Interesting story.  I would use the phrase "lightning strikes" rather than lightnings. I hope this helps.  Bon travail!

2018 was the most stormy year in France (please correct this text)

2018 Was the Stormiest Year in France (please correct my text below)**

According to Meteorage, a French company founded in 1987 to monitor a network "lightning detection" system, the country endured more than 700,000 lightning strikes over 296 stormy days, this past year. The lightning hits measured over that period were far beyond the 695,000 hits, which was the previous record, counted in 1995. In fact, lightning in 2018 was so numerous that it had already surpassed the prior record as early as the month of September.*

How can we explain this phenomenon since this past year’s record heat is not the only the answer? Many other unusual weather-related conditions contributed to create such intense and violent storms, notably, the months of spring experienced significant temperature contrasts between their warm- and cold-air fronts. Similarly, rapidly falling temperatures, along with its abundant humidity, quickly followed the summer heat waves.

Thus, given that the most severe storms naturally happen during the change of seasons, when different air masses are colliding, it is not surprising to learn that May, with 180,000 lightning strikes (three times the average amount!), was the most active month, while August 9, with 41,000 lightning strikes, was the day that experienced the most hits of the year.

Hi, Cilla. Good job! Whatever changes I made were simply to allow your sentences to sound more natural to my native ears, for ease of reading and continuity, to allow your post a better ending. I hope that this helps you. J.

P.S. Fantastic use of the picture! It greatly adds to your post!

*I wanted to know what Meteroage was—a magazine, a newspaper, a society, etc., since I thought its identity relevant to your post, so I googled it. Instead, I not only found out the above information, but also realized that I could then eliminate that rather lengthy and awkward initial sentence which leaves the reader thinking that those facts are unrelated. I think that it reads much better this way, as well as conveys the information in a more concise and logical manner.

**In English grammar, with most two-syllable adjectives (in this case—stormy) we form the comparative form with more and the superlative with most. Therefore, I understand why you titled your post as so. However, the other part of that rule is that if the two-syllable adjective ends with –y, we change it to ‘i, then add –er for the comparative form (stormier), or add –est for the superlative form (stormiest).  Thus, the reason for my change of your title.

2018 was the most stormy  (not wrong, just not the normal way to say it) stormiest year in France (please correct this text)

It can be hard to know where to put the word only https://onlinewritingtraining.com.au/2018/07/only-position-in-a-sentence/

Amount is for things we don't count (a lot/a little/a liter/a kilo so for things like love, water and weight); number is used for counting (2 people/50 dogs/20 hats) https://grammarist.com/usage/amount-number/ 

2018 was the most stormy year in France (please correct this text)

How can we explain this phenomenon? The heat is not only the reason. Many conditions are required to create storms. In the spring, there are significant temperature differences between warm and cold air. In the summer, heatwaves followed by sudden drops in temperature can also cause this phenomenon.

Thank you. Cilla.