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Rainfall Patterns In The South West

November 23, 2014

By Paul Homewood

 

 

Whenever wet weather hits England, as often as not it is the South West which bears the brunt of it, as Somerset discovered last winter. But is it actually getting wetter in that part of the country, or is rainfall becoming more extreme?

 

The Met Office maintain a regional precipitation series, which includes a set for SW England & S Wales, as per this map.

 

Region definitions for EWP

 

Using this data, let’s first look at annual trends.

 

 

image 

Figure 1

 

The record annual total of 1395mm was set in 2012, but this was only 3mm more than in 1960. Meanwhile, nothing much seems to be happening to the trend.

The wettest group of years were between 1874 and 1883, when every single year was above 1000mm.

 

Are we seeing any trends in extreme rainfall patterns though? Let’s first take a look at the wettest months.

 

 

image

Figure 2

 

Since 1873, there have been 31 months with over 200mm of precipitation. The record of 273mm was set in Nov 1929 (followed by 232mm in December). The next highest was 251mm in Dec 1934.

The wettest month last winter was January, with 235mm, but this only ranks the 4th wettest month.

There is evidence of clustering, with a relative absence of extreme wet months before 1900, and again in the 1970’s. But there is no evidence of anything unusual occurring in recent years, nor that extreme wet months are becoming more common or extreme.

 

 

We can also test whether rainfall is becoming heavier by looking at rainfall per rainday.

 

image

Figure 3

 

Daily rainfall data is only available since 1931 from the Met Office, and this shows that the long term trend is actually to lower rainfall amounts, on the days when it rains. This rather flies in the face of conventional that the opposite is occurring.

The peak seen in the 12-month figure in early 2013, is not exceptional compared to earlier periods. Indeed, as Figure 4 illustrates, we are seeing an increase in the number of days when it does rain, rather than more rain when it does.

 

image

Figure 4

 

 

Finally let’s look at the really extreme rainfall days.

Figure 5 shows that, although the number of days with more than 25mm of rain has increased since the 1940’s, we only back where we were in the 1930’s.

 

 

image

Figure 5

 

 

.

 

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Figure 6

Analysis of days with over 30mm again shows some clustering, but nothing out of the ordinary in recent years. The wettest day was 28th July 1969, when 57.9mm fell across the region. The Met Office’s report for the month noted:

 

image

http://www.metoffice.gov.uk/media/pdf/e/q/Jul1969.pdf

 

 

 

 

 Summary  

There is nothing in the data that suggests the South West is getting wetter, or experiencing more extreme rainfall. We are continually told that global warming is leading to heavier rainfall, but as far as this part of the country is concerned the evidence exposes this as no more than a myth.

I actually find it extraordinary that the Met Office are not doing this sort of analysis themselves. Instead they obsess about theories and models.

 

 

Sources

All data from the Met Office

http://www.metoffice.gov.uk/hadobs/hadukp/data/download.html

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12 Comments
  1. saveenergy permalink
    November 23, 2014 6:11 pm

    Thanks Paul, keep up the good work,

    You say “I actually find it extraordinary that the Met Office are not doing this sort of analysis themselves. ”

    They probably do, but the answers are not on message so they don’t publish

  2. A C Osborn permalink
    November 23, 2014 6:46 pm

    They rely on people not bothering to do so and also those old enough having memories too poor to remember the 30s, or any other past weather event/conditions that disproves their “Unprecedented” bullshit.
    As savenergy says anything not “on message” gets ignored.

  3. November 23, 2014 8:13 pm

    You won’t see the correct pattern unless you follow the Hale solar cycle, or try a running average of 22 year.

  4. November 24, 2014 8:48 am

    Living in the South West, in the middle of Devon, I have noticed how variable the amount of rainfall is across the county. Last winter there were floods in certain parts of Devon, but we had no periods of rainfall to write home about. Trying to measure rainfall across Devon must be as thwart with problems as trying to measure “average temperature”. You would need thousands of measurement locations to try to get a believable average amount of rainfall for the county, and even then, the uncertainty would be large.

  5. November 24, 2014 12:00 pm

    I have to ask: is the step seen in Fig 4 around 2008- 2000 significant.? Rise in global temps following El Nino leading to more cloud cover/rainy days in the West Country?

    Please consider.

    NoName

    • November 24, 2014 12:10 pm

      Good point. But it could also indicate a change in weather patterns to more of a flow from the Atlantic.

      • November 28, 2014 9:24 am

        Thanks Paul: Could such a change in the weather pattern be driven by El Nino? What other measurements of say wind, atm pressure, jetstream cloud cover etc could be studied to see if there is a possible relationship to the idea I voiced?

        thanks,
        NoName

      • November 28, 2014 11:24 am

        It was the sort of thing that HH Lamb excelled in. He would analyse the number of days with westerly winds, for instance.

        I am not aware the Met Office keep this sort of data for public analysis now. The one thing they do have is sunshine hours.

        Interestingly, (for the UK as a whole), these show a step up around 1990 in all seasons but summer.

        Thanks for raising it. It’s worth doing a post on.

  6. November 24, 2014 12:50 pm

    According to HadUKP, annual rainfall in the South West and Wales was as high in the late 19th Century as it is now.

    The 30 year moving average annual rainfall, was almost identical for the period 1910-39 (1064mm), to that for 1984-2013 (1067mm).

    In view of the close correlation between annual rainfall and the frequency of heavy rain days, it is almost certain that such days were as frequent then as they are now.

    It’s just as well that you are doing this analysis, because I don’t trust the MO to do it, if it disproves climate change.

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