Radiation is a process by which energy moves through a medium or vacuum without the movement of any molecules and without heating any medium through which it passes. The quantity of energy radiated from a grain surface increases with increased surface temperature and neighbouring grains increase their temperature by absorbing the radiation emitted. Because the higher-temperature grains radiate more energy, radiation results in a net transfer of energy to the lower-temperature grains. For coarse dry soils heat conduction is low and 10-20% of heat transfer can be due to radiation. Generally though, radiation plays a negligible role for heat transfer in soils.
Vapour moves towards a lower vapour pressure by a molecular process known as diffusion (fundamentally the same process that redistributes contaminants in static water as described in Chapter 6, Section 6.3.1.). There is no need for there to be a conventional pressure differential, only a difference in the concentration of the vapour. Vapour pressure decreases with decreasing temperature and with decreasing relative humidity. That is, cold and dry areas attract vapour. Typically, at microscale, water evaporates at the warm end of a pore and condenses at the cold end, thereby transferring latent heat from the warm to the cold end of the pore.
Convection is energy transfer by macroscopic motion of fluid (liquid and gas) particles. The fluid motion is the result of a force. The force may be due to a density gradient or due to a pressure difference generated by, for example, a pump, by gravity or induced differences of density. The moving particles bring their high or low energy with them. In soils, typically, the moving particles which transport heat are water molecules. Density gradients may be due to a gradient of water or of salt content. Because the density of water varies with temperature, the density gradient may be due to temperature gradient only.
