How many days of thunder do we have in the UK? 

The heatwave ended with a bang for many of us earlier this week as thunderstorms hit the UK after several weeks of record-breaking hot and dry weather. According to the Met Office’s next generation lightning location system, there were 26,718 lightning strikes over UK land between Sunday and Wednesday, which equates to more than half of all UK lightning strikes (49,439) during the past twelve months.  

These thunderstorms produced torrential downpours in some parts, leading to surface-water flooding and travel disruption, with much of the rainwater unable to permeate the hard ground. Lightning can also cause wildfires, damage to infrastructure and even fatalities. Due to these impacts, it’s important to understand how frequently thunderstorms happen and how that has changed over time.  

Thunderstorms can bring torrential rain leading to flooding.

How do we detect thunder and lightning? 

We can calculate how far away lightning is by counting the number of seconds between the flash of lightning and the sound of thunder, and then dividing by five. That gives us the distance in miles from the lightning: 1 mile for every five seconds.  

The Met Office has operated some form of long-range very low frequency lighting location network to detect and locate the source of lightning events since the 1940s. Nowadays it can obtain data about thunder and lightning from observation stations and voluntary observers, as well as from its lightning location system.  

This data helps meteorologists provide warnings for the public, the aviation sector and other industries of hazardous weather associated with thunderstorms and potential impacts. But it doesn’t help to determine long-term trends accurately for the whole of the UK, because of data limitations and changes to the location of sensors over time.  

Lightning is a huge electrical discharge that flows between clouds, from a cloud to air, or from a cloud to the ground.

Days of Thunder study  

The Met Office recently conducted a Days of Thunder (DoTs) climatology study to show how the frequency of thunderstorms varied over a 30-year period for the UK from 1990 to 2019 per month, season and year.  

The National Climate Information Centre at the Met Office has a database of DoTs per month from volunteer observer stations, dating back to 1949. But the number of stations have varied from eight in 1949 to around 500 at its peak. The study was based on 25 stations that continuously reported DoTs between 1990 and 2019, alongside lightning location data generated from long-range lightning location networks. 

The thunderstorm study introduced a new method of normalising the long-range lightning location data using observations of audible thunder. It was also the first thunderstorm study to generate a gridded dataset for such a long time period, focusing exclusively on the UK. The study could be extended beyond 2019, as more data becomes available, allowing trends to be analysed in the future.   

The  figure shows the 30-year average days of thunder separated by season, with the colour scales showing the number of days of thunder per year.

How frequently do we get thunderstorms in the UK? 

Overall, there was a reduction in the number of days of thunder in the UK per year between 1989 and 2019, according to the study. Thunderstorms increased in the north of the UK and decreased in the south. Summer was the most active thunderstorm season and the winter the least active.  

Met Office Observations Scientist, Ed Stone, who led the study said: “Our study has shown that in the United Kingdom we are less likely to see a thunderstorm today than we were 30 years ago. Although we have not attempted to investigate the changes shown, is possible that some of the trends seen are due to decadal-scale oscillations in the atmosphere.  

“The findings may also help us to predict future lightning activity as part of climate change. We hope that the methods presented can support significant further research using other climatological data.”  

These graphs show how the average number of days of thunder has changed during the 30-year period year. The solid grey line is UK land and sea area; the solid black line is all UK land area, and the dashed grey line is UK sea area. The black dotted line and black dashed line are south UK Land and North UK Land area respectively.

What is the relationship between lightning and climate?  

The relationship between lightning and climate change is still being studied by climate and weather scientists. A Science study in 2014 predicted that global warming will increase the frequency of lightning strikes by about 12% for every degree rise in global temperature. 

Between the two 30-year periods 1961-1990 and 1991-2020, the mean temperature of the UK rose by 0.8C from 8.3C to 9.1C. Scientists will need to undertake further research to understand why the increase in the UK’s mean temperature appears not to have led to a corresponding increase in lightning activity; in contrast to the 2014 study. 

Ed Stone of the Met Office added: “Thunderstorms are among the most impactful elements of weather in the UK. Understanding more about their frequency has important implications for increasing our knowledge of future climate change and severe weather events.”

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