The difference between the maximum and the minimum values in a day is called the diurnal range of atmospheric temperature for that day.
The diurnal range of temperature (A-B) of both land and sea surfaces are greater than that of the air (C-D) above each respective surface. However, the diurnal range of the surface temperature of the land is greater than that of the sea. For some land surfaces the range may be tens of degrees celsius, whereas for deep sea areas it is less than 1°C. The diurnal range of air temperature above each surface also shows this contrast.
The diurnal cycle of temperature of a surface and the air above may be analyzed in terms of the gains and losses of energy experienced by each medium as follows:
Sunrise to midday: When the surface temperature increases, the receipt of solar radiation progressively increases to a maximum value at midday. Simultaneously the emission of long wave radiation from the surface increases. Although the surface is absorbing long wave radiation from the atmosphere, the net long wave radiation represents a loss of energy by the surface. However, the solar radiation absorbed during this period more than compensates for this loss.
Midday to sunset: When the surface temperature decreases, the solar radiation received and the net longwave radiation loss both decrease progressively.
Sunset to sunrise: The surface temperature continues to decrease but less rapidly. The net longwave radiation loss from the surface continues but in decreasing amounts.
The changes in air temperature during the daily cycle (Fig. 3.4) principally depend upon the processes of conduction, convection and turbulence, and thus the influence of the underlying surface.
From one hour after sunrise to 1400/1500 LMT The air gains energy through conduction when in direct contact with the surface. Convection and turbulence then ensure the transfer of energy to greater heights.
From 1400/1500 LMT to sunrise. The temperature gradient between the air and the surface immediately below is such that conduction, aided by turbulence, results in a loss of energy by the air. Its temperature decreases rapidly at first, then more slowly, reflecting the decreasing rate of change of surface temperature during the same period. Between sunrise and one hour later. Although the surface experiences a small gain in energy during this period, the air above continues to lose energy, achieving a minimum temperature 1 hour after sunrise.
The significant difference between the values of the diurnal range of land and sea surface temperatures may be attributed to a number of factors: