A
Horizontal movement of air in the lower atmosphere due to wind.
Large body of air with approximately uniform temperature and water vapour throughout.
Instrument that determines altitude using atmospheric pressure.
Instrument used to measure wind speed and, in some cases, wind direction.
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.
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.
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.
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.
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.
B
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.
Scale that estimates wind speed based on the effects we feel, see and hear (human observations).
Very strong and cold wind loaded with snow, some lifted off the ground.
Upward force which lifts a parcel of air. This happens when the air parcel is less dense than the surrounding environment.
C
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.
Artificial or constructed waterway designed to carry water.
Wispy, feathery-looking cloud formed high in the atmosphere and made of ice crystals.
Free from cloud, fog, mist or haze.
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.
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.
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.
Line joining points of equal value on a surface.
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.
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.
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.
Indicator of moisture in the atmosphere used by the agricultural industry to help with decisions about when to spray crops.
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.
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.
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.
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.
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 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
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.
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.
Zone of relatively low pressure along the equator, including across northern Australia.
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).
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.
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.
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.
Flat or nearly flat land subject to flooding that is next to a stream or river.
Warm, dry wind generally blowing downslope on the lee side (sheltered from the wind) of a mountain.
The boundary between air masses with different characteristics. See also:
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.
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.
If hail larger than 5 cm across is expected, we use the words 'giant hail' in our forecast.
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.
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.
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.
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.
Highest tide level predicted under average meteorological conditions and any possible combination of astronomical conditions. Often referred to as HAT.
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.
Aboriginal and Torres Strait Islander communities have seasonal calendars based on traditional weather, climate and environmental knowledge passed from generation to generation.
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.
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.
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.
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
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.
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.
Breeze blowing from land towards the sea, due to the relative warmth of the sea.
If hail larger than 2 cm across is expected, we use the words ‘large hail' in our forecast.
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.
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.
Atmospheric circulations of low pressure that rotate clockwise in the southern hemisphere. Low pressure systems generally bring stronger winds, unsettled conditions, clouds and rain.
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.
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).
Average level of bodies of water (the sea, lakes and so on) from which elevation can be measured.
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.
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.
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.
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.
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.
Large, agriculturally important region in southeastern Australia. It is defined by the catchment areas of the Murray and Darling rivers.
N
Most abundant gas in air, comprising 78% by volume. It is colourless and odourless.
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.
Sky completely covered with cloud.
Cloud expected to cover 100% of the sky for most the day.
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
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.
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.
Height and direction of the swell with the highest energy. This is sometimes called the dominant swell.
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.
R
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).
Amount of moisture in the air. It's a percentage of the maximum moisture the air could hold at that temperature.
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.
S
Breeze blowing from the sea towards land, due to the relative warmth of the land.
Water temperature close to the surface of the sea. It's usually measured 1–20 m below the sea surface.
See Wind waves.
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.
Height and direction of the swell with the second-highest energy.
Relating to earthquakes.
Weather with the potential to affect safety and wellbeing or cause damage to property and infrastructure.
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 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.
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.
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.
One of the 4 seasons of the year. In the southern hemisphere it's September, October and November.
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.
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.
Tide that combines storm surge with normal tide levels.
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.
Low cloud forming a uniform layer.
Flow of water in streams, rivers and other channels. Also called '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.
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.
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.
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 day on which thunder is heard at least once in a location.
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.
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.
Ozone concentration in the atmosphere above a point.
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.
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.
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.
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.
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.
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.
V
Gaseous form of a substance.
Rain that evaporates before it reaches the ground.
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
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.
Leading edge of a relatively warm air mass moving into a region of cooler air.
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.
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.
Moving air in a continuous succession of gusts, lulls and changes of direction.
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.