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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 descends.
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 okta.
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 millibars
(mb).
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 A measure 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 particular
location.
Rain ShadowThe 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" and
"fog."
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
atmosphere state.
Wind movement of air caused by changes in temperature and air
pressure. Winds are always identified by the compass direction from which they
blow.
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