No heatwave in Reading in July 2016 – but two in December 2015!

By Roger Brugge

Maybe the title of this piece may seem rather odd – but then ‘what is a heatwave?’

Perhaps surprisingly, in the UK there is no accepted definition for a heatwave. The Met Office tend to use the World Meteorological Organization definition of a heatwave which is “when the daily maximum temperature of more than five consecutive days exceeds the average maximum temperature by 5 degC, the normal period being 1961-1990” [Link 1].  In the popular press the impression is gained that a heatwave is a hot spell lasting at least two days.

In practice, the definition of a heatwave varies from country to country and, rather like a drought, it can depend upon the use to which the definition is being put. Thus humidity, minimum temperature and other meteorological factors relating to human physiology can also be included in a definition. The WMO definition presumably refers to the 1961-1960 period as this is the current standard 30 year averaging period used in many areas of meteorology. Arguably, the actual temperature itself should figure in the definition – the public might take some convincing that the South Pole might experience a heatwave!

Here in Reading we have a weather station that has been operational on the Whiteknights campus since 1968 – prior to that records have been kept closer to the centre of the town along the London Road since 1901. The relative urban (London Road) – rural (Whiteknights) differences of the two sites make it difficult to directly compare day-to-day temperature measurements between the sites.

So, I thought I would examine daily maximum temperatures as observed at Whiteknights and employ this definition of a heatwave:

A period of five or more consecutive days on each of which the daily maximum temperature is more than 5 degC higher than average for the period 1981-2010. The long-term average is calculated here on a day-to-day basis.

Results at Reading

This definition produced 35 heatwaves in the 48.5 years since 1 January 1968, with some interesting results:

• There were just 3 heatwaves during 1968-1980 – of which the longest (and also that for the entire 48.5 years) was that lasting for 17 days from 22 June 1976.
• The second-longest heatwave lasted for just 11 days from 3 August 2003.
• During 1981-1990 there were 7 heatwaves, with 6 in the period 1991-2000. During 2001-2010 we had 10 heatwaves, with 9 since the beginning of January 2011.
• 16 heatwaves have begun during one of the summer months (June-August), while 14 have started at some time during March-May (i.e. in spring).
• The only autumn heatwave was the six-day long one starting on 28 September 2011, while there have been four that started during winter months – namely those starting on 11 February 1998, 11 December 1998, 15 December 2015 and 25 December 2015.

So, we had a heatwave starting on Christmas Day last year. In fact had the temperature not dropped slightly on 23-24 December (the maximum temperatures were just 4.5 degC and 4.3 degC above normal on these days, respectively), December would have had a 16 day heatwave!

The bullet points above indicate a tendency towards an increasing number of heatwaves – as might be expected in a warming climate. This is confirmed by Figure 1.

Figure 1. The number of heatwave days during 1968-1991 (blue), 1992-2015 (red) and 1968-2015 (black). Notes that monthly totals of less than 5 days are an indication of a heatwave spread over two months.

Figure 1 shows that the second half of the period produced more heatwave days than the first half, the exception being in June due to the remarkable events of the summer of 1976 [Link 2] – which are still talked about to this day! Moreover, the warmth of December 2015 also stands out –see also [Link 3], while both April and May seem to fare better than August.

Figure 2. The number of heatwave days each year during 1968-2015, along with the ten-year running mean plotted on the final year of the ten-year period.

Looking at the annual heatwave day totals (Figure 2) suggests less of a warming trend in recent years than does Figure 1. With the exception of 1976, there were less than 10 heatwave days each year until 1989; since then it is difficult to see much of a trend in the 10 year running mean count – 1989 and 1995 led to a peak in the running mean by 1996 but since then the running-mean has declined (slightly).

‘Proper heatwaves’

Of course, in the public’s mind a UK heatwave is one with high temperatures, and not just large temperature anomalies – i.e. the South Pole argument mentioned earlier.

If we examine the 35 heatwaves and rank them according to the average maximum temperature or the highest temperature achieved during the heatwave, then the results are those shown in Table 1.

Table 1. Top nine of the 35 heatwaves, ranked by average of highest maximum temperature.

Heatwave start date  (ymd)	Heatwave duration, days	Highest temperature, °C		Heatwave start date (ymd)	Heatwave duration, days	Mean maximum temperature, °C
2003 Aug 3	11	36.4		1995 Jul 29	6	31.6
2006 Jul 16	7	35.3		1976 Jun 22	17	31.4
1976 Jun 22	17	34.0		2003 Aug 3	11	31.2
1995 Jul 29	6	33.6		2006 Jul 16	7	31.2
1989 Jul 20	6	33.2		1989 Jul 20	6	30.6
1983 Jul 11	7	31.6		1983 Jul 11	7	30.5
2006 Jun 30	5	31.4		2009 Jun 28	5	29.7
2009 Jun 28	5	30.4		2006 Jun 30	5	29.6
2013 Jul 13	7	30.2		2013 Jul 13	7	29.4

 

This table highlights the appearance of the same nine heatwaves in both sets of rankings. Omitting the entry of 1976, then the other years that appear are those of 1983, 1989, 1995, 2003, 2006 (twice), 2009 and 2013.

With only one year from the period 2010-2016 in the list, maybe this has fuelled the idea that we are having a run of poor summers without any heatwaves?

Further reading

[Link 1] http://www.metoffice.gov.uk/learning/learn-about-the-weather/weather-phenomena/heatwave

[Link 2] http://www.metoffice.gov.uk/news/releases/2016/heat-wave

[Link 3] https://blogs.reading.ac.uk/wcd/2015/12/18/not-yet-christmas-but-has-spring-sprung/