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
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On Wed, 1 Oct 2003 08:37:01 +0100, Mike Tullett
wrote:

That wouldn't surprise me at all. Don't forget there is a very large heat
source only an inch or so beneath the grass minimum thermometer - the still
warm soil, especially after such a warm summer. At night, there will be a
considerable flux of heat upwards to the suraface and the soil will be
radiating more LW radiation than the screen top (there will be an added
supply of heat by simple conduction from the soil to the overlying air).
Some of this radiation will be intercepted by the grass min thermometer and
it will react accordingly.

....which reinforces your earlier point that frost tends to form on
exposed car roofs and upward facing glass surfaces before the ground.
The car's upper bodywork is a relatively small heat sink; the car
structure and the air inside it tend to insulate the car roof from the
ground heat source.

--
Dave

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.

"Mike Tullett" wrote in message


On 30 Sep 2003 14:31:28 -0700, Michael McNeil in
. com wrote:
snip
Besides which; the bit about condensation and evapouration is not
quite right.

I think you will find I am right in this situation, as Bernard has
indicated and explained.

Mind you, there is one place where both can occur at the same time - clouds
composed of a mixture of water droplets and ice crystals - as are many in
our latitudes. It is common for the droplets to be evaporating, at the
same time as the ice crystals are growing by deposition of water vapour
derived from the droplets. i.e. the ice crystals grow at the expense of
the water. The air is super-saturated with respect to ice but not with
respect to water.


I can't help thinking this is an oversimplification of events not unlike
the presumptions about the moon in a recent thread.

About the condensation: as long as there is room for deposition there
will be condensation. It is an ongoing process. The only difference with
the clouds and the screen top is location. If the screen was in a
balloon in a cloud.....


--
Posted via Mailgate.ORG Server - http://www.Mailgate.ORG

In uk.sci.weather on Wed, 1 Oct 2003 at 08:37:01, Mike Tullett wrote :

But is the above exposure, or that inside the screen, more typical of
what garden plants will experience?

Very much the former, as plant tops can "see" the sky whereas that in the
screen cannot. A similar process occurs at the top of a forest canopy and
at the tops of clouds at night, thus leading them to become less stable at
that height.

If the lower temperature is more typical of what plants experience, and
will be damaged by, then surely gardeners might be mislead into leaving
their delicate plants unprotected, when they should be covering them up?
--
Paul Hyett, Cheltenham, England

On Thu, 2 Oct 2003 06:43:41 +0100, Vidcapper in
wrote:

Very much the former, as plant tops can "see" the sky whereas that in the
screen cannot. A similar process occurs at the top of a forest canopy and
at the tops of clouds at night, thus leading them to become less stable at
that height.

If the lower temperature is more typical of what plants experience, and
will be damaged by, then surely gardeners might be mislead into leaving
their delicate plants unprotected, when they should be covering them up?

I agree, if all they are going by is the forecast minimum "air"
temperature. A forecast of a "ground" (now seemingly called a grass frost)
is a better indicator.
I well recall one 28th June, many years ago, just before graduation here.
The flower beds were looking good and one in particular had just been
planted with marigolds. A clear night soon followed the planting and
whilst the air temp stayed above zero, the grass minimum was well below.
Result next morning - every plant was black.

--
Mike posted to uk.sci.weather 02/10/2003 17:54:57 UTC
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