30 °C days in Reading

By: Roger Brugge

The temperature in the Reading University Atmospheric Observatory peaked at 32.3°C on Saturday 29 June 2019. Press stories were full of pictures of people sunning themselves across parts of the United Kingdom in glorious sunshine – yet not far across the English Channel even higher temperatures were causing problems of all kinds as temperatures rose 10°C (or more) higher in places than they did in Reading. As Table 1 shows, this was one of the highest June temperatures in the Reading record.

Table 1: The highest June temperatures on record at the University of Reading since 1908.

We seem to expect 30°C to be reached in any good summer these days, but just how common is such a temperature in the Reading record?

Daily observations have been made on Whiteknights campus since 1968 – prior to them measurements were made on the (slightly warmer – due to its location in a built-up area) London Road campus. Much of this blog, therefore, restricts the analysis to the past 52 years.

30°C has been reached, sometime in this period, in each of the three summer months. (In the time when records were kept at London Road, 30°C was reached on seven dates in May – peaking at 31.9°C on 29 May 1944.) Peak temperatures at Whiteknights each month are as follows:

  • June: 34.0°C on 26 June 1976
  • July: 35.3°C on 19 July 2006
  • August: 36.4°C on 10 August 2003.

Since 1968, temperatures have reached 30°C on 17 days in June, 59 days in July and on 34 days in August. The earliest occurrence in the year of the ‘magical number’ was on 18 June (in 2017 when 30.3°C was recorded) while the latest was on 24 August (in 2016 when 30.1°C was reached). So, this year’s 32.4°C is nothing out of the ordinary in some respects.

Figure 1: The number of days each summer when the temperature reached 30°C in Reading. Data for 2019 are valid to 30 June.

Figure 1 shows the annual incidence of such 30°C temperatures. Unsurprisingly, there is a lot of variation from year-to-year. The summer of 1976 stands out, however: 30°C was reached every day for the fortnight of 25 June to 8 July – while the summer of 1995 with nine 30°C days has since come the closest to surpassing that year in the record. There is a slight trend towards an increase of the number of 30°C days each year, from 1-2 in 1968 to 2-3 days each year nowadays. If we remove all the 30°C data in 1976, then the expected value in 1968 would be under 1 day per year. Note also that the year 2019, and the four previous ones, have all attained 30 °C – the first time that five consecutive years have reached this mark.

Despite the warming trend observed in other aspects of Reading (and UK) temperatures, no such trend can be obviously seen in Figure 2, which shows the peak value achieved by the 30°C days.

Figure 2: The highest summer temperature observed in years when 30°C has been reached in Reading. Data for 2019 are valid to 30 June.

Figure 3: The range of dates each summer in Reading with temperatures reaching 30°C. Note that the periods shown may actually contain several spells of 30°C+ temperatures, with cooler days in between.

Figure 3 shows the date ranges each summer during which 30°C has been reached – here there is a suggestion that the 30°C ‘season’ is now starting earlier (e.g. 2017) and ending (e.g. 2016) later than it used to, especially if the unusual summer of 1976 is removed as an outlier.

Finally, perhaps the most remarkable feature of the recent heatwave day was the change in temperature leading into and leaving, Saturday 29th. Maximum temperatures were 24.2°C on the 28th, 32.3°C on the 29th and 22.7°C on the 30th, corresponding to 24-hour changes in the maximum temperature of +8.1 degC and -9.6 degC in successive 24-hour periods. The first of these changes was caused by a change in wind direction and, consequently, air source into the 29th – France and the near continent had been suffering from unusually high temperatures for several days before this hot air reached Reading. The second change was the result of a cold front (albeit a dry affair in Reading) that crossed from the west overnight 29th/30th.

Such 24-hour changes in maximum temperature are relatively rare in summer – in the 111-year period 1908-2018 there have been 174 changes of 8°C or more over two days in the summer months (June-August), with just 29 of these involving the onset or cessation of 30°C temperatures. The largest 24-hour changes involving one day over 30 °C were as follows:

  • 1 degC change, 22-23 August 1918 (30.3°C to 17.2°C)
  • 6 degC change, 7-8 July 1970 (30.6°C to 20.0°C)
  • 5 degC change, 6-7 June 1942 (30.2°C to 19.7°C)
  • 0 degC change, 21-22 June 2017 (32.5°C to 22.5°C)

All these large changes involved a sudden cooling that marked the ending of a 30°C spell; the largest change involving the onset of a 30°C spell (before 2019) was one of 7.6°C (from 22.7°C to 30.3°C) on 11-12 July 1912.

Many hot spells in Reading tend to build up over several days as hot air from a southerly source gradually becomes established over southern England; a sudden end to a hot spell is often marked by a thundery breakdown and noticeable temperature drop.

No pair of these 29 summer temperature changes occurred in two successive 24-hour periods with one day reaching in excess of 30°C. So, the June 2019 heatwave really did come and go within little more than 24 hours in Reading – as the spike in the bold red line in Figure 4 confirms.

Figure 4: Daily maximum and minimum air temperatures, and grass minimum temperatures, in June 2019 in Reading compared to the 1981-2010 daily averages. The spike in the maximum temperature followed some unusually cool days around mid-month.

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