On Wed, 01 Oct 2003 08:21:29 +0100, John Dann in
wrote:

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

The thermometer is almost as exposed to outgoing long wave radiation as a
grass minimum thermometer and will behave in a similar fashion. The same
effect is seen when frost forms on car roofs, even though the "air
temperature" is above zero.


That explanation doesn't make complete sense to me though that
probably reflects my own lack of understanding and/or may be just
semantics. I'd always imagined that the car roof frost was because the
roof was a more effective radiator than the air and, in radiating
proportionately more energy than it received, cooled more quickly than
the surrounding air. *In other words, the 'outgoing long wave
radiation' from the ground is not a cooling agency in any sense*, the
temperature of any material is determined simply by the balance of
incoming radiation it receives and the amount of outgoing radiation it
emits; 'outgoing long wave radiation' from the ground is just one
such input and in this context probably a fairly negligible one.

* to indicate my emphasis

snip

Hi John

I have snipped the rest of your excellent explanation. I don't see any
disagreement at all - it was my rushed use of words last night that may
have caused you to misunderstand me. I was using the grass minimum only as
an analogy (indicating it behaved in a similar manner to the car/screen
roof as the "active surface" where radiation exchange occur), but wasn't
suggesting for one moment that outgoing LW radiation from the ground was a
factor in determining the car/screen top temperature.

You rightly say it is all down to the radiation balance on any surface, be
it the grass surface or the car/screen top. The only thing I'd add would
be the energy input to the sensor at grass level from the underlying warm
soil, as I have just done in my reply to Paul Hyett.

--
Mike posted to uk.sci.weather 01/10/2003 08:31:49 UTC
Coleraine
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