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Big fall in very warm summer days

PostPosted: 2015 Sep 02, 11:28
by Dick Lovett
Summer 2015 has produced just 4 days here when the temperature reached at least 25C, continuing a dearth that had become particularly noticeable since 2006. Using the same threshold temperature of 25C, the total number of these 'very warm' summer days over these last 9 years is 67, and compares markedly with 126 days during the previous 9 years, 1998-2006, and as many as 154 days over the period 1989-1997.

Re: Big fall in very warm summer days

PostPosted: 2015 Sep 03, 14:25
by Martin Rowley
... Not sure if these diagrams will back-up your case or confuse! It's a case of comparing apples with pears perhaps . . .

However, I have two series of summer indices which I maintain - based on JJA sunshine & 24hr maxima: one based on Kew Observatory / Heathrow and the other Southampton (Mayflower Park) and Hurn. Lots of 'massaging' to achieve consistency between the series / differing instruments / site change etc., but the running mean (10 year / plotted on last year) shows that there has been a hesitation in the generally upward trend in summer 'goodness' since the late 1970s/early 1980s, and that for the 'west London' cluster shows a slow decline in values since the 10-year peak that ended 2003.

KewHeathrow_SumIndex.jpg
KewHeathrow_SumIndex.jpg (37.63 KiB) Viewed 1357 times

Re: Big fall in very warm summer days

PostPosted: 2015 Sep 03, 16:06
by Graham Easterling
A similar trend here in Penzance. Days >25C are very variable year to year here, 0 so far in 2015!

However, taking days per annum > 20C you get
Average for 1996-2005 - 72 Days
Average for 2006-2015 - 45 Days (average may rise 1 day as we have Sept/Oct to go)
A considerable difference.

Mean Summer temperature (June/July/Aug)
1961-1990 15.7C
1981-2010 16.2C
2006-2015 16.0C - the recent summer cooling confined to July/August.