How does the subsurface earth temperature vary around the world?
The subsurface temperature will vary due to the soil type: sand, clay, loam, silt and the degree of saturation of the soil, soil thickness, the thermal conductivity of the soil, the surface temperature (time of the year) and the albedo of the surface.

Temperature changes within soil become less variable with depth.

So far I have only found information for two regions: the US-Virgina and Cyprus. Following is a series of curves for the US. A similar diagram is available for Cyprus, but its locked in a pdf file.

In Cyprus, the soil temperature at 3 m depth varies between 18 °C and 20 °C. In the US, at a depth of 12 feet (3.6 m) the temperature varies between 60 °F and 65 °F (15.5 °C and 18.3 °C).

These results are similar, but it needs to be noted both locations are in temperate climate zones. The latitudinal extent of Virginia is between 36.5° and 39.5° North and Cyprus is 35° North.



From research undertaken in Australia, the temperature at depth in a soil is also dependent on the air temperature and hence the latitude. At a depth of 1 m the soil temperature is 35 °C at latitude 10° South and 12 °C at latitude 45° South. There will be a corresponding difference at 5 to 10 m depth.



The British Geological Survey states:


  About half of the solar radiation received by the Earth is absorbed at the surface. As a result, the ground temperature shows seasonal fluctuations to depths of about 15 m where the temperature is approximately equal to the mean annual air temperature(8 - 11° C in the UK). Below this the ground temperature increases at, on average, 2.6 °C per 100 m due to heat flowing from the interior of the Earth. Mean temperatures at 100 m depth in the UK vary between about 7 - 15°C.