The unlikely winners in the UK heatwave

Much of Europe, particularly France have endured a week of record breaking temperatures, in what has been for them a memorable heatwave that is still ongoing as I write (30 June 2019). Thankfully in the UK, despite the constant hype about just how hot it’s going to get, the heatwave conditions have now been swept away by a cold front and a change of air mass. But just what defines a heatwave. The Met Office describe a heatwave as :

An extended period of hot weather relative to the expected conditions of the area at that time of year, which may be accompanied by high humidity.


Specifically they say that a heatwave in the UK occurs when:

A location records a period of at least three consecutive days with daily maximum temperatures meeting or exceeding the heatwave temperature threshold. The threshold varies by UK county, see the UK temperature threshold map below

Courtesy of UKMO

Usually other weather services across the world stipulate that a heatwave has to last five days or more and not just three days laid down by the Met Office. Five day long heatwaves do occur in the UK but obviously more rarely the the shorter three day ones. So did anywhere in the UK actually record a heatwave this week using the new Met Office criteria?

Well the short answer to that is only three out of 249 stations across the UK and Ireland did manage to register a “heatwave” in the last week, and two of the three occurred in the most unlikely places.

SYNOP data courtesy of OGIMET

As you can see Keswick in Cumbria and Castlederg in Northern Ireland, along with Hurn in Dorset were the only stations to record three days above the appropriate heatwave threshold. I say ‘appropriate threshold’ because the Met Office have not only shortened the number of days, but they have also set a variable threshold that depends on the county you’re in (see map above). In calculating the statistics for my map and because I don’t have access to GIS to identify the county, I’ve simplified which threshold to use (25, 26, 27 or 28°C) by the distance (15, 80 & 120 NM) that each station is from the centre of London. It may come as a surprise to some that places like Heathrow, although recording the highest temperature of the year (34.0°C) didn’t record a heatwave.

I think a much better way of defining idf a heatwave had occurred would be to use daily maximum temperature anomalies for each station, calculated on at least 30, if not 50 years of data. An anomaly of 10°C above the LTA for that day would then count as a “heatwave” day, count three consecutive “heatwave days” in a row, and you have yourself a 24 carat heatwave!

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