On Sun, 29 Apr 2007 09:30:37 +0100, "Bernard Burton"
wrote:

No, I mean the ground temperature. The ground surface temperature reacts
quickest to the cloud cover, as there is usually a flux of heat from deeper
in the ground towards the surface. The ground surface temperature under
clear skies will fall rapidly until the heat loss by radiation is balanced
by the heat flux from lower in the ground (ignoring latent heat). The whole
ground temperature structure and air temperature structure will eventually
reach equilibrium in which the sum of all the heat losses is balanced by the
gains, provided that the external conditions (eg cloud cover, wind speed,
incoming radiation (from sun)), remain constant.

What Paul is talking about is the possibility that the temp rises when
the clouds come over. If all that happens is cloud cover, with no
other effects present, then the mere presence of cloud cover cannot
cause the temperature to rise.


--

"Perhaps the meek shall inherit the Earth, but they'll do it
in very small plots - about 6' by 3'."
-- Robert A. Heinlein

On Apr 29, 1:36 pm, (Citizen Bob) wrote:
On Sun, 29 Apr 2007 09:30:37 +0100, "Bernard Burton"

wrote:
No, I mean the ground temperature. The ground surface temperature reacts
quickest to the cloud cover, as there is usually a flux of heat from deeper
in the ground towards the surface. The ground surface temperature under
clear skies will fall rapidly until the heat loss by radiation is balanced
by the heat flux from lower in the ground (ignoring latent heat). The whole
ground temperature structure and air temperature structure will eventually
reach equilibrium in which the sum of all the heat losses is balanced by the
gains, provided that the external conditions (eg cloud cover, wind speed,
incoming radiation (from sun)), remain constant.

What Paul is talking about is the possibility that the temp rises when
the clouds come over. If all that happens is cloud cover, with no
other effects present, then the mere presence of cloud cover cannot
cause the temperature to rise.

But it does. That is the whole point of this thread.

Tudor Hughes, Warlingham, Surrey.

On 29 Apr 2007 05:42:09 -0700, Tudor Hughes wrote:

What Paul is talking about is the possibility that the temp rises when
the clouds come over. If all that happens is cloud cover, with no
other effects present, then the mere presence of cloud cover cannot
cause the temperature to rise.

But it does. That is the whole point of this thread.

I thought the purpose of this thread was to confirm or deny that such
an effect occurs - and explain why.

There is nothing presented thus far which does either of those.


--

"Perhaps the meek shall inherit the Earth, but they'll do it
in very small plots - about 6' by 3'."
-- Robert A. Heinlein

On Sun, 29 Apr 2007 12:33:20 GMT, Citizen Bob wrote in


Clouds absorb and re-radiate downwards the upwelling long-wave radiation
from the surface. When cloud cover replaces clear sky the surface radiation
continues, but can no longer escape to space. The air temperature near the
surface rises towards the ground temperature until a new balance is
achieved.

But how can that cause a temperature rise?

Soil heat flux, as explained by Bernard, is still happening and in an
upward direction. If there was equilibrium *before* the cloud (i.e. upward
soil heat flux approx = net outgoing long wave radiation), the arrival of
cloud will reduce the loss of energy from the soil, yet the same heat is
arriving from below, so the surface warms, until a new equilibrium
temperature is reached.

--
Mike Tullett - Coleraine 55.13°N 6.69°W posted 29/04/2007 19:50:55 GMT

In uk.sci.weather on Sun, 29 Apr 2007, Citizen Bob wrote
:

Notice that I specifically referred to nights *without* frontal
activity, though...

How can "clouds come over" if there is not some kind of frontal
activity.

Well, fog forms without frontal activity...
--
Paul Hyett, Cheltenham (change 'invalid83261' to 'blueyonder' to email me)

On Apr 29, 1:33 pm, (Citizen Bob) wrote:
On Sat, 28 Apr 2007 23:57:06 +0100, "Bernard Burton"

wrote:
Clouds absorb and re-radiate downwards the upwelling long-wave radiation
from the surface. When cloud cover replaces clear sky the surface radiation
continues, but can no longer escape to space. The air temperature near the
surface rises towards the ground temperature until a new balance is
achieved.

But how can that cause a temperature rise?

--

"Perhaps the meek shall inherit the Earth, but they'll do it
in very small plots - about 6' by 3'."
-- Robert A. Heinlein

There is another source of heat - the air some distance
above the ground. On a radiation night this air will be warmer than
that in the lowest layers. If there is any turbulence at all there
will be some downward flux of heat to the lower layers which are no
longer being cooled by contact with the ground. In the case of a flat
calm and a strong inversion this particular mechanism would not work,
but there would still be heat flux from just below the ground surface.

Tudor Hughes, Warlingham, Surrey.

On 29 Apr 2007 15:48:43 -0700, Tudor Hughes wrote:

There is another source of heat - the air some distance
above the ground. On a radiation night this air will be warmer than
that in the lowest layers. If there is any turbulence at all there
will be some downward flux of heat to the lower layers which are no
longer being cooled by contact with the ground. In the case of a flat
calm and a strong inversion this particular mechanism would not work,
but there would still be heat flux from just below the ground surface.

Let's try this experiment. Take a tall jar and put it on a warm
surface. Cap it with a transparent top. Let it come to equilibrium and
measure the temp at the bottom of the jar. Then cover the transparent
top with an opaque lid without otherwise doing anything else.

Would you expect the temp at the bottom to rise?

In the initial condition, the system is open, and in the latter
condition it is closed. Does closing an open system cause the temp to
rise?

I wonder if this simple experiment would yield meaningful results. Put
a pot on the stove with water in it. Bring the bottom water up a some
few degrees and put a glass top on the pot. Shut off the stove. When
the temp at the bottom stabilizes, cover the glass top with an opaque
item. Will the temp at the bottom rise?


--

"Perhaps the meek shall inherit the Earth, but they'll do it
in very small plots - about 6' by 3'."
-- Robert A. Heinlein