# Glossary

This glossary is here to help you understand some specific meteorological terms.

## 1. Dew Point

The dew point is the temperature to which air must be cooled to become saturated with water vapor. When further cooled, the airborne water vapour will condense to form liquid water (dew). When air cools to its dew point through contact with a surface that is colder than the air, water will condense on the surface.

## 2. Geopotential height

Geopotential height or geopotential altitude is a vertical coordinate referenced to Earth's mean sea level, an adjustment to geometric height (altitude above mean sea level) that accounts for the variation of gravity with latitude and altitude. Thus, it can be considered a "gravity-adjusted height".

The geopotential height is interesting in meteorology because it allows to measure constant pressure levels. It roughly represents the height above sea level of a pressure level. For example, if a station reports that the 500 mb [i.e. millibar] height at its location is 5600 m, it means that the level of the atmosphere over that station at which the atmospheric pressure is 500 mb is 5600 meters above sea level. This is an estimated height based on temperature and pressure data.

A high geopotential height is often associated to an anticyclone and a low geopotential height corresponds to a depression.

## 3. Pseudo-adiabatic Potential Temperature of The Wet Bulb

It indicates the temperature of an air particle would have if it is raised by an adiabatic process until its condensation level, then brought down to a 1000hPa level by a pseudoadiabatic process, while remaining satured without evaporation during its descent. This parameters allows to compare the temperature of samples of air that are at different altitudes.

This parameter is very useful in meteorology to track the dynamic areas of a depression.

## 4. Vertical Velocity

The vertical velocity is the vertical speed of a mass of air. The displacement is expressed in Pa (in meteorology, the vertical levels are often expressed in isobar levels, see Weather Models for more details). The higher the height, the lower the pressure. If the vertical velocity is positive, there is an upward current (ex : the earth surface is warmed by the sun). The opposite corresponds to a downdraft (ex : an cold air mass passes above a warmer ground).

## 5. Wind Components

The U and V wind components are the two horizontal wind speed vector components :

• U : from west to east

• V : from south to north