As well as the poor weather this summer, the suppression of
temperatures compared to typical contemporary summer temperatures
(since say 1989) is also noticeable. Yesterday in a warm humid airmass
the SE "only" reached 26C, a value more typical of such airmasses
occurring in May. On no day this week in London is even the 1961-90
mean of 22-23C forecast to occur, despite a lack of any real cold air
plunges from the north. The last time the more typical contemporary
pattern of temperatures being generally a degree or so above the
1961-90 average was the first 20 days of June, in the earlier stages
of the wet spell. So as suggested by someone else a couple of weeks
ago is the wet ground, or moisture content of the air, responsible for
this?

Nick

On Jul 16, 11:17 am, wrote:
As well as the poor weather this summer, the suppression of
temperatures compared to typical contemporary summer temperatures
(since say 1989) is also noticeable. Yesterday in a warm humid airmass
the SE "only" reached 26C, a value more typical of such airmasses
occurring in May. On no day this week in London is even the 1961-90
mean of 22-23C forecast to occur, despite a lack of any real cold air
plunges from the north. The last time the more typical contemporary
pattern of temperatures being generally a degree or so above the
1961-90 average was the first 20 days of June, in the earlier stages
of the wet spell. So as suggested by someone else a couple of weeks
ago is the wet ground, or moisture content of the air, responsible for
this?

Nick

None of the above. The answer is cloud. The high surface
temperature possibilities in a warm airmass will only be realised if
the sun comes out. You can get 564 dam thicknesses in October but if
it remains cloudy you will be stuck at about 15°C. Yesterday this
place was one of the warmest in the country largely because the storms
missed it and the sun shone for a little longer than in many other
places. The maximum was 25.8°C but it soon dropped when the cloud
from a storm skirting the area obscured the sun.

Tudor Hughes, Warlingham, NE Surrey, 556 ft.

On 16 Jul, 13:15, Tudor Hughes wrote:

None of the above. The answer is cloud. The high surface
temperature possibilities in a warm airmass will only be realised if
the sun comes out.

Fully agree. The very wet state of the ground surface is another
factor in limiting the efficiency of conversion of what inbound solar
radiation there is to sensible heat.

Last July, with the ground very dry after months of below-normal
rainfall, solar radiation could be converted into sensible heat very
rapidly. This year, it's having to warm up wet ground and that's
taking quite a bit of the available energy in doing so - waterlogged
soil as you know has a very high Specific Heat Capacity compared to
dry soil.

This factor is so important that I doubt whether even if a synoptic
situation 'just right' for August 2003/July 2006 heatwave conditions
suddenly arrived tomorrow whether we'd see maxima approaching the
36-38°C we saw in those months. Of course, a few warm days at this
time of year would rapidly dry out surface layers and (all else being
equal) we'd see successive daily maxima rising quickly as a result.
August 2003 produced very high temperatures despite a fairly wet end
to July in many parts of south-eastern England, after a few days for
the soil to dry out.


Stephen
Stratfield Mortimer, Berkshire