VMS Links » Glossary Friday, February 24, 2017
 Glossary

 

Commonly used Terms used in both short and long term Forecasts
 
Climate Change
Climate change is a change in the statistical distribution of weather over periods of time that range from decades to millions of years. It can be a change in the average weather or a change in the distribution of weather events around an average (for example, greater or fewer extreme weather events). Climate change may be limited to a specific region, or may occur across the whole Earth.
 
Cloud
Mass of water droplets or ice crystals caused by water vapour in the atmosphere condensing or freezing
 
Convection
The process generally associated with warm rising air and the formation of cloud. Local breezes, wind and thunderstorms are a result of convection in the atmosphere.
 
Convergence Zone
Convergence zone usually refers to a region in the atmosphere where two prevailing flows meet and interact, usually resulting in distinctive weather conditions. An example of a convergence zone is the Intertropical Convergence Zone(ITCZ), a low pressure area which girdles the Earth at the Equator. Another example is the South Pacific convergence zone that extends from the western Pacific Ocean toward Vanuatu, Fiji, Samoa or French Polynesia.
 
Cyclone
An atmospheric circulation that rotates clockwise in the southern hemisphere and anti-clockwise in the northern hemisphere. Cyclones are areas of lower pressure and generally associated with stronger winds, unsettled conditions, cloudiness and rainfall.
 
Drizzle
Fairly uniform precipitation (rain) composed exclusively of very small water droplets (less than 0.5 mm in diameter) very close to one another.
 
Flash Flood
Flood of short duration with a relatively high peak discharge.
 
Fine
No rain or other precipitation (hail, snow etc.). The use of fine is generally avoided in excessively cloudy, windy, foggy or dusty conditions.
 
Flood
A flood occurs when water inundates (covers) land which is normally dry.
 
Front
The boundary between air masses having different characteristics.
 
Cold Front
In some regions along the polar front, cold dense air advances equatorwards, causing warm air to be forced aloft over its sloping surface. This portion of the polar front is known as a cold front.
Cold polar air is replacing warm tropical air.  
 
Warm Front
In other regions along the front, warm air of lower density moves polewards, sliding over its sloping surface. This portion is called a warm front.
Warm tropical air replaces cold polar air.
 
High pressure
An atmospheric circulation that rotates anti-clockwise in the Southern Hemisphere and closkwise in thenorthern hemisphere. Anticyclones are areas of higher pressure and are generally associated with lighter winds and fine conditions.
 
Hurricane Force Winds
Wind speed exceeding 64 knots. This is usually common in Tropical cyclones.
 
Low pressure
An atmospheric circulation that rotates clockwise in the southern hemisphere and anticlockwise in the northern hemisphere. Cyclones are areas of lower pressure and generally associated with stronger winds, unsettled conditions, cloudiness and rainfall.
 
Rain
Precipitation of liquid water drops greater than 0.5 mm in diameter.
 
Ridge
A ridge is an elongated area of high pressure. It is indicated by rounded isobars extending outwards from an anticyclone and has associated with it a ridge line. The pressure at a point on the ridge is higher than at an adjacent point on either side of the line.
 
Showers
Precipitation, often short-lived (but may last half an hour) and heavy, falling from convective clouds. Usually begin and end suddenly.
 

Temperature
A physical quantity characterising the mean random motion of molecules in a physical body.
 
Trade winds
East to southeasterly winds (in the southern hemisphere) which affect tropical and subtropical regions. During the summer season in Vanuatu, the trade winds are usually lighter, while in the Winter Season, trade winds are stronger.
 
Trough
A trough of low pressure is an elongated area where atmospheric pressure is low relative to its immediate surroundings.
 
Thunderstorm
Sudden electrical discharges manifested by a flash of light (lightning) and a sharp rumbling sound. Thunderstorms are associated with convective clouds (Cumulonimbus) and are more often accompanied by precipitation. They are usually shortlived and hit on only a small area.