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The top of the soil is very sensitive to changes in the air temperature. Even down at 2"
below the surface, the soil temperature can fluctuate by 20°F between night and day in
a semi-arid climate such as that in Fort Collins, CO (for example).
However, under identical conditions, the temperature 12" below the surface might only
fluctuate by 2°F. The deeper you go, the less the temperature changes during a day,
or even a year. Below a certain depth (typically at least 15'), the soil is isothermal.
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In the course of a normal clear day, the upper layers of the soil respond quickly to the air
temperature changes. Further down, the warming or cooling takes longer to effect the soil. So
the diurnal signal peaks earlier at shallow levels, and lags behind at deeper levels.
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Snow acts like an insulating blanket when accumulated on the ground. The deeper the snow,
the more it insulates the surface (and below) from the air temperature. For example, the
surface air temperature could be 0°F, but the top layer of the soil could still be
34°F if under a thick blanket of snow. It can become impossible to identify a diurnal
signal even in a 2" soil temperature plot!
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In the plot to the right, soil temperatures are shown at depths of 2", 6", and 12" in Fort
Collins during a clear month (November 2006), a month with partial snowcover (December
2006), and a month with total snowcover (January 2007). At the end of November, 7.4" of snow
fell, but only 6" accumulated on the ground. This snow muted the diurnal temperature signal,
but was melted by the middle of December, and the diurnal signal commenced. However, just a
week later, over 1.5' of new snow fell and completely insulated the ground from the air
temperature.
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