How do you estimate the temperature 20 feet belowground?

[marcus]

it is nice and quiet down here in the basement

 

[Integral]

If I recall correctly, surface temperatures are one season out of phase with air temperature. So the maximum ground temps occur in the fall, having been heated by the long days all summer. When day lengths fall below 12 hrs the ground begins to radiate more energy at night then it gains during the day. Given that you should be able to see what is happening on the surface.

At some depth there are several factors at work. Soil density and type determine the rate of heat propagation, surface temperatures set the scale. A general rule of thumb is ~2m of depth is like a season so you will see further damped seasonal variations with depth. At 20ft ~7m your temperature should be about 3 seasons out of phase with the air, but at that depth the variations will be pretty small. The absolute temperature will depend on the absolute difference in your local air temperature over the year and the soil type.

Now knowing all of this it is safe to say that the best way to determine your temperature is with a thermometer!