According to Météo France, on average, fifteen storms affect France each year.
One in ten can be described as “strong” according to the criterion used by the organization.
In meteorology, the storm has very specific characteristics.
Definition of a storm
The storm is caused by violent atmospheric disturbances occurring on land and sea: an extended area in which low pressures combine with fast winds.
These are “synoptic” systems, their diameter varies between a few hundred and a few thousand kilometers (on French territory it is less than 1000 km) and their lifespan is approximately one week.
Typically, storms only last a few hours at any given point since they move so quickly.
Those which form above the Atlantic go up to 100 km/h.
In the strict meteorological sense of the term, at sea and on land, we call a storm an atmospheric depression which creates an average wind greater than 90 km/h, that is to say a wind of force 10 or greater on the scale of Beaufort (this scale measures the intensity of the wind and has 13 degrees).
Storms therefore differ in intensity depending on wind speed.
For example, we speak of a “strong storm” at 10 Beaufort, a “gale” storm at 11 Beaufort, and a hurricane at 12 Beaufort.
On the other hand, if the wind only reaches storm intensity for a short period of time (a few seconds), it is called a “storm gust” or “squall”.
Generally, the storm is accompanied by other phenomena such as thunderstorms with lightning and thunder, tornadoes and hail or rain.
The causes and formation of storms
A storm forms when a current of warm air from the sea meets a current of cold, dry air coming from the land.
The collision between these two air masses with different temperatures and water content creates violent winds, but also storm clouds such as cumulonimbus clouds.
The greater the temperature difference between the two air masses, the more violent the winds.
Storms occur most often during fall and winter, about 80%, in temperate regions, because the oceans are still relatively warm while the polar air is already cold.
Linked to storm phenomena, they are rare and less severe in summer.
When the two air masses meet, the warm, lighter air rises above the cold air, and a meteorological front forms (meteorological fronts are the areas of contact between two air masses distinct).
This contact zone can extend several hundred kilometers long, or even several thousand.
It is the thermal contrast and various other properties of air masses that cause strong upward and downward movements, a phenomenon called atmospheric convection.
Under the influence of air masses
and heat exchange, the front is the place of numerous atmospheric manifestations: it is here that events characteristic of the storm are formed, such as winds, rain or other abundant precipitation, but also intense electric shocks .
This strong electrical activity becomes evident when lightning strikes, resulting in lightning and thunder.
Its appearance depends on different factors such as the degree of atmospheric ionization, the type and concentration of precipitation.
The different types of storms
Depending on the characteristics of the two air masses at the origin of its creation, the storm can be accompanied by several phenomena.
This allows us to distinguish different types of storms:
Thunderstorms and electrical storms are characterized by the presence of lightning and thunder coming from cumulonimbus clouds.
Mid-latitude storms are formed outside tropical areas, on land or at sea, when two air masses with opposite temperatures, a cold front and a warm front, meet.
Tropical storms and hurricanes are observed at tropical or subtropical latitudes.
Average winds are between 62 and 117 km/h for the storm, beyond that, it is no longer a depression but a tropical cyclone.
Blizzards and blizzards are two phenomena with precipitation in the form of snow.
The blizzard causes heavy precipitation, intense cold and blowing snow and winds of more than 40 km/h for at least 4 hours.
Hailstorm: hailstone precipitation, or balls of ice that form at high altitude in a storm system.
The size of hailstones varies depending on pressure and temperature between the size of a pea and that of a grapefruit.
Sand or dust storms that occur most often in arid or semi-arid regions, sometimes reaching as far as Europe.
The wind then moves the grains of sand at a speed greater than 40 km/h.
Also read: What are the weather phenomena linked to snow?
It is with more or less precision that it is possible today to predict the various types of storms, their frequency, the time of occurrence and the regions a priori threatened.
On the other hand, their triggering and their trajectory cannot be predicted in advance, only as they progress.
The devastating effects of storms
Because of the strong wind gusts and intense rain it can bring, a storm poses a safety risk.
In Europe, the damage caused by storms amounts to several dozen deaths and hundreds of millions of euros per year (report from the Ministry of Ecology and Sustainable Development).
Those directly affected may suffer a slight injury or physical damage which in the most serious cases may lead to death.
Most of the time these injuries or deaths are due to impacts by various objects thrown by the wind, trees falling on houses or vehicles, floods or even landslides.
People can also find themselves homeless due to damage to their homes.
The storm often damages the economy of the affected territories.
Thus, strong winds from storms affect buildings and cause significant material damage: private or public buildings, industrial and transport infrastructures suffer damage that must subsequently be repaired.
On the coasts, waves can damage infrastructure (sea walls, jetties, etc.).
All road, rail and air transport is affected by heavy precipitation and strong winds, often over a fairly long period.
This economic cost is also increased by the interruption of traffic (road, rail, air) and disruptions to activities.
One of the sectors most affected by the storms is the agricultural sector which suffers loss of income resulting from damage to livestock, livestock and crops.
Also read: How the 1999 electroshock changed everything in storm management
Although the storm is a natural phenomenon, it has an impact on the environment, more or less significant depending on the strength of the wind and precipitation.
Generally, ecosystems, including forests, are resilient to storms, which they have always faced.
In ecology, and in forest ecology in particular, the uprooting of trees caused by storms is considered a normal and essential disturbance to the silvigenetic cycle.
However, fauna and flora can be indirectly affected by a storm when it causes, for example, a shipwreck and therefore more or less serious and extensive pollution of the coastline, or when transport is impacted on land, potentially leading to contamination of the lands.
Storms cannot be prevented, but individual and collective risk prevention measures can mitigate their consequences.