Three principal sources of heat.
The earth’s crust receives its supply of heat from three principal sources, viz.:
1. From the sun through its rays;
2. From the interior of the earth through conduction; and
3. From various physical and chemical processes which take place in it and create heat.
Heat through sun’s rays.
Dark soils absorb more heat than light-coloured soils.
Capacity for heat is greater in damp and fine-grained soils.
Evaporation and condensation of aqueous vapour produce the greatest effect in fine-grained soils.
Dealing with the upper layers of the crust, it may be said that, besides the intensity of the sun’s rays, the temperature also depends on a variety of properties possessed by various kinds of soil, amongst which latter may be mentioned the absorption of heat, which is much greater in dark than in light-coloured soils; the heat conductivity and the capacity for heat, which lead to higher temperatures in damp and fine-grained soils; and finally the evaporation and condensation of aqueous vapour, which tend to prevent extremes of heat and cold and which likewise produce the greatest effects in fine-grained soils.
A fine-grained damp soil does not get so hot, but retains the heat better.
It follows from these observations that a coarse-grained, dark coloured and dry soil will show the highest and lowest temperatures, whereas a fine-grained damp soil does not get so hot but retains the heat better.