After a clear night, fence posts and car tops are
decorated with ice crystals, and the grass is frosty.
Why are these
places cooler than elsewhere?
At night, there is no sunlight to keep things warm.
Meanwhile, the earth and everything on it is constantly radiating
invisible infrared light.
On clear
days and
nights, this light has a straight shot out to the blackness of
outer space, and this is removing energy.
The result is that the tops of exposed objects become much colder than
the air.
You will notice that frost does not form under evergreen trees or
on cloudy nights -- the leaves and the clouds block the escape
of infrared light.
