The Rain Gain Refrain

Is the recent UK rain exceptional? Guest post by Charles Duncan

[Update 31/12/12. Clarification added on final graph]

Watching a Channel 4 programme about recent weather, and specifically the claimed increase in rainfall in the UK. Devon and Cornwall in the South West have been particularly badly hit.

Source: http://www.thisisexeter.co.uk (click for link)

I went on the Met Office’s website and downloaded the UK rainfall data, available at daily resolution from 1/1/1931.

I looked at the most rain in any one day during each month and the total rainfall in each month.  Because the data is so variable, I calculated the 5 year rolling averages, shown below in red and black.

Rain 1931-2012

Looking at detail for the South West I calculated rolling quarterly and 12 month averages:

Rain 2012 SW Q&Y

SW rainydays

This years monthly totals for the South West are plotted below with the average monthly rainfall and one standard deviation on either side of the average.

Rain 2012 SW

Whilst looking at rainfall data, I thought I’d look to see if there was a trend in the number of days of rain.  It’s actually very quick and easy to do.  I downloaded rain data from:

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

I imported it into Excel, then at the at the right hand end of the data created a column labelled “year” and one “dry days”.  I just copied down the year from the left hand column, and used COUNTIF(range, 0) to count the number of dry days in each month, then did a pivot table to get yearly data.  The result was dramatic:

Graph showing the number of completely dry days (as measured by the available instrumentation).

Dry Days

SW, SW and Central, and they all look the same as the method of measurement up to the 1990s could not discriminate <1mm in accuracy.[Clarification/correction added]. The Met Office clearly changed their instrumentation to a method that could measure <1mm per day accurately; there was a gradual update to automated gauges with a 0.2mm resolution in the 1980s and 1990s. If one plots “less than 1 mm of rain” there is no trend. It shows there has been no change when the previous measurement criterion was used.

Graph showing the number of dry days with <1mm of rain.

less than 1mm

Despite claims of increased rainfall it doesn’t look like there’s anything exciting in the data.  I suspect the problem (increased flooding and therefore apparent increases in rainfall) lies with 1) building on flood plains, and 2) covering every flat surface with tarmac or concrete for car parks, roads etc, thereby reducing the area of soil to absorb said rain.

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10 Responses to The Rain Gain Refrain

  1. sirgeogy says:

    Super work, thanks for this!

  2. sirgeogy says:

    Could we also investigate the intensity of rainfall events, not just maximums or rainy days? It is sometimes claimed that rainfall events are becoming more intense and that an increased rainfall rate (mm/hr) of individual storms might be an additional factor causing floods. Is this data available from the met office for investigation? http://www.rgsweather.wordpress.com

    • Verity Jones says:

      There’s a 2008 summary from CRU/UEA here that looks at intensity. The authors suggest a small increase in Winter and Autumn intensity, as Total Amount and Average on Wet Days, but this could be accounted for by increased probability of ‘catching’ a heavy shower due to an increased number of rain gauges:

      The different colours depict periods with different rain gauge coverage; for example the early (purple) period was based on 37 rain gauges, while the most recent (blue-green) period was based on 544 rain gauges.

      I guess that doesn’t quite answer your question. I think I did see something on the Met Office site about challenges of measuring short duration rainfall rate.

  3. John F. Hultquist says:

    “2) covering every flat surface with tarmac or concrete . . . ”

    Runoff events have been studied and plotted as hydrographs. Urbanization results in higher peaks, less lag time between the center of mass of the rainfall and center of mass of runoff. The peak, though higher, is shorter in time. Some notes here with a standard labeled diagram (scroll down):
    http://www.uwstout.edu/faculty/scotta/sect5.cfm

    The effect of urbanization on storm runoff from two catchment areas in
    North London — M.J. Hall
    http://iahs.info/redbooks/a123/iahs_123_0144.pdf

  4. Charles

    Increased flooding due to more people, more Tarmac and building on flood plains are undoubtedly big reasons for the scenes we have witnessed. Also people are far less tolerant of flooding and whereas they may have shrugged their shoulders and coped years ago it is now expected that they are protected. This water has to go somewhere however.

    I live near the met office and use their archives and library frequently. The weather events of the current era are dwarfed by the ferocious weather our ancestors sometimes had to cope with. The wheel always turns and it is difficult to see how our modern infrastructure will cope when faced with truly historic weather events

  5. intrepid_wanders says:

    Hi Verity.

    Great article! I am totally stuck on the “Number Dry Days” chart, for it indicates a methodology change or something else very interesting (since it is around the 1997-8 Super El Nino). Does anyone have an interesting explanation for this step function (other than the CAGW wet/dry/hot/cold theory of everything)?

  6. alexjc38 says:

    Interesting analysis and a timely reminder that there’s not much new in all this talk of “weird weather”. Yesterday there was a TV programme “Wild Weather in 2012″ on BBC1 and the description on the BBC website includes this: “The programme looks at the impact of the climate all around England, ensuring that 2012 goes down in the record books as one of the worst weather years we have ever had to face.”

    But a glance back through history tells us we’ve often had far worse, as climatereason says. Here in London alone there was dreadful flooding in 1928 and 1947 – also the 19th century appears to have been regularly punctuated with extreme weather events caused by rainfall and flooding impacting the Thames valley, as this web page amply illustrates:
    http://www.thamesweb.co.uk/windsor/windsorhistory/floods1875.html

    Another thing that made me wonder during yesterday’s programme was the suggestion that the impact of frequent rain on the activities of bees and other pollinating insects is threatening to lead to “ecosystem collapse”. I’m no ecologist, but going by the severity and regularity of wild weather in times past, my suspicion is that if the ecosystem was such a fragile flower, it would have collapsed long, long ago.

    • Verity Jones says:

      That’s what makes me so cross about this vein of reporting/programming. Broadcasters (and the media in general) find it useful to have very short memories when reporting AGW.

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