#Reigate August weather summary: cool and wet summers the new normal?

September 6, 2015 — Leave a comment
2015-09-05_22-11-17

Reigate August 2015

August Reigate Summary Statistics

  • Average temp 16.7C
  • Tmax 29.5C
  • Tmin 7.1C
  • Total rainfall 99mm
  • Sunshine 116.4 hours
  • Average pressure 1014mb
  • Max gust 29mph
  • average wind 16mph
  • dominant wind direction SOUTH

August in Reigate, like the rest of Southern and SE England, came out slightly cooler than average at nearly 17C with about twice as much rainfall than the long term average with a rainfall total of 100mm.  (Possibly* half of this fell in one deluge on 24 August when the town centre flooded quite notably from intense rain falling in a few hours (see post below)).

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South East England rainfall August 2015: 177% of long term average

Across SE England the MetOffice official records showed the rainfall total of 103mm was 177% of the August 1961-1990 long term average, so nearly double the usual total in some locations, especially near the south coast (100% being the average monthly total for August in this case). Don’t forget that August is often a wet month and that August 2014 had 84mm.  Nevertheless, if you think our Summers are getting cooler and wetter then, yes, you could well be right!  Read on to find out more.

The mean 500mb pressure pattern for August above shows a deep trough in the Atlantic, dug unusually far to the south for the time of year, and a continuation of the dominant high pressure over Europe from July, nudged further east maintaining the heat wave in Europe, especially Central Eastern Europe.  This looks like it should have promised a hot August for SE England with an average upper flow from the south and a surface mean flow from the SE. Unfortunately this pattern did not deliver any heat of note, but it did deliver occasional heavy showers and some thunderstorms, with notably torrential rain showers delivering big precipitation totals in a matter of hours.

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August 2015 heat stayed in Europe and away from UK

Despite the average southerly and SE flow bringing occasional thundery Spanish Plumes, the real heat stayed stubbornly on the continent and only fleetingly wafted temperatures exceeding 25C across the SE.  While France regularly baked in 30C+ daytime heat, Reigate and the SE could only struggle to Tmax temperatures of 25C and only once nudged 30C. Notably, the mean temperature was a tad below the 1981-2010 average but above the 1961-1990 average, showing how recent decades have been warmer.  So August was either above or below the long term average, depending on what LTA you choose. This is a small example of how weather statistics can be presented by the media to suit any argument regarding climate change.

The August monthly Central England Temperature (CET) came out at 0.1C above the long term average but this hides the cooler than average Tmax attained in a disappointing month.  This again illustrates how headline figures hide many subtle interpretations of weather statistics.  Overall, August was nearly average from the perspective of long term Central England Temperatures, with the East coming out slightly above average compared with a cooler West.  Maximum temperatures were widely lower than usual for August.

At 116.4 hours, sunshine was also only average or a tad below the 1961-1990 average.

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sunshine duration August 2015

The dominant wind direction this August was from the SOUTH (i.e. a southerly wind).  The highest rainfall totals experienced in Reigate are brought, on our 4 year “long term average”, by southerly winds.  This makes sense because mean southerly winds are from warm source regions, travelling across Biscay and the Channel, and are frequently associated with humid warm sectors that precede fronts where heavy thundery showers can occur.

Our more frequent prevailing Westerly / SW winds bring our familiar frontal rain but this often peters out before reaching the South East as most rain is dumped over the western hills of the UK.  In contrast, warm southerly winds containing more water vapour arrive laden with precipitable water (PWAT) ripe for torrential convective downpours across Southern and SE England. The Downs (both the North and especially the South Downs) can also have a marked orographic effect enhancing this southerly rainfall pattern, whereas they have little impact on frontal rain from the west.  This warm humid southerly set-up was responsible for the Reigate deluge on 24 August, also known as a Spanish Plume.

Finally, research shows that UK Summers have got cooler and considerably wetter since 2000.  Cooler by just 0.4C (this despite some warm years) but total summer precipitation has increased by some 50mm over the last 15 years and the 10 year moving rainfall average is on the rise, most likely due to torrential rainfall events. This change to rainfall being delivered in torrential but sporadic events is in-line with climate change predictions.

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Reigate summers have got cooler and wetter since 2000

Rainfall challenge! *Our local rainfall totals are proving mighty tricky to verify at the moment.  We use three sites for measuring rainfall locally: an automatic tipping bucket rain gauge at RGS, a manual CoCoRaHs rain gauge at the same site and a local sister site in town. Unfortunately they rarely agree and sometimes vary quite considerably. Also, the manual rain gauge is used to measure monthly totals because emptying it everyday (during the holidays for example) is not always possible. Calibration of the AWS and regular rain gauge measurements is therefore an immediate target. Meanwhile, all rainfall figures are available on request and posted in these summaries along with official MetOffice rainfall figures for the South East.

Refs

https://xmetman.wordpress.com/2015/09/04/summers/

http://www.metoffice.gov.uk/climate/uk/summaries

http://www.metoffice.gov.uk/climate/uk/summaries/2015/august

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