5 questions on storms and lightning

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Did you know?
There are between 6 and 10 million lightning strikes on the planet every day1. While they are necessary for the planet to maintain an electrical balance, their inherent danger puts lightning in 25th place in the world in terms of causes of death2...

1) Why do thunderstorms happen?

As an ongoing phenomena in the world, thunderstorms form as soon as three conditions are met:

It is the only cloud that generates storms, and it is responsible for phenomena such as hail and lightning.

Heatwave and thunderstorms
The start of thunderstorms on 8 June 2014 in Yvelines (France - left) and on 13 May 2017 in the Rhone Valley (France - right).

Popular belief:

Heat does not always give rise to storms: these phenomena are rare or even non-existent in most deserts, and we do not necessarily experience storms during heatwaves, such as the one in June 2019 in Europe.

Desert areas are clearly identified on this lightning strike map created using GLD360 data.

>> Learn more: Read the educational article on Météo-France about storms

2) How does a thunderstorm form?

Lightning is the phenomenon that we systematically encounter during a thunderstorm. It is an electrical discharge schematically linked to the collision of particles that make up the cumulonimbus cloud.
These lightning strikes occur in the cloud or between two clouds: we talk about inter- and intra-clouds and roughly 10% of these will reach the ground in the phenomenon we know as lightning.
Although lightning occurs systematically, other phenomena can form if particular conditions occur. This is what happens in the case of hail, as well as in rarer cases such as “TLE”: Transient Luminous Events or transient light phenomena, which form the subject of recent research.

Popular belief:

The majority of electrical discharges we perceive do not “fall from the sky”, but follow a reverse path.
Our eyes only see the “return arc”, which is the only visible discharge and which follows a path from the ground to the cloud in almost 99% of cases!
As proof of this, take a look below at the slow-motion video in the sequence filmed by our camera. You may need to watch it several times, even at 7,000 frames per second…

A short story:

A flash is so brief that it usually lasts only a few milliseconds. Nevertheless, a group of researchers, including our technical director Stéphane Pedeboy, has highlighted the existence of much longer phenomena, in particular a lightning flash lasting 16.73 seconds in Argentina in 2019, which was validated as a world record by the World Meteorological Organization (W.M.O) in 2020 3

>> Learn more: Discover the official study behind these records

3) When?

Some information to remember about thunderstorm occurrences. There are:

The majority of electrical activity is concentrated in the summer months.

In schematic terms in respect of Europe4, we find:

There are, however, many disparities, especially on the Iberian Peninsula where the distribution is more equal between spring and autumn, and in northern countries where lightning strikes in autumn are less frequent than elsewhere.

However, the point in common is the low proportion of winter thunderstorms in our European latitudes, which generally does not exceed 3% of annual activity.

Popular belief: :

Even though they are rare during this period, it would be wrong to ignore the risk of thunderstorms in winter.
As early as the 1990s, we found that the electrical current from lightning was much higher in winter than in other seasons.
Of the annual number of very violent lightning flashes—a threshold we set at 100 kiloamperes (kA) — nearly 60% are detected in winter compared with 9% in summer and 10% in spring.

>> Learn more: Discover our latest publication about the geographical distribution and seasonality of intense lightning strikes in Europe.

4) Where?

Some parts of the planet are spared thunderstorms, particularly desert and polar regions, while others are hit almost daily in equatorial zones.

Europe does not experience this type of extreme lightning strike. However, there are great disparities between countries, as well as between regions and even within specific areas such as the départements in France.

Nîmes, for example, experiences 30 times as many lightning strikes as Quimper on average, which is perhaps less surprising than the difference in lightning strikes between Gentilly and Saint-Mandé located in the same département. In fact, the latter sees nearly 4 times as many lightning strikes as its counterpart!

Popular belief:

The famous saying that lightning never strikes twice in the same place does not, as you might expect, stand up to reality. Worse, the opposite often happens, as shown by the intense lightning strikes in certain areas such as the Pic du Midi in the Pyrenees Mountains.

Vidéo Dataviz

A short story:

It will not have escaped the notice of some readers that Saint-Mandé is home to the headquarters of Météo-France, but we say that's entirely by chance! 😊

5) How many?

Nearly 2.5 billions lightning strikes were detected worldwide in 20195 , a figure that can be compared to the 10 million or so detected over a large part of Western Europe 6.
The average density of lightning strikes in France is in the region of one lightning strike per km²/year, although in reality this value can exceed 5 strikes/km²/year in the south-east of the country.
Despite obvious strong territorial disparities, this indicator allows a comparison to be made with our neighbouring countries. Belgium, Spain and Germany are close to this national average level, Switzerland is slightly above this, and Italy more than twice as high, whereas the United Kingdom does not see half this number. In addition to this indicator of the number of lightning strikes, we are also interested in how violent they are, particularly those that can exceed 200,000 amperes.
In France, around 500 such strikes are detected each year, and it can be seen that many losses do indeed correlate with the presence of high amplitude…

Stéphane Schmitt
File written by Stéphane S. He is our expert and trainer in storm risks.
He is (hyper)active in many organizations to reinforce awareness actions regarding storm risk. He contributes in the normative field and through numerous studies published during international conferences.

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1 & 5 Source : GLD360 Vaisala global network
2 Source : sutdy by the National Safety Council : “injury facts”
3 Press release on the World Longest Lightning Bolt
4 Analysis based on the Météorage and Euclid networks covering a large part of Europe.
6 Source : Euclid network data including data from the Météorage network