Heatwaves

Photo 1: Sunburnt Lettuce (12th July 2025)

Recently, there has been a lot of media coverage on the topic of heatwaves and whether we are experiencing a greater number of and/or more intense events. It has been quite the task this year to keep our flowers and vegetables adequately irrigated following a dry Spring and a hot dry Summer (so far). Our water butts (~2500 litres) are nearly empty despite also using grey water from the kitchen sink to supplement our watering regime. 

The World Meteorological Organisation (WMO) defines a heatwave as 5 consecutive days where the daily temperature exceeds the long-term average maximum daily temperature by at least 5 ℃ (the threshold temperature).

Many countries adapt this standard to suit local conditions. So, for example, the UK Meteorological Office defines the heatwave threshold temperature as the mean daily maximum temperature at the mid-point of the meteorological summer (July 15th), based on the 1991 - 2020 climate averaging period, plus 5 ℃. And because we are not used to hot weather in the United Kingdom, we only require 3 days at these elevated temperatures for a heatwave to be declared!

To complicate matters, summer temperatures vary quite a lot throughout the UK (due to its maritime climate) with the western and northern regions being somewhat cooler than the south-eastern part of Britain. The map (Figure 1)below displays the heatwave threshold temperatures for the United Kingdom where Herefordshire (circled) has a designated heatwave temperature threshold of 26 ℃.

Figure 1: Heatwave Threshold Temperatures for UK

I thought I'd check my Davis Weather Station records to see how many days surpassed the heatwave temperature threshold for Herefordshire (>26 ℃) over the 2020 - 2025 period (see Figure 2). Note that the 2025 data is up to, and including, July 14th only. At first glance, you could be forgiven for thinking there is a downward trend in the number of hot (>26 ℃) days in Hereford but that would be ignoring the fact we are currently only halfway through meteorological summer as I write this. We still have more than 6 weeks of Summer to go. Weather being weather, the distribution of hot days (>26 ℃) throughout the Spring-Summer-Autumn period varies from year to year. 2020 was atypical insomuch as hot days were evenly distributed between all 7 months (April - Oct). In 2022, 80% of hot days (>26 ℃) occurred in July and August while, in 2023, 30% of the hot days occurred in September (a so-called Indian Summer). However, generally, it is the three summer months (June, July, August) where we find the bulk of hot days.

Figure 2: Number of Days, by Month, when the Maximum Daily Temperature >26 ℃

In Figure 3, there is a graphical display showing the number of days, by month, when the maximum daily temperature was 30 ℃ or more. Bearing in mind that the data for this year is incomplete, it is still possible that 2025 will see the highest number of super hot days (>30 ℃).

Figure 3: Number of Days, by Month, when the Maximum Daily Temperature >30 ℃

Of course, hot days on their own do not make heatwaves; we have the other criterion that the daily maximum temperature equals or exceeds the threshold temperature for a minimum of three consecutive days. To identify heatwaves, I plotted out daily maximum temperatures for the April - September period for every year (2020 - 2025) using a minimum temperature cut-off (26 ℃). The plots are shown below (Figures 4 - 9). Click on the plots to enlarge. When three or more lines are close together, we have a heatwave. For example in Figure 4, there are four heatwaves (one 5-day, two 4-day and one 3-day). The longest heatwaves (8-day) were in 2021 & 2022 (Figures 5 & 6).

Figure 4: Daily Maximum Temperatures, April to Sept 2020

Figure 5: Daily Maximum Temperatures, April to Sept 2021

Figure 6: Daily Maximum Temperatures, April to Sept 2022

Figure 7: Daily Maximum Temperatures, April to Sept 2023

Figure 8: Daily Maximum Temperatures, April to Sept 2024

Figure 9: Daily Maximum Temperatures, April to mid-July 2025

The number of heatwaves in each of the last six years is summarized in Figure 10. Heatwaves used to be rare events in the UK but are now rather mundane. Over such a short time period it is not possible to pick out a rising trend. However, it should be remembered that the 2025 data is up to and including the 14th July only.

Figure 10: Number of Heatwaves in Hereford (2020 - 2025)

If we only consider the number of heatwaves up to the middle of July then the bar graph looks a little different ...

Figure 11: Early Season Heatwaves up to mid-July

It is the fact that we've not seen so many heatwaves this early in the summer season before that is giving cause for concern. Combined with the exceptionally dry weather we have had these past few months, it has put enormous stress on the agricultural sector, the water supply industry and nature itself.