It's the end of a scorching day: the air seems still, the streets remain warm, and the heat weighs like an invisible burden on your shoulders. Suddenly, a distant rumble is heard, the sky darkens, and massive dark clouds invade the horizon. Why does sweltering heat so often summon these violent thunderstorms?
It all begins with the accumulation of warm, humid air near the ground. Under the effect of solar radiation, the temperature rises and the atmosphere becomes unstable: the warm air, being lighter, rises while the cooler air at higher altitudes descends. This density difference creates strong convective currents.
Illustration image Pixabay
As the warm air rises, it gradually cools; upon reaching a certain level, the water vapor it contains condenses into droplets and forms cumulus clouds. If the air remains humid and the heat intense, these cumulus clouds grow and accumulate more water: they then develop into cumulus congestus, and eventually into towering cumulonimbus clouds, the true "thunderstorm factories."
The formation of a heat thunderstorm also requires a continuous supply of energy: the more the sun has heated the ground during the day, the more the air near the surface is charged with potential energy (referred to as "CAPE," Convective Available Potential Energy). When this energy is released, it violently fuels air movements, causing strong winds and instability within the clouds, resulting in hail, lightning, and torrential rains.
Sometimes, an additional triggering factor comes into play, such as a light sea breeze, an incoming cold front, or terrain that forces the air to rise. This "orographic lift" can be enough to initiate convection and trigger the storm.
Lightning is born from the friction and separation of electric charges between water droplets and ice crystals inside the cloud. When the potential difference becomes too great, a discharge occurs: this is the thunderclap heard after seeing the lightning.
Ultimately, these thunderstorms play a crucial role: they release the stored heat and humidity, abruptly cooling the atmosphere. But they can also cause localized flooding, hail damage, and dangerous wind gusts. At the first rumble, it's best to seek shelter and avoid any outdoor activities.