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