The thermal diffusivity, a (m2/s), is the ratio between thermal conductivity (X) and thermal capacity (c):
a = X/c (4.3)
It, thus, measures the ability of a material to conduct thermal energy relative to its ability to store thermal energy. Soils of large a will respond quickly to changes in their thermal environment, while materials of small a will respond more sluggishly. From a physical point of view the thermal diffusion of a medium is indicative of the speed of propagation of the heat into the body during temperature changes. The higher the value of a, the faster propagation of heat within the medium.
For example, during sunny days the pavement surface temperature will show strong daily oscillation and in soils and pavement materials with a high thermal diffusivity this oscillation penetrates to a greater depth.
