In Britain in summer an anticyclone will mean heatwaves during the day. At night, however, as there are no clouds, heat will be quickly lost. The ground will cool sufficiently to cause condensation of water vapour in the descending warm air and mist or heavy dew may form. This will clear quickly in the morning sun. After a few days, a layer of hot air builds up at ground level, which eventually will give rise to thunderstorms, ending the anticyclone.
In winter the longer nights combined with clear skies leads to intense cooling of the land. There is an increased risk of dew, frost and thicker, more extensive fog patches which may be slow to clear or even persist.
Under very calm conditions, both frost and fog may persist for several days. An anticyclone's very stable conditions and little air movement means that pollution is trapped at low levels, resulting in very poor air quality such as smogs.

Adiabatic cooling/heating: Back to contents
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: Back to contents
A large body of air that has similar temperature, pressure and moisture properties.

Anticyclone: Back to contents
High pressure system in which air descends to give calm conditions and clear skies. Associated with summer heatwaves and winter frosts and fogs.

Atmosphere: Back to contents
The envelope of air surrounding the Earth and bound to it by gravity.

Atmospheric pressure: Back to contents
Pressure (force per unit area) exerted by the atmosphere on any surface by virtue of its weight.

Clear sky: Back to contents
Sky with a total cloud cover of less than one okta.

Climate: Back to contents
Long-term (50 year) weather averages.

Cold front: Back to contents
The "leading edge" of a relatively cold air mass.

Continental climate: Back to contents
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): Back to contents
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): Back to contents
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: Back to contents
The physical process by which a liquid or solid substance is transformed to a gas; the opposite of condensation.

Fog: Back to contents
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: Back to contents
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: Back to contents
Water vapor content of the air.

Isobar: Back to contents
A line of equal or constant pressure. Measured in millibars (mb).

Maritime climate: Back to contents
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: Back to contents
A front that is formed as a cold front overtakes a warm front and lifts the warm air completely off the ground.

Okta: Back to contents
A measure of cloud cover (in fractions of eight) on a synoptic chart.

Precipitation: Back to contents
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 Shadow: Back to contents
The dry region on the leeward side of a mountain (the side sheltered from the wind).

Relief rainfall: Back to contents
Formed when air is forced to rise over relief features such as hills or mountains. Cooling and condensation occurs as the air rises.

Seasonality: Back to contents
Periodic fluctuations in the climate related to seasons of the year e.g. wet winters, drier summers.

Smog: Back to contents
A word currently used as a synonym for general air pollution. It was originally created by combining the words "smoke" and "fog."

Synoptic chart: Back to contents
A weather chart reflecting the state of the atmosphere over a large area at a given moment.

Temperature: Back to contents
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: Back to contents
Maximum minus the minimum temperature for a particular location.

Warm front: Back to contents
The forward edge of an advancing warm air mass that is rising over cooler air in its path.

Warm sector: Back to contents
The zone of warm air within a depression.

Water vapour: Back to contents
Water substance in vapour (gaseous) form; one of the most important of all constituents of the atmosphere.

Weather: Back to contents
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: Back to contents
Movement of air caused by changes in temperature and air pressure. Winds are always identified by the compass direction from which they blow.