Soil temperature follows the changes in the air temperature with a time lag that increases with soil depth. Close to the soil surface, the variation in temperature is larger than, for example, half a meter deep in the soil. In summer, the soil surface warms up much more than the deeper soil layers. The temperature in soil surface can reach 15 –20°C, while deeper soil layers stay around 10°C. In midwinter, the temperature remains above zero in deeper soil layers, approximately at +1–2°C. The snow cover efficiently insulates the soil surface, keeping the temperature close to zero.
Soil temperature is measured with similar systems as air temperature but the spatial variation in the soil is large (i.e. measurement in a single point are not likely to represent the whole soil). The soil temperature has also a vertical profile and therefore, displacement might lead to measurements of another horizon that was wanted. On the other hand, it is hard to define in unhomogeneous soil in a repetitive manner. Here you can view and download soil temperature measurements made at Hyytiälä Forestry Field Station.