Local variations of pressure, temperature, and composition in the troposphere
are manifest as ‘weather’. A small region of air is termed a parcel. First, we note that
a parcel of warm air is less dense than the same parcel of cool air. As a parcel rises, it
expands adiabatically (that is, without transfer of heat from its surroundings), so it
cools. Cool air can absorb lower concentrations of water vapour than warm air, so the
moisture forms clouds. Cloudy skies can therefore be associated with rising air and
clear skies are often associated with descending air.
Local variations of pressure, temperature, and composition in the troposphereare manifest as ‘weather’. A small region of air is termed a parcel. First, we note thata parcel of warm air is less dense than the same parcel of cool air. As a parcel rises, itexpands adiabatically (that is, without transfer of heat from its surroundings), so itcools. Cool air can absorb lower concentrations of water vapour than warm air, so themoisture forms clouds. Cloudy skies can therefore be associated with rising air andclear skies are often associated with descending air.
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