Thunderstorms in the UK
Slow motion cloud-to-ground lightning near Bintree (Norfolk) on 25 July 2019 (Credit: Chris Bell)
What is lightning and thunder?
Lightning is a huge electrical spark (discharge) that flows from a cloud (called a cumulonimbus or thunderstorm) to the ground, or within and between clouds, or from the cloud to air. Thunder is the sound produced by the rapid heating of the air by a lightning flash. The air expands explosively and contracts rapidly, producing sound waves.
The UK Met Office provides useful resources to explain how thunderstorms, lightning and thunder form:
There are many US websites which explain the science as well as lightning safety:
Cloud-to-ground lightning over Norwich (Norfolk) on 19 July 2017 (Credit: Dan Holley)
Where in the UK is the greatest risk of being struck and when?
Thunderstorms may occur in any part of the UK and in any month although southeast England and the summer half year, especially the months of May to August inclusive, give rise to the highest incidence of thunderstorms and resulting lightning. Although there is significant annual variability in the occurrence of thunderstorms, the map below highlights the long-term annual number of thunder days for the 30-year period 1981-2010. However, there are some limitations of the 'thunder days' statistic as discussed by the following articles (contact Jonathan Webb for further details):
Prichard, R. J (1985) The spatial and temporal distribution of British Thunderstorms. J. Meteorology, UK, 10, 227-230.
Webb, J. D. C (2014) Thunderstorm Division review for Britain and Ireland 2013. Int. J. Meteorology, UK, 39, 135-140 (section 5).
Principal sources of data include: TORRO and TCO Thunderstorm observers' reports, Climatological Observers Link station data (courtesy of Roger Brugge), and the Monthly Weather Report of the Met Office (published until 1993). Data has been checked for consistency between adjacent stations; however note well that the means for less populated areas will be based on fewer available observations, also, that as thunder observation requires a "24 hour watch" to be kept on the weather, a tendency for under-recording is likely.
Average number of days with "thunder heard", 1971- 2010
||Under 5 days
Potentially dangerous cloud-to-ground strikes make up only one-quarter of all lightning generated by thunderstorms. Most other lightning happens wholly within the cloud and is visible only as a brightening of the cloud (often called 'sheet lightning').
Each year the United Kingdom, Ireland and the surrounding seas typically experience 200,000 to 300,000 lightning counts. A 'thunderstorm day' may experience as many as 10,000 but on exceptional days more than 50,000 can occur, as happened on 28 June 2012 when there were 64,000 strikes.
There were extensive thunderstorms over the rest of Europe on that day too as shown below:
Maps of current lightning strikes are available from Netweather