Dartmoor writes:
big snip
Then one day somewhere hits 40.1C. I'm wondering if it is worth a
bet? What odds would the bookies give now on such an event?


.... looking back at August 2003, when we set the previous record (either
the Kew/38.1 or Brogdale/38.5°), temperatures over France did hit 40°C,
but only just and these stations had the 'benefit' of having been in the
middle of a country 'cooking' for several days in a slack airflow, with
no relatively cold water surface anywhere nearby. Even the short
sea-track across sea area Dover might be enough to cap temperatures; in
France over those days (6th - 10th August), they had a stagnant,
cauldron-like baking, yet the numbers of stations with =40.0 were not
abundant. So on that basis I'd say it's going to be difficult to get a
40+ figure in the immediate future (a handful of years say)

.... however ...

Analysing the Heathrow average *highest maxima* between the 1980s and
2000s (the only station I've got data to hand), then the changes between
those two decades a-

June: means +1.8degC (sd increased from 2.06 in the 1980s to 2.67 in the
2000s)
July: means +1.1degC (sd increased from 3.15 (1980s) to 3.35 (2000s)
August: means +2.2degC (sd increased from 2.05 (1980s) to 3.18 (2000s)

Heathrow's highest recorded temperature was set on 10th August, 2003 at
37.9°C

So, given the 38-ish all-national highest as above and assuming a
similar change over the next 20 years, we must surely look to an
'under-pinning' figure of 39-ish (the lowest of the changes of mean),
and applying just the *mean* change in August (+2.2degC), would suggest
that 40-ish must be achievable within the next two decades ... applying
a couple of standard deviations and the '40' figure could be smashed.

At the moment, I'd go for at least two instances of 40+ within the next
20 years somewhere in 'lowland' SE Britain - and that might be
conservative.

Martin.


--
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