This influence is due not only to its well known insulating properties but also to the moisture it provides to the ground during the thaw period. Vegetation can exert a similar insulating effect, protecting the underlying ground from weather extremes that cause high rates of heat transfer to and from the atmosphere. Other terrain features such as slope orientation, can have significant effects. In general, slopes with a southern exposure have a higher average ground temperature than north-facing slopes. There are numerous publications available on the effect of terrain features on ground temperature 3 .
Water content is a variable property.
The response of the ground to temperature variations is further complicated by the changes that occur on freezing. In addition to the fact that volumetric heat capacity and thermal conductivity of ice differ from those of water, the ground may heave during freezing and settle during thawing. It is beyond the scope of this Digest to discuss these complex interrelationships. Fortunately, in nature many factors tend to compensate each other so that it is usually possible to use relatively simple formulae to estimate the limits within which soil temperatures will fluctuate. The amplitude of a temperature variation at the surface is normally about equal to that of the corresponding one for air.