1. Soil texture
Coarse (sandy) soils conduct more heat, but have less heat capacity
Fine-grained (clay) soils “hold onto” water

2. Heat Flow through Soil
Vertical heat flux in soil or rock is down-gradient “diffusion”
Formulate change in storage as a flux divergence:
Combining heat flux with change in storage

3. Diurnal Variations of Soil Temperature
Huge range near surface
25 K diurnal cycle at 0.5 cm
Max T around 2 PM
Damped and delayed with depth
Only 6 K diurnal range at 10 cm’
Max T about 6 PM
Negligible diurnal cycle at 50 cm
Similar phenomena on seasonal time scales

4. Soil Temperature Variations
Assume periodic forcing of period p (e.g., diurnal or seasonal cycles, ice ages, whatever).
Response of T(z) is also periodic, but damped and delayed with depth relative to surface forcing
“Penetration depth” (e-folding) of temperature oscillations forced by surface periodicity depends on period of forcing and physical properties of material
p= 1 day D ~ 10 cm p= 1 yr D ~ 1.5 m p= 10,000 yr D ~ 150 m