In the marine environment, surface wind generates waves locally. Strong surface winds that come with intense storms can cause large swell waves.
See Peak wave period.
Waves which have travelled from the area where they were caused (for example, by storm activity and wind). They may travel thousands of kilometres from their origin before dying away. There can be swell waves even if the wind is calm and there are no sea waves.
The minimum length of data necessary to separate a pair of constituents according to the Rayleigh criterion.
The average period of the revolution of the Moon around the Earth with respect to the Sun or the average interval between corresponding phases of the Moon. The synodical month is 29.5305888 mean solar days long. Also known as lunation or lunar month.
Chart showing lines of equal pressure (isobars), corrected to mean sea level, over a broad area (for example, Australia). They're based on the observations taken every 3 hours by weather observers and automatic weather stations. Also called weather maps.
Used to describe weather features in the lower atmosphere that range in size from several hundred to several thousand kilometres across. For example, it's often used about high and low pressure systems over mid-latitude regions.
The condition whereby the Sun, Earth and Moon are in alignment. See also quadrature.
T
Section of the Earth's crust which moves in relation to the other plates. The amount of movement is usually measured in centimetres each year.
How hot or cold something is, measured with a thermometer.
A harmonic constituent with three periods in a constituent day. See also species.
A layer in which the temperature decreases significantly (relative to the layers above and below) with depth. The principal ones are designated diurnal, seasonal, and main thermocline. A common feature of thermoclines is the presence of internal tides.
The layer of the atmosphere located between the mesosphere and outer space. It is a region of increasing temperature with height, and includes all of the exosphere and most of the ionosphere.
The sound emitted by rapidly expanding gases along the channel of a lightning discharge.
A day on which thunder is heard at least once in a location.
Thunderstorms are associated with convective clouds (cumulonimbus) and are more often accompanied by precipitation. They are often short-lived and usually occur over localised areas. Aviation hazards associated with thunderstorms include wind shear, loss of visibility, low cloud, lightning strikes and possible large hail. In aviation forecasts and reports it is coded as TS.
Thunderstorms are cumulonimbus clouds with lightning and thunder. They often come with rain and hail, are usually short-lived and affect a relatively small area.
For terms describing thunderstorms in forecasts see:
Rain with thunder. An area of rain that contains thunderstorms.
A level of the sea defined by some phase of the tide, from which depths of water and heights of tide are calculated.
In relation to a watercourse, the maximum upstream location on that watercourse at which a tidal variation in water level is observed.
The various tidal water levels, such as low water or mean sea level for example, are known collectively as tidal planes. Also known as tidal datums, tidal levels, tidal elevations or datum planes.
Tidal planes are defined at the tide gauge – a specific location – but for practical purposes they are considered points on a continuous surface. The full list of tidal planes is extensive and different countries define the tidal planes differently. For legal definitions, the appropriate regulatory authority or document should be consulted.
Where the tide moves up and down the lower reaches of a river, a volume, known as the tidal prism, of fresh water is displaced each tidal cycle.
The tidal prism takes its name from the fact that the front between fresh and salt water is often inclined to the vertical, with the downstream edge of the fresh water riding over the salt.
This term is used in various contexts, including:
- Coastal aquifers – in this case, the rise and fall of the tide is often accompanied by a delayed and reduced oscillation of water level in nearby wells.
- The bringing of nutrient-rich offshore water into the shallower regions – this usually involves a relatively large flood tide bringing water up and into a bay or other semi-enclosed area, where it mixes with water from previous high tides before draining more slowly back to the open ocean. Submarine canyons across the continental shelf may also cause a rectified flow with a net increase of nutrients in the upper layer.
A generic term for a type of large-scale wave generated by the gravitational forces of the Sun and Moon on the ocean. Sometimes this term is used incorrectly as a synonym for tsunami.
A device for measuring sea level.
Timings refer to local time in each state/territory.
Early in the morning
Expected before 7 am
In the morning
Expected between 1 am and 11 am
In the late morning
Expected between 9 am and noon
During early afternoon
Expected between noon and 4 pm
During the afternoon
Expected between 1 pm and 9 pm
In the evening
Expected between 6 pm and midnight
Later in the evening
Expected after 9 pm
Tall, rapidly rotating column of wind 5–1000 m in diameter. Tornadoes are attached to the base of a cumulonimbus or large cumulus cloud and can cause damage on the ground. They may form water spouts when they happen over water.
See also Funnel cloud.
In relation to a water sample, the total concentration of all forms of nitrogen found in the water sample. This includes nitrate, nitrite, ammonia-N and organic forms of nitrogen.
Ozone concentration in the atmosphere above a point.
In relation to a water sample, the total concentration of all forms of phosphorus found in it.
In relation to a water sample, the measure of the particles mixed in it.
Combined height of the sea and the swell that mariners experience on open waters. Refers to the average height of the highest third of the waves.
Also known as combined sea and swell, or significant wave height.
A vertically developed cumulus cloud, often a precursor to cumulonimbus. In aviation forecasts and reports it is coded as TCU.
Rainfall observers report a trace of rain when a little water can be seen in the rain gauge, but there is less than 0.1 mm in total.
This is often reported as 'tce' or 'tr' in rainfall bulletins.
Rainfall amounts between 0.1 mm and 0.2 mm are reported as 0.2 mm in rainfall bulletins.
Predicted path of the centre of a tropical cyclone.
East to southeasterly winds (in the southern hemisphere) which affect tropical and subtropical regions, including the northern areas of Australia. During the monsoon season in northern Australia, the easterly trade winds are replaced by moist northwesterly (monsoonal) winds from the Indian Ocean and southern Asian ocean waters.
In the northern hemisphere the trade winds blow east to northeasterly.
So in both hemispheres, they tend to blow from the east to the west and towards the equator.
Located at 23.5 degrees north, it is the most northern point on the earth where the sun is directly overhead (on 21 June).
Located at 23.5 degrees south, it is the most southern point on the earth where the sun is directly overhead (on 22 December).
At latitudes near the maximum declination of the Moon (which varies between 18.3° and 28.6° latitude north and south over the course of the nodal cycle) the diurnal tides are greatest when the moon is near maximum declination.
These so-called tropic tides are the equivalent of the more common spring tides, with the beat frequencies being diurnal instead of semi-diurnal. The range (peak to peak distance between high and low tide) at the time when these diurnal tides are greatest is known as the tropic range.
An air mass that forms in the tropics or subtropics. Maritime tropical air is produced over oceans and is warm and humid, while continental tropical air is formed over arid land and is very hot and dry.
Very intense low pressure system that forms over warm ocean waters in the tropics (low latitudes). It brings damaging winds, torrential rain, large waves and storm surges.
An area of organized convection, originating in the tropics or occasionally the subtropics, that maintains its identity for 24 hours or more, but has no closed wind circulation. It is often the first developmental stage of a tropical cyclone.
The region of the earth located between the Tropic of Cancer, at 23.5 degrees north and the Tropic of Capricorn, at 23.5 degrees south.
The boundary zone or transition layer between the troposphere and the stratosphere.
The lowest layer of the atmosphere. Is characterised by clouds, weather and a decrease in temperature with increasing altitude.
Elongated area of low pressure. On a weather map a trough shows as a dashed line, which often extends outward from a low-pressure centre.
Generally, the wind changes direction as you cross from one side of a trough to the other.
The measure of the light-scattering properties of water. This is an indicator of the presence of suspended solids.
Irregular fluctuations occurring in fluid motions.
Term used in the northwestern Pacific for a tropical cyclone.
U
Electromagnetic radiation with wavelengths shorter than visible light, but longer than x-rays. Exposure to too much UV radiation from the sun can cause skin cancer.
Time as defined by the rotation of the Earth and determined from diurnal motions reflecting this rotation.
It is expressed in respect of 0° meridian, and used as a standard for comparison of global phenomena. Also known as Greenwich Mean Time (GMT) and Zulu time (military term). See Coordinated Universal Time (UTC).
If an air mass is unstable it contains 'parcels' of air that are warmer or more humid than the surrounding air. This causes them to rise, forming cloud. Unstable air can help thunderstorms develop.
The total amount of terrestrial electromagnetic energy emitted from the Earth's surface that irradiates a specified unit horizontal surface from underneath.
Longwave irradiance emitted by the Earth's surface that irradiates a specified unit horizontal surface from underneath.
In the context of tides, upwelling (upwards movement of water) can occur as a result of periodic flow over uneven topography, especially submarine canyons on the continental shelf.
Upwelling is more often associated with alongshore winds combined with the Coriolis effect, or spatially divergent wind fields over the open ocean. Tidal upwelling can also lead to significant flux of nutrients into the photic zone.
Information about the treatment and reuse of urban wastewater.
Information about water in the urban stormwater drainage system.
Information about wastewater that is disposed of through a sewerage system.
Information about water supplied by urban water utilities.
V
An anabatic wind which forms during the day by heating of the valley floor. As the ground becomes warmer than the surrounding atmosphere, the lower levels of air heat and rise, flowing up mountain slopes.
A test designed to detect flaws in the mechanical operation of tide gauge chart recorders.
A measurement is taken of the positive distance between a fixed point near the top of the gauge, down through the stilling well to water level.
The sum of this distance, which is a maximum at low tide, and the tide gauge reading should be constant through a full tidal cycle. The sum when plotted against the measured distance (with the latter plotted on the vertical axis) should therefore be a vertical line. Deviations from the straight line can be interpreted as faults such as backlash in the gauge mechanism or scaling error.
See dodge tide.
Gaseous form of a substance.
The difference between the amount of moisture in the air and how much moisture the air can hold when it is saturated.
One of many deviations in the Moon's orbit caused by changes in the solar gravitational potential during the course of orbit, which produces variational constituents. See also evection.
A clockwise shift in the wind direction.
Rain that evaporates before it reaches the ground.
A measure of the opacity of the atmosphere. It is the greatest distance one can see prominent objects with unaided normal eyesight.
Rules that govern civil aircraft flight under conditions that allow navigation by visual reference to the earth’s surface.
Rotating mass of air or water, such as a tropical cyclone.
Measure of local rotation or spin in the atmosphere at a specified level. In weather analysis it usually refers to rotation around a vertical axis.
W
A disruption of the airflow behind a moving aircraft that produces turbulence.
Deep east-west air circulation caused by the contrast between the warm waters of the western Pacific and the cooler waters of the eastern Pacific.
The horizontal movement of warmer air into a location.
Leading edge of a relatively warm air mass moving into a region of cooler air.
Section 125 of the Water Act 2007 defines water information as:
any raw data and metadata, or any value added information product, that relates to:
- the availability, distribution, quantity, quality, use, trading or cost of water; or
- water access rights, water delivery rights or irrigation rights; and includes contextual information relating to water (such as land use information, geological information and ecological information); or
any metadata relating to data of a kind referred to above
and includes contextual information relating to water (such as land use information, geological information and ecological information).
An area (however described) used by an agency of the Commonwealth or an agency of a State for the purposes of managing surface water or ground water.
Examples of water management areas:
- river basins
- catchments
- aquifers
- water sources
- sustainable diversion limit resource units.
A body of water that is naturally occurring, is impounded by a constructed barrier or is within an excavation, and includes ponds, swamps, lakes, wetlands, weirs, dams, basins, reservoirs and open pits.
Water in its gaseous form.
Atmospheric pressure caused by water vapour. This is one way of measuring humidity. At a given temperature, an increase of water vapour in the air corresponds to an increase in humidity.
The depth of water that would result if solid precipitation were melted.
Height difference between the wave crest and the preceding trough.
Average horizontal distance between successive crests (or troughs) of a wave pattern.
Average time between successive crests (or troughs) of waves.
Index to measure thermal stress – or how comfortable you might feel under certain atmospheric conditions. It takes into account wind, air temperature, humidity, and radiation from the sun and nearby hot surfaces.
Annual season in the tropics that is warm, humid and wet. In northern Australia, it's from October to April. In some parts of our tropics, such as Western Australia, the wet months are only from about January to March.
Temperature measured using a thermometer with the bulb wrapped in wet muslin. The evaporation of water from the thermometer has a cooling effect, so the temperature is less than that measured by a dry-bulb (unmodified) thermometer.
A small-scale, rapidly rotating column of wind, formed thermally and thus most likely to develop on clear, dry and hot afternoons. Often called a dust devil when made visible by the dust, dirt or debris it picks up. In aviation forecasts and reports it is coded as PO.
Occurs when clouds and the earth’s surface seem to blend, resulting in the horizon becoming erased.
Moving air in a continuous succession of gusts, lulls and changes of direction.
The product of the average wind speed and the period over which that average speed was measured.
Wind direction and/or speed change over a vertical or horizontal distance.
Speed at which wind is blowing.
Our measured wind speeds are generally averaged over 10 minutes at a standard height of 10 m above open, flat ground or sea.
The 10-minute average wind speed is also what we forecast.
Wind speed is given in kilometres an hour in land areas and knots in marine areas.
Wind gusts are stronger than average wind speeds.
Waves generated by the current (and recent past) wind in the area. Also called sea waves.
Measure of the cooling effect of the wind on body temperature.
Prolonged period of average wind speeds exceeding 40 km/h during the day.
One of the 4 seasons of the year. In the southern hemisphere it's the 3 coldest months – June, July and August.
Y
Four different types of year are of significance to tides.
Sidereal year – the period taken by earth complete a single orbit of the sun, 365.2564 mean solar days (msd).
Tropical year – measured in relation to the beginnings of the various seasons (successive vernal equinoxes). It is slightly shorter than the sidereal year as a consequence of precession. The axis of the earth is tilted at about 23½° degrees to the perpendicular of the orbital plane. The axis slowly precesses about the perpendicular, in the manner of a 'sleeping top'. Each precession takes 26,000 years. If a single precession was completed in just one day, then we would experience four seasons in a single day. This means that the seasons advance 1/26,000th part per sidereal year faster than they would without precession, and the tropical year is therefore only 365.2422 msd.
Anomalistic year – the period between successive perihelions. Just as the anomalistic month is slightly longer than a sidereal month, the anomalistic year, 365.2596 msd, is slightly longer than a sidereal year. Of the three types of year, the anomalistic is of greatest importance in tides. The longitude of the sun undergoes a complete cycle in one tropical year.
Julian year – defined as 365.25 days.