All this recent talk of the current heatwave in France got me to thinking what was the easiest way at looking at the frequency and duration of heatwaves in the UK. Enter the central England temperature series, which since 1878 has listed the daily maximum composite temperature. So before we can go any further we need a definition of what constitutes a “heatwave”, because of course all countries use different criteria. But as we are British and proud of it there’s no better source of an exact definition than the Met Office, so for the next two paragraphs I’m going to quote verbatim from the Met Office.
What is a heatwave?
A heatwave is 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.
What is the definition for a UK heatwave?
A UK heatwave threshold is met 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
So all that I’ve had to do is sift through a 140 years of maximum temperature data from the daily CET series, recording any consecutive spells when the temperature was 25°C or more for three or more days. The threshold is the same one used by the Met Office for areas away from the greater southeast of England, as outlined in the last two paragraphs and the map above. Here’s the table of annual totals ranked by total heatwave days from 1878 up till now that I produced. Interestingly this year has still to produce a maximum CET of 25°C or more at the time of writing (27 June 2019).
So the first thing to notice from the table is that 1976 come in at only joint second place with 27 days, equal to the total number of heatwave days for last year (2018), and a full 5 days behind the very warm summer of 1995. There are four years from the 21st century in the top 10 which maybe in itself is statistically significant.
Long term trends
So the trouble with the idea of using a fixed threshold of 25°C for the whole series since 1878, is that summers [JJA] in the CET series have warmed by 1.0°C in that time, this is obviously going to throw up more “heatwave” events for recent years than in the late Victorian era, which sounds obvious but maybe not the best way of identifying a “heatwave” at the time. The best way to identify a heatwave is to maybe use daily anomalies for the whole period (1878-2018) but that’s a step too far for this article.
So graphing the annual number of heatwave days, adding a five year moving average and also a couple of linear trends for the whole series and for the last 30 years, this is what you get.
So what does the bar chart and linear trend reveal? Well there has been a 45% increase in the number of “heatwave days” in central England since 1878. This 45% equates to 3.1 extra days of “heatwave” per year on average, increasing from just 2.5 days in 1878 (which interestingly doesn’t make the criteria of three days for a heatwave) to 5.6 days in 2019. The trend over the last 30 years (in blue) has shown a slight decrease. Make of this what you will. All I know is that heatwaves have always occurred and as they will continue to occur in the future. There was a scarcity of heatwaves in the 1960’s and mid 1980’s, but at other times, around the millenium for example, they were a regular feature of any summer.