The BBC Bristol weatherman said last night that the lowest temperature
this morning would not be just before dawn, but just after. He didn't
have time to explain why, so could someone here please help me see why
this should be.

TIA

Hugh

--

Hugh Newbury

www.evershot-weather.org

"Hugh Newbury" wrote in message ...

The BBC Bristol weatherman said last night that the lowest temperature
this morning would not be just before dawn, but just after. He didn't
have time to explain why, so could someone here please help me see why
this should be.

Hi, Hugh,
It's all about balance of radiation. When the incoming short wave from the
sun becomes greater than the outgoing long wave from the ground,
temperatures will rise. Normally, under clear skies and light wind it's
about 30 minutes after sunrise. Explains why hoar frost remains in the shade
all day and sun doesn't melt snow that much if skies are clear.
HTH
Best wishes,
Ken

On 23/11/11 14:58, Ken Cook wrote:
"Hugh Newbury" wrote in message ...

The BBC Bristol weatherman said last night that the lowest temperature
this morning would not be just before dawn, but just after. He didn't
have time to explain why, so could someone here please help me see why
this should be.

Hi, Hugh,
It's all about balance of radiation. When the incoming short wave from
the sun becomes greater than the outgoing long wave from the ground,
temperatures will rise. Normally, under clear skies and light wind it's
about 30 minutes after sunrise. Explains why hoar frost remains in the
shade all day and sun doesn't melt snow that much if skies are clear.
HTH
Best wishes,
Ken

Thanks so much, Ken

Hugh

In deepest winter I would expect the minimum to be about 1hr after
sunrise (all things being =)

Also just think, under clear skies and it gets dark at 1545hrs and
sunrises 0820hrs that is a long time in which there is no source (sun)
to heat the ground and there is a lag affect of the earth still losing
heat before the effect of a rising sun kicks in and temps start to
recover

On Nov 23, 3:27*pm, Hugh Newbury wrote:
The BBC Bristol weatherman said last night that the lowest temperature
this morning would not be just before dawn, but just after. He didn't
have time to explain why, so could someone here please help me see why
this should be.


Here is what NASA believes -

"The Earth spins on its axis about 366 and 1/4 times each year, but
there are only 365 and 1/4 days per year. " NASA

So expand it out to 1461 days across the 4 year calendar cycle or
count the days from Mar 1st 2008 until Feb 29th 2012. and the actual
number of rotations is 1461 whereas NASA has it at 1465.In each one of
those days,the temperature rises and falls with the rotation of the
Earth to the central Sun with 1461 rotations matching 1461 days.

So,the primary weather fact of all is daily temperature rising and
falling,something people here would find it impossible to operate
without yet seemingly readers will tolerate something as cruel as 1465
rotations in 1461 days which actually argues against the cause of the
daily temperature fluctuations.

Even I do not know what to say other than there is little reason to be
annoyed with me,I simply point out that we inherited a particularly
bad flaw from the late 17th century which went too far with modeling
dynamics using clocks.








TIA

Hugh

--

Hugh Newbury

www.evershot-weather.org

On Nov 24, 7:39*am, oriel36 wrote:
On Nov 23, 3:27*pm, Hugh Newbury wrote:

Here is what NASA believes -

"The Earth spins on its axis about 366 and 1/4 times each year, but
there are only 365 and 1/4 days per year. " NASA

Wait a minute there - a sidereal year (time for Earth to complete its
orbit with respect to the fixed stars) has about 365.256 solar days,
but 1 more sidereal day than that - because there is exactly one whole
axial revolution of the Earth to catch up. Lots of references
available for this.

On Nov 23, 2:27*pm, Hugh Newbury wrote:
The BBC Bristol weatherman said last night that the lowest temperature
this morning would not be just before dawn, but just after. He didn't
have time to explain why, so could someone here please help me see why
this should be.

TIA

Hugh

--

Hugh Newbury

www.evershot-weather.org
-----------------------------------------------------------------------------------------------------------------------------------------------
Hello Hugh,

Ken's explanation is probably the main reason why temperatures don't
rise immediatedly at sunrise,
but there is another factor, transport of cold air, that can delay the
rise in temperature and even produce the legendary 'post dawn
freeze'.
This is highly dependent on locality.
After sunrise when the solar flux is sufficient to warm the surface,
the surface temperature inversion begins to erode.
This allows movement of any surface air which might have been trapped
in cold air pools or frost hollows.
It is likely to drain to lower land, producing a temporary dip in air
temperature.

Len Wood
Wembury, SW Devon, 83 m asl
---------------------------------------------------------------------------------------------------------------------------------------------------

On 24/11/11 09:35, Len Wood wrote:

....

-----------------------------------------------------------------------------------------------------------------------------------------------
Hello Hugh,

Ken's explanation is probably the main reason why temperatures don't
rise immediatedly at sunrise,
but there is another factor, transport of cold air, that can delay the
rise in temperature and even produce the legendary 'post dawn
freeze'.
This is highly dependent on locality.
After sunrise when the solar flux is sufficient to warm the surface,
the surface temperature inversion begins to erode.
This allows movement of any surface air which might have been trapped
in cold air pools or frost hollows.
It is likely to drain to lower land, producing a temporary dip in air
temperature.

Len Wood
Wembury, SW Devon, 83 m asl
---------------------------------------------------------------------------------------------------------------------------------------------------

Great! Thanks, Len.

Hugh

"Len Wood" wrote in message
...

On Nov 23, 2:27 pm, Hugh Newbury wrote:
The BBC Bristol weatherman said last night that the lowest temperature
this morning would not be just before dawn, but just after. He didn't
have time to explain why, so could someone here please help me see why
this should be.

TIA

Hugh
-----------------------------------------------------------------------------------------------------------------------------------------------
Hello Hugh,

Ken's explanation is probably the main reason why temperatures don't
rise immediatedly at sunrise,
but there is another factor, transport of cold air, that can delay the
rise in temperature and even produce the legendary 'post dawn
freeze'.
This is highly dependent on locality.
After sunrise when the solar flux is sufficient to warm the surface,
the surface temperature inversion begins to erode.
This allows movement of any surface air which might have been trapped
in cold air pools or frost hollows.
It is likely to drain to lower land, producing a temporary dip in air
temperature.

Hi, Len,
It has the effect here of stopping the overnight katabatic which has kept
the temperature higher. The resultant calmer air allows rapid cooling for a
short time before the sun takes over. During, clear, calm light wind
situations, we have the lowest temperatures just before the katabatic sets
in and stirs things up, around an hour after sunset, and again just after
the katabatic stops. Most of the night is too windy for any lower readings.
It's a question I have been asked more than once. In marginal situations,
car windscreens can be damp at sunrise and frozen 30 minutes later!
Best wishes,
Ken