diamond geezer

 Thursday, July 19, 2018

It last rained in East London at 9pm on 9th June, some 40 days ago.
That's highly unusual, but not unprecedented.

For the avoidance of doubt, I'm using data from a weather station in Wanstead. This has recorded no measurable rainfall since a light shower almost six weeks ago.

It's not the case that the whole of London has been dry over this period. A heavy shower crossed northwest and central London on 4th July, but completely missed the south and east. Then last Friday evening a massive downpour followed much the same path, dumping over half an inch of rain on areas like Harrow, Ealing and Finchley. At my normally-favoured weather station in Hampstead, it was the wettest day of the year so far. But further to the east and south we got nothing, just as before, and so our drought rolls on.

It might rain tomorrow, if a threatened plume of thunderstorms rolls up from the continent, or everything might miss again, and 2018's unusually dry spell continue.

Droughty facts

In the UK, absolute drought is defined as a period of at least 15 consecutive days or more where there is less than 0.2 mm (0.008 inches) of rainfall. By that definition, East London entered drought on 26th June, and is still there. Hampstead didn't meet these conditions until 2nd July, and fell out of absolute drought two days later.
This definition isn't especially hard to meet. Over the last five years, Hampstead has been in absolute drought in July 2013, March 2014, September 2014, August 2016, April 2017, June 2017 and July 2018.

June 2018 was an unusually dry month, one of the five driest since records began in 1910. According to the Met Office, Central Southern England and Southeast England recorded just 3.0mm of rainfall, 6% of the expected total. Essex only had 1.7 mm (4% of average), Dorset 2.0mm (4%) and Middlesex only 0.7 mm (2%)... provisionally the driest June on record.

As for Summer 2018, we're now halfway through - a period which has coincided almost perfectly with the very dry spell. It's the driest start to summer on record for the UK, averaging 47mm across the country, but only 10.8mm for Southern England (a mere 6% of what you'd expect). Of course this could all change, and likely will, if the second half of summer doesn't match up.

The most famous drought of recent times was the summer of 1976, made more intense because it followed a dry summer and autumn in 1975, and an exceptionally dry start to 1976. We've not had that dry backstory this year, so no hosepipe bans have kicked in, nor are standpipes likely. Other recent droughts include July 1983, July 2006 and April 2011, but nothing that's bleached the grass quite like now.

It was dry and sunny on St Swithin's Day this year - of course it was - which might hint at 40 further dry days to come. This nugget of weather lore is based on the truth that the jetstream often brings persistent weather conditions to the UK in the summer months, be that prolonged high pressure or changeable days with bands of rain. It's never once been true, but had St Swithin's Day been June 10th, and if we can hold out until Saturday with no rain, a 40 day dry spell will have been achieved.

The British record for the longest period of drought is held, perhaps surprisingly, by Mile End in east London. An exceptionally dry spring in 1893 led to an amazing 73 consecutive days without rain from Saturday 4th March until Monday 15th May. High pressure was firmly in control across much of northwest Europe, bringing harsh frosts overnight but also some unusually warm days. We'd have to hold out until 22nd August 2018 without a drop of rain to beat Mile End's 1893 record.

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