1.3. Air Temperature
The rate of heating and cooling of the surface of the earth is the main factor determining the temperature of the air above it. The air is transparent to almost all solar radiation, which therefore has only an indirect effect on air temperature.
The air layer in direct contact with the warm ground is heated by conduction; this heat is transferred to the upper layers mainly by convection and with the turbulence and eddies in the air. Currents and winds bring large masses of air into contact with the earth's surface, to be warmed in this way.
At night and during winter the surface of the earth is usually colder than the air, on account of longwave radiation to the sky, and so the net heat exchange is reversed and air in contact with the ground is cooled.
The annual and diurnal patterns of air temperature thus depend on the variations in surface temperature. In this respect wide differences exist between land and water surfaces. Great bodies of water are affected more slowly than land masses under the same conditions of solar radiation. Therefore land surfaces are warmer in summer and colder in winter than sea surfaces on the same latitude. The air masses originating over these surface differs accordingly. The average temperature of air is higher in summer and lower in winter over land than over the sea.
A change in altitude also alters the temperature of the air. When a mass of air rises, as for instance when it is pushed up a mountain, it moves from a higher to a lower pressure region and so expands and is cooled. Conversely, when an air mass descends it is compressed and heated. These are known as adiabatic cooling and heating processes and the rate of temperature change is about 1 deg С per 100 metres in altitude (5-4 deg F per 1000 ft).
When water vapour condenses to form liquid droplets the latent heat evolved provides energy to heat the air or to reduce cooling. Therefore when condensation occurs in rising air, the rate of cooling is decreased for as long as the condensation continues. In the free atmosphere, air temperature decreases with altitude up to the stratosphere. The decrease, which is known as the "lapse rate", varies with the season and the time of day, but averages about 0-6 deg С per 100 metres (3-6 deg F per 1000 ft). During the day the lapse rate is greater near to the ground, owing to the conductive heating of the lower air layers in contact with the earth. The heated air expands, becomes less dense and tends to rise, making the lower air layer unstable and so constantly mixing with the upper layers.
During the night, especially when the sky is clear, the earth's surface cools appreciably more than does the air and so, near the ground, the lower air layers are colder than those above them. This results in a reversal of the normal vertical temperature gradient near the ground and the phenomenon is known as surface "inversion". As the colder, lower air layer is heavier than the warmer air above it, the air becomes stable under inversion conditions and all vertical movement is suppressed. The conditions promoting surface inversion are long nights, clear skies, dry air and the absence of wind.
An inversion can also be produced when masses of cold and warm air meet and the warm air is lifted above the cold air mass. This is a dynamic inversion.
The cold air near the ground tends to concentrate in low areas, such as valleys, and there the temperature may be several degrees lower than over higher ground.
Pressure differences over the earth cause the migration of air masses; air at a temperature acquired in one region may move to an area at a different temperature, altering the prevailing conditions. Thus a subtropical air mass moving towards the poles causes an elevation in temperature while a polar air mass reduces the temperature in the regions en route.