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.
--
Paul Hyett, Cheltenham, England

On Tue, 30 Sep 2003 08:02:10 +0100, Vidcapper in
wrote:

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.

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.

Both the screen top and grass blades are the active surfaces as far as the
radiation is concerned.

--
Mike posted to uk.sci.weather 30/09/2003 18:01:10 UTC
Coleraine
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"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.

Hi Paul,

Well, the thermometer outside was in effect a wet bulb thermometer. water
evaporates from the bulb of the thermometer cooling the bulb and therefore
causing the temperature to be lower. The one inside was a dry bulb
thermometer. The difference between the two would have helped you calculate
the humidity of the air.

HTH

A

On Tue, 30 Sep 2003 18:24:21 +0000 (UTC), Andrew Bond in
wrote:

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

Well, the thermometer outside was in effect a wet bulb thermometer. water
evaporates from the bulb of the thermometer cooling the bulb and therefore
causing the temperature to be lower.

This effect is mainly due to radiation exposure and *nothing* to do with
being a wet bulb. It way well become wet over time, but that will simply
be a result of condensation on the exposed cooler surfaces of screen and
thermometer. If condensation is occurring evaporation must be zero.

--
Mike posted to uk.sci.weather 30/09/2003 18:43:53 UTC
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Just to chip in with my pennyworth..
Mike is correct. The radiation to which he refers is 'outgoing' from the
thermometer. The thermometer inside the screen (the radiation screen) is
protected from both incoming and outgoing radiation effects. If the outside
thermometer is found to be wet in these conditions, it will be condensation
(dew) causing the wetness, due to the thermometer being at a lower
temperature than the dew point of the air surrounding it. As the change in
state from vapour to liquid releases latent heat, the thermometer will be
slightly warmed by this process, but not enough to offset the radiative
cooling to any great extent. As Make says, evaporation will be zero in this
scenario, so wet bulb cooling is not a factor..

--
Bernard Burton
Wokingham, Berkshire, UK.


Satellite images at:
www.btinternet.com/~wokingham.weather/wwp.html
"Michael McNeil" wrote in message
om...
Mike Tullett wrote in message
...

This effect is mainly due to radiation exposure and *nothing* to do
with
being a wet bulb. It way well become wet over time, but that will
simply
be a result of condensation on the exposed cooler surfaces of screen and
thermometer. If condensation is occurring evaporation must be zero.

Your are saying that the radiation it recieves on top of the screen is
greater than that inside and that this radiation is a coolant?

Besides which; the bit about condensation and evapouration is not
quite right.

Mike Tullett wrote in message ...

This effect is mainly due to radiation exposure and *nothing* to do with
being a wet bulb. It way well become wet over time, but that will simply
be a result of condensation on the exposed cooler surfaces of screen and
thermometer. If condensation is occurring evaporation must be zero.

Your are saying that the radiation it recieves on top of the screen is
greater than that inside and that this radiation is a coolant?

Besides which; the bit about condensation and evapouration is not
quite right.

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.

--
Mike posted to uk.sci.weather 30/09/2003 22:48:34 UTC
Coleraine
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In uk.sci.weather on Tue, 30 Sep 2003 at 19:01:10, Mike Tullett wrote :
On Tue, 30 Sep 2003 08:02:10 +0100, Vidcapper in
wrote:

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.

The thermometer is almost as exposed to outgoing long wave radiation as a
grass minimum thermometer and will behave in a similar fashion.

It actually read lower than the grass minimum, that night.

The same
effect is seen when frost forms on car roofs, even though the "air
temperature" is above zero.

But is the above exposure, or that inside the screen, more typical of
what garden plants will experience?
--
Paul Hyett, Cheltenham, England

On Tue, 30 Sep 2003 19:01:10 +0100, Mike Tullett
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.

But in the example here, I would have described the space inside the
screen as a buffered environment in that the thermal capacity of the
screen material must be slowing the rate at which the microenvironment
inside the screen adapts to change in the temperature of the
surrounding outside air. For any surface inside the screen such as a
themometer, its temperature would probably (?) be governed primarily
by radiative processes (ie rather than conduction/convection) and
would see more incoming radiation from the internal surfaces of the
screen (being at a somewhat higher temperature than the surrrounding
air) than would be the case outside the screen during the nightly
cooling process. Such a thermometer would therefore read a higher
temperature. I'd also guess that this would be effectively a kinetic
process rather than one of thermodynamic equilibrium in that if a
minimum air temperature was reached and sustained for some hours such
that the screen material reached equilibrium with the air then the
reverse might apply during subsequent morning warming, ie with the
inside reading lower than the outside.

Finally, these effects would likely only be significant during calm
conditions - any significant breeze would substantially increase
convective equilibration and therefore minimise temperature
differences.

But maybe my brain isn't in gear yet this morning!

JGD

On Wed, 1 Oct 2003 07:35:44 +0100, Vidcapper in
wrote:
snip
The thermometer is almost as exposed to outgoing long wave radiation as a
grass minimum thermometer and will behave in a similar fashion.

It actually read lower than the grass minimum, that night.

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.

The same
effect is seen when frost forms on car roofs, even though the "air
temperature" is above zero.

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.

--
Mike posted to uk.sci.weather 01/10/2003 07:37:01 UTC
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