diamond geezer

 Saturday, August 08, 2020

2020's second heatwave is now underway. But what precisely is a heatwave.

Until last year the Met Office didn't have a precise definition. And now, just in time, they do.
A UK heatwave is met when a location records a period of at least three consecutive days with maximum temperatures meeting or exceeding a heatwave temperature threshold.
Three days matches the World Meteorological Organisation definition, and that bit's nothing new. The key addition is what the temperature threshold is... and no it's not 30°C, it's not even the same threshold across the country.

In Greater London the threshold temperature is 28°C. If the temperature reaches or exceeds 28°C anywhere in London for three consecutive days that's a heatwave in the capital. But the threshold is 27°C for the counties surrounding London - from Norfolk to Wiltshire and Nottinghamshire to Kent. A further ring of counties from Lincolnshire to Dorset (plus the Isle of Wight) has a threshold of 26°C. The rest of England and Wales, and the whole of Scotland and Northern Ireland, has a threshold of 25°C.

For example, daily temperatures of
26°C, 28°C, 29°C, 25°C, 28°C
would be a heatwave in Bristol or Bradford
but not in Birmingham or Brighton
(because the temperature didn't top 27°C for three consecutive days)

The reason for these different thresholds is because benchmarks are set according to local climatological conditions. What counts as a heatwave in Glasgow is very different to what counts in Greenwich because expectations of heat in those locations are very different. Three days of temperatures over 25°C will spark a weather warning in Manchester but not in Mayfair.

These heatwave thresholds have been based on "the 90th percentile of the climatological distribution of daily maximum temperature". The 90th percentile, if you'd like an example, would be the equivalent of the cut-off point for the three warmest days of a month. In this case the Met Office looked at July temperatures over the period 1981-2010 and calculated the value exceeded only 10% of the time. This was 22.8°C in Edinburgh, 23.1°C in Belfast, 26.8°C in Cardiff and 28.9°C in London. They also decided to set a minimum threshold value of 25°C... hence the stepped approach of 25°C/26°C/27°C/28°C. [research paper]

Heatwaves ought to be expected in 30–50% of years, according to the Met Office, except in Scotland, Northern Ireland and northern England where they're rather rarer. The majority of heatwaves last for no more than 7 days, but longer heatwaves tend to occur somewhere in the UK in 10–20% of years. 85% of heatwave days occur in July or August - the high summer season - because mid July to early August tends to be the hottest period of the year.

Thus far this year in London we've had two heatwaves, one at the end of June and one now. Last week's high of 37.8°C was only part of two consecutive days over 28°C, not three, so officially counts as a 'hot spell' rather than a heatwave. Looking back over the last decade I can find four years with no heatwaves (2011, 2012, 2014, 2015), three with one heatwave (2010, 2013, 2016), two with two (2017, 2019) and one with three (2018). The longest of these heatwaves was in July 2013 and lasted seven days. But I've only been able to look at data for Hampstead, rather than the hottest spot in London, so my figures are definitely underestimates.

UK temperatures over 35°C, as we had yesterday, are also very rare. The Met Office reckons "every five years on average", but since 1976 we've recorded them in 1990 (37.1°C), 1995 (35.2°C), 2003 (38.5°C), 2006 (36.5°C), 2015 (36.7°C), 2018 (35.6°C), 2019 (38.7°C) and "the last two Fridays" (37.8°C, 36.4°C). Research into climate change suggests we could see 35°C "almost every other year" by the end of the century under a high-emissions scenario, although recent weather makes it look like we've reached that point already.

Whatever, yes there is a heatwave across much of England at present, and it'll continue until the daily maximum temperature dips back below 25°C, 26°C, 27°C or 28°C as appropriate. Best get used to it.

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