"echinosum" wrote in message
...

'jbm[_2_ Wrote:
;865058']Ground frost
Sometimes the air temperature at night dips to 3 or 4 degrees, but the

forecaster still warns of a 'ground frost' and the need to de-ice your
car
in the morning. This is because the ground can reach freezing while the
air
temperature remains above.

It is the same as the way on a sunny day the sun can heat your car so
that it is so hot you can't touch it, though the air temperature may be
only 20C or so. Likewise, the entire black night sky is a source of
cold (just like the sun is a source of heat) that can cool the ground,
or your car, to below the temperature of the air, though the effect is
typically not as powerful as for the sun. I think typically your car
windscreen frosts over before hoar forms on the grass. One term for
this is "radiative frost", as the cold comes from radiation from the
sky, as opposed to "convective frost", where the cold is communicated
by convection (ie movement) of the air.

Reducing radiative frost is why plants with overhanging evergreen
branches can survive the winter much better than those that can "see"
the whole black sky. One of the beneficial effects of being against a
wall, as opposed to an open situation, is that it reduces the amount of
black sky the plant can "see".

Oh my! The things you learn here!!