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Uplift of Air

The basic mechanism for raising air temperature occurs at ground level with the heating of the surface by the Sun. Consequently, temperatures generally fall with increasing altitude above the Earth's surface. Such a temperature profile in the lowest 10 km of the atmosphere allows significant uplift of air to take place, generating much of the world's weather.

When a packet of air near the Earth’s surface is heated, it rises, being lighter than the surrounding air. This type of air uplift is called convection. Whether or not an air packet continues to rise will depend upon how stable the surrounding air is. As convection continues, air pressure begins to fall, and the air packet expands. Such expansion consumes heat energy and results in a fall in temperature. After sufficient cooling the dew point is reached and condensation occurs in the form of clouds. If the atmosphere is fairly stable, convection will be limited. Cumulus clouds often form in such atmospheric conditions. If the atmosphere is particularly unstable, uplift of air will continue to much greater altitudes, and huge towering cumulonimbus clouds may form, generating significant rainfall or hail.

Uplift of air also occurs along fronts, when huge masses of air come together from different directions and with different temperatures. They cannot mix together immediately owing to their different densities, any more than two liquids like water and oil. Mixing takes time. In the meantime, the lighter, warmer air mass begins to rise above the cooler, denser one. The boundary between the two air masses is called a front. Fronts are accompanied by clouds of all types, and very often by precipitation.

In addition to convection and frontal uplift there is a third lifting mechanism which produces cloud, and sometimes precipitation on its own, or enhances cloud and precipitation which already exists. This is the necessary lift air must make to surmount large obstacles which obstruct its passage. On an otherwise clear day, lift over hills and mountains may be enough to produce clouds over their tops. As the air descends over the other side, the clouds may dissipate. If the air is fairly humid, considerable precipitation may be generated over hills and mountains. Once past these obstructions, precipitation ceases as the air warms up and condensation returns to its vapour state. On the leeward side of mountain ranges, rain shadows can exist where little precipitation penetrates.