Soil Temperature Investigation
One winter, Journey North staff dug through the snow and stuck in a soil thermometer. When the air temperature was 2°F. below zero, the soil two inches down was 27°F. above zero! What was going on?
In the winter, soil temperatures tend
to be higher than air temperatures. (See B,
below. "Ground surface" is the air temperature.)
Soil can hold heat better than air does. It is also insulated by soil above
it, vegetation, and snow. In fact, the deeper you go, the more insulation and
the higher the average temperature, to a point. From about 30 to 200 feet below
the surface, the soil temperature is relatively constant (about 55 degrees
temperatures (and shallow soil temperatures) also fluctuate more
than temperatures in deeper soils over the course of a day or year. (See A and B,
It takes deeper soil layers longer to respond to changes in the surface that
are caused by daily and seasonal changes in the amount of the sun's energy
reaching the earth. By the time you reach about 30 feet deep, soil temperatures
are fairly constant.
the summer, the whole cycle reverses! (See B,
In the spring, air and soil temperatures both rise, but the soil warms up more
slowly. So by the time summer comes, the soil temperature is lower than
the air temperature. And the deeper you go in the summer, to a point, the cooler
the soil is.
Soil temperatures over three days at different depths.
shallow depths, soil temperatures fluctuate a great deal.
deeper you go in the soil, the more constant the temperatures.
Soil temperatures during a year at different depths. (Click
for a larger image.)
the winter, the deeper the soil, the warmer the temperature.
This reverses in the summer.
general, deeper the soil, the more constant the temperatures.
Deep in the Earth!
If you could dig deeper than 400 feet in the earth, you'd find that the
temperature begins to rise again. (That's because radioactive elements
deep in the earth are slowly disintegrating and producing heat.) Initially,
it increases by an average of 1.5 degrees per 100 feet (or 25 degrees
C per kilometer). At 24 miles below the earth's surface, the temperature
averages 1,800 degrees F. The center of the earth (about 4,000 miles
down) is closer to 8,000 degrees F.