If the temperature of the ground falls below the dew point of the air,
the air deposits on the cooler surface moisture in the form of small
drops of water, which we call dew drops. Where the temperature of the
ground becomes cooler than the air above it, a rapid cooling by
radiation on a clear night has taken place; and if the dew point or
frost point has been reached by the ground, the air just above the point
is several degrees warmer.