- 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:
