Rainfall Patterns In The South West

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.

 

 

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

 

 

 

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.

 

 

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.

 

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.

 

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.

 

 

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:

 

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