Vidcapper wrote in message ...
That a thermometer placed on top of a white screen on a clear night will
read a lot lower than one inside? I wouldn't have expected it to make
*that* much difference.

I tried it on the 24th, and it read -2C, as opposed to +1.5C inside.

Hi,

I've replied directly to your post, although I have seen the other
explanations as well, and just wanted to add my "two penneth".

To start with, the air, on a clear night, is cooled by conduction, as
it is in contact with the surface of the Earth. Thus, on a clear, calm
night, the ground cools first, and then cools the air. The air doesn't
cool by it's own radiation.

Thus, a thermometer exposed to the sky will also cool by losing
radiation whereas the one in the screen will be isolated from the sky,
and thus it's outgoing radiation will be absorbed and re-emitted back
to it by the screen. Thus, it will measure the temperature of the air,
whereas the one on the top of the screen will cool and lose it's heat,
and thus register a lower temperature.

The roof of a car will always have frost on it before other areas on
the ground, as it is isolated from the heat of the sub-soil.

It must always be remembered when comparing temperatures, especially
air temps, that the ground temperature controls the air temperature
and not vice versa (although this is not the case when a rapid change
of airmass occurs).


cheers,

Paul K.