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Wet Augusts!

September 16, 2015

By Paul Homewood 

  

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Philip Eden’s columns in the Sunday Telegraph are always worth a read, but often raise more questions than they answer.

Last week he pointed out that, in theory, rainfall is heavier in August than the other summer months because that is when the seas are warmest. In particular, he says, the West Country is likely to be most affected.

Ross-on-Wye is one of the longest running Met Office stations running in that part of the country, so what do August rainfall trends there have to show us since 1931, the earliest that the published Met Office data goes back to?

 

image

http://www.metoffice.gov.uk/pub/data/weather/uk/climate/stationdata/rossonwyedata.txt

 

 

Volatility from one year to the next is evidently very marked, but there seems to be no long term trend at all. The wettest August was in 1977, and the top six all occurred in the last century.

The wettest 10-year stretch was between 1945 and 1954.

If sea temperatures really are such a dominating factor, this would suggest that they are no higher now than they were then.

 

Moreover, we also know that even heavier rainfall occurred prior to 1931 in Ross. As the British Rainfall Publication for 1912 shows, August recorded 6.22 inches there, or 158 mm, much higher than anything in recent years.

 

image

http://www.metoffice.gov.uk/media/pdf/c/2/British_Rainfall_1912.pdf

 

Five years later, in 1917, almost as much fell again, this time 5.23 inches. This equalled the “record” of 133 mm set in 1977.

 

image

http://www.metoffice.gov.uk/media/pdf/t/f/British_Rainfall_1917.pdf

 

 

 

    

Perhaps Mr Eden might care to explain!

 

 

FOOTNOTE

Just for completeness, here is the chart of August rainfall for SW England/S Wales since 1910.            

1912 and 1917 stick out like sore thumbs!

 

 

image

http://www.metoffice.gov.uk/pub/data/weather/uk/climate/datasets/Rainfall/date/England_SW_and_S_Wales.txt

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9 Comments
  1. September 16, 2015 1:40 pm

    Stats for my area (Stratford / Greenwich / Wanstead) show there has been a general fall in August rainfall since 1797.

    Wettest was 1799 with 192.1mm. Driest was 1995 with just 0.7mm. The month can lurch from one extreme to the other from year to year. Average rainfall for August (1981-2010) is 50.2mm

    Top 10 wettest
    1 1799
    2 1878
    3 1960
    4 1941
    5 1912
    6 1832
    7 1829
    8 1879
    9 1881
    10 1931

    Top 10 driest
    1 1995
    2 1940
    3 1818
    4 1976
    5 2003
    6 1947
    7 1819
    8 1849
    9 1899
    10 1975

    • Billy Liar permalink
      September 16, 2015 2:32 pm

      My rule-of-thumb with rainfall is that it is a long tailed distribution. Wherever the peak of the distribution falls, the distribution will include zero rainfall on one side and will extend past 3 times the peak of the distribution on the other side. Rainfall above 3 x the peak of the rainfall distribution is unusual, but unlikely to be unprecedented. Rainfall more than 6 x the peak of distribution is notable.

      What does the distribution for August look like at Wanstead?

      • September 16, 2015 5:46 pm

        Unfortunately I cannot access my data in the cloud – I’ll get back as soon as…

  2. September 16, 2015 2:56 pm

    The behaviour of the jet stream must also be a factor as explained here.
    http://www.netweather.tv/index.cgi?action=jetstream-tutorial;sess=

    Ed O’Toole’s summary says:

    The position of the jet stream over the UK determines the type of weather we experience.
    If the polar front jet is situated significantly to the south of the UK we will experience colder than average weather.
    If the polar front jet is situated to the north of the UK we will experience warmer than average weather.
    If the polar front jet is situated over the UK we will experience wetter and windier than average weather.
    If the polar front jet has a large amplification then cold air will travel further south than average and warm air will travel further north than average.
    The direction and angle of the jet stream arriving at the UK will determine what source of air (i.e. cold, dry, warm, wet, from maritime or continental sources) the UK experiences.

  3. September 16, 2015 5:46 pm

    “Ross-on-Wye is one of the longest running Met Office stations running in that part of the country,” From where I sit (Nr. Truro) Ross On Wye hardly seems to qualify as “West Country”!

  4. bit chilly permalink
    September 16, 2015 11:46 pm

    would be nice to see overlays of the north atlantic oscillation and the atlantic multidecadal oscillation over that last graph paul .it looks like one of those graphs that appears to have similarities to other climate variables.

  5. bit chilly permalink
    September 16, 2015 11:48 pm

    just noticed a remarkable correlation with recruitment of the gadoid species in the north east atlantic in there as well paul .will have a further dig into that aspect .

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