Adiabatic cooling/heating The rate of cooling (by
expansion) as an air parcel rises. It is approximately 1 degrees Celsius per 100
metres. It is also the rate of heating (by compression) as an air parcel
Air mass A large body of air that has similar
temperature, pressure and moisture properties.
Anticyclone High pressure system in which air descends
to give calm conditions and clear skies. Associated with summer heatwaves and
winter frosts and fogs.
Atmosphere The envelope of air surrounding the Earth and
bound to it by gravity.
Atmospheric pressure Pressure (force per unit area)
exerted by the atmosphere on any surface by virtue of its weight.
Clear sky Sky with a total cloud cover of less than one
Climate Long-term (50 year) weather averages.
Cold front The "leading edge" of a relatively
cold air mass.
Continental climate A climate with a high temperature
range away from the influence of the sea. Winters will be colder and summers
warmer compared to a coastal location for the same latitude.
Depression (cyclone, low, low-pressure area) Area in the
atmosphere in which the pressures are lower than those of the surrounding region
at the same level. In its development a depression usually has the following
phases. A wave (young) depression forms and moves along a front. Mature
depressions have well-developed warm sectors and both cold and warm fronts. An
occluded depression is that within which there has developed an occluded front.
Dew point (dew-point temperature) The temperature to
which a given parcel of air must be cooled in order for saturation to occur.
When this temperature is below 0 °C, frosts form.
Evaporation The physical process by which a liquid or
solid substance is transformed to a gas; the opposite of condensation.
Fog Saturated air with visibility below one kilometre.
Fog differs from cloud only in that the base of fog is at the Earth's surface
while clouds are above the surface.
Front The meeting point between two air masses of
different density. Since the temperature is the most important regulator of the
atmosphere density, a front almost invariably separates air masses of different
temperature. When warmer air replaces the colder, it is a warm front, and a
front is a cold one when the opposite occurs.
Humidity Water vapor content of the air.
Isobar A line of equal or constant pressure. Measured in
Maritime climate A climate with a low temperature
range influenced by proximity to the sea. Winters will be warmer and summers
cooler compared to a continental location for the same latitude.
Occluded front A front that is formed as a cold front
overtakes a warm front and lifts the warm air completely off the ground.
Okta Ameasure of cloud cover (in fractions of
eight) on a synoptic chart.
Precipitation Any of all of the forms of water
particles, whether liquid or solid, that fall from the atmosphere and reach the
ground. The forms of precipitation are: rain, drizzle, snow, snow grains, snow
pellets, diamond dust, hail, and ice pellets.
Prevailing wind The most common wind direction for a
Rain Shadow The dry region on the leeward side of a
mountain (the side sheltered from the wind).
Relief rainfall Formed when air is forced to rise over
relief features such as hills or mountains. Cooling and condensation occurs as
the air rises.
Seasonality Periodic fluctuations in the climate related
to seasons of the year e.g. wet winters, drier summers.
Smog A word currently used as a synonym for general air
pollution. It was originally created by combining the words "smoke"
Synoptic chart A weather chart reflecting the state of
the atmosphere over a large area at a given moment.
Temperature A physical quantity characterizing the mean
random motion of molecules in a physical body. In other words, it is a measure
of the degree of hotness or coldness of a substance.
Temperature range Maximum minus the minimum temperature
for a particular location.
Warm front The forward edge of an advancing warm air
mass that is rising over cooler air in its path.
Warm sector The zone of warm air within a depression.
Water vapour Water substance in vapour (gaseous) form;
one of the most important of all constituents of the atmosphere.
Weather The state of the atmosphere, mainly with respect
to its effects upon life and human activities. As distinguished from climate,
weather consists of the short-term (minutes to about 15 days) variations of the
Wind movement of air caused by changes in temperature
and air pressure. Winds are always identified by the compass direction from
which they blow.