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.