In recent days, Italy has been crossed from North to South by bad weather caused by the so-called “Halloween cyclone” (or “All Saints’ storm” as it has been defined journalistically), i.e. a disturbance of Atlantic origin that will bring wind, rains And thunderstorms in most cities. But, from a meteorological point of view, what is a thunderstorm and how does it form?
This is an atmospheric phenomenon associated with rain showers which, to form, requires humidity, instability and upward currents, i.e. a forcing that forces thewarm, humid airpresent in the lower layers, a go out.
By the way, if we were to find ourselves in the middle of a storm without an umbrella, the best option would be to run to get wet as little as possible, since we would spend less time in the rain.
So let’s see what thunderstorms are, how they form and what are the elements that trigger them.
What is a thunderstorm and what causes it: the factors that trigger it
By definition, thunderstorms are local-scale, short-term phenomena associated with heavy rain showersand sometimes even hail, often accompanied by strong winds, with manifestations of thunder and lightning and very high expenditure of potential energy.
In general, for trigger a storm is needed three factors main:
- thehumidityespecially in the layers of the atmosphere closest to the ground;
- a unstable air masswhich tends to rise upwards with convective motion forming large clouds (called cumulonimbus);
- from the updraftswhich force the hot air to rise.
While the first two factors are common to all thunderstorms, the mechanism that leads to the vertical lifting of a mass of hot air (the upward current, in fact) can occur in different ways:
- Due to a front lift: in this case, thunderstorms are triggered when a warm air mass and moist it comes abruptly stripped from a cold front. In practice, the cold air associated with a disturbance collides with the warm, humid air and undermines it, lifting it upwards. These types of storms, called cold front storms, can be particularly intense due to the thermal contrast between air masses.
- Due to orographic uplift: in case gods are present mountainous reliefsthe warm and humid air that meets the slopes of the mountain range is forcibly lifted, triggering convective motions.
- Due to a thermal factor: when the sun warms the air present near the ground, the latter, being warmer and therefore lighter, will tend to rise upwards. This is the mechanism that triggers the so-called “heat storms”, typical of the summer period.
How the storm cycle works
To trigger, therefore, a storm needs various elements. But, once they are present, how does the storm cycle? As the warm, humid air rises upwards and reaches altitudes between 800 and 1500 metres, it progressively cools down to condense water vapor present inside it. As a result, billions of rain droplets are formed, which form a cloud and become larger and larger heavy.
Simply put, at that point the cold, denser and heavier air precipitates together with the rain that had formed previously, with strong downward (and therefore downward) gusts associated with a decrease in temperature. In other words, when the weight becomes greater than the thrust ascending (upwards), the drop begins to fall, dragging towards the Bass part of the surrounding air.
The storm, thus, has reached its phase maturity: At its maximum development, a large storm cell can release enormous quantities of rain or hail, associated with thunder, lightning and violent gusts of wind, up to devastating tornadoes or tornadoes.
When the cold descending currents reach the surface of the ground and spread in all directions giving rise to gusts of wind, then the dissolution phase of the storm: the air reaching the ground interrupts the supply of warm and humid air, with the cumulonimbus weakening until it dissipates, in a time that can vary from 15 minutes to several hours.
