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:
- Specific heat capacity: The energy required to raise the unit mass of a substance through 1 K is termed specific heat capacity. For pure water it is 4.18 J/g/K, whereas for any soil mass it is substantially smaller, the absolute value being dependent upon the soil type and its liquid water content.
- Transparency to solar radiation: The depth to which solar radiation penetrates a water mass depends on the amount of solid material contained within it. In pure water the shorter wavelengths within the solar spectrum may penetrate to 100m before being absorbed, while the longer wavelengths are absorbed by the upper layers. However, solar radiation will only penetrate the first few millimetres of soil. The depth depends upon the grain size of the soil, the longer wavelengths penetrating further than the shorter ones. Since a given amount of solar radiation will be absorbed by a greater mass of water than of land, the increase in water temperature will be correspondingly less.
- Evaporation: The energy absorbed by a surface in the form of solar radiation may be used in the process of evaporation. For a sea surface the amount of energy involved is large, the remaining energy being available to increase the temperature of the water. In contrast the reverse conditions tend to exist for a land surface.
- Turbulence: The generally turbulent nature of sea water aids the distribution of energy to greater depths, thus contributing to its smaller diurnal range of surface temperature compared with thatof the land.
To summarize, the small increase in sea surface temperature is a result of its high specific heat capacity value, relative transparency to solar radiation, and the processes of evaporation and turbulence. The surface temperature of the land decreases more rapidly than that of the sea (Fig. 3.4), since the overall loss of energy experienced by the land surface is only moderated by the relatively small amount of energy gained through conduction from sub-surface levels and the air above. In contrast, when the surface layer of water cools, it becomes denser and sinks, being replaced by less dense warmer water from below. This process, termed convective overturning, will occur, provided that the water is at a temperature greater than that of its maximum density, (pure water 4°C). As a result, the sea surface temperature decreases very slowly, but overall there is a loss of energy from the water mass. Finally there are a number of other factors which affect the value of the diurnal range of temperatures, which are particularly significant for a land area:
- In mid and high latitudes, the range is greater in the summer due to the longer hours of daylight.
- Clear skies throughout a 24 hour period result in a greater range than overcast conditions.
- Advected air, which is non-systematic, also affects air temperature and the diurnal range.