It hails instead of raining in a specific location when water droplets at high altitudes freeze as they enter a colder layer of air near the surface, forming hailstones that fall to the ground.
Hail originates in the heart of thunderstorms, more specifically in large cumulonimbus clouds, which are these super dense clouds with strong vertical development. Inside, there's quite a mix: water droplets rise and fall multiple times due to powerful vertical currents, and they transform into ice when passing through the very cold areas of the cloud. With each round trip, the ice thickens a little more until it forms a hailstone heavy enough to fall to the ground. In short, it all comes from a big churn of air and water in hyper-agitated clouds.
To have hailstones instead of raindrops on your head, there must be a strong instability of the air. This means that warm, humid air near the ground rises rapidly to high altitudes where it is very cold. The greater the temperature difference between the air at ground level and that at altitude, the more conducive it is to severe thunderstorms and hail. Another important factor is upper-level winds. Strong air currents aloft promote the formation and maintenance of hailstones by carrying them multiple times through cold areas, where they gradually grow larger. Finally, air particularly loaded with humidity facilitates the creation of ice because it brings a large amount of water that will freeze quickly when it suddenly reaches high altitudes.
The formation of hailstones largely depends on how air circulates inside a thunderstorm. When a cloud develops, warm updrafts push water droplets upward, where the air is very cold. These droplets freeze, becoming small ice crystals. The stronger these updrafts are, the longer the crystals stay aloft: this allows them to grow by accumulating other droplets and gradually transforming into hailstones. When the weight of the hailstones exceeds the updraft force, they fall, and then the downdrafts of cold air come into play, which accelerate their descent and quickly eject them from the cloud. If the updrafts are particularly strong, giant hailstones can form, capable of causing significant damage upon reaching the ground.
The risks of hail often increase in regions at higher altitudes, because the air at altitude is colder, which helps hailstones form easily without melting. Additionally, mountainous terrain plays a significant role: when a mass of air encounters a mountain, it is pushed upwards, cools rapidly, and easily forms unstable clouds capable of producing hail. However, even in flatter regions, certain places like canyons can promote the formation of strong local updrafts, conducive to the development of large hailstones. Finally, large bodies of water (lakes, seas) heating certain areas locally can also amplify these phenomena by bringing warmth and moisture conducive to more violent storms, and thus more hail.
The diameter of a hailstone can reach the size of a tennis ball or even larger. The largest hailstone ever recorded measured about 20 centimeters and fell in South Dakota, USA, in 2010!
Hail generally falls in summer and spring rather than in winter. This is explained by the better atmospheric conditions (strong updrafts and instabilities) that favor its occurrence during these seasons.
The term 'hail shower' is used exclusively to refer to a fall of hailstones with a diameter exceeding 5 millimeters. Below this size, it is more appropriate to refer to it as sleet.
Modern weather radars can detect hailstones forming within a cloud. This detection sometimes allows for local alerts to be issued a few minutes before the hail reaches the ground.
The prediction of hail mainly involves observing certain meteorological factors: atmospheric instability, humidity, vertical temperature variations, as well as the presence of intense updrafts in cumulonimbus clouds. Modern weather radars can also detect characteristic signatures of hail within the clouds.
The size of hailstones largely depends on the number of ascents and descents that occur within the thunderstorm cloud. The more intense and sustained the updraft, the more a hailstone can make multiple trips inside the cumulonimbus, accumulating additional layers of ice and thereby increasing its diameter.
Sure! Here’s the translation: "Yes. Certain geographical areas are particularly susceptible to hail due to local conditions such as the proximity of mountain ranges, the presence of particularly unstable zones that promote severe storms, or specific local interactions between terrain and atmospheric fronts."
Thunderstorms produce hail when the updrafts are strong enough to lift water droplets high into the frozen regions of the cloud, allowing for the formation of hailstones. A thunderstorm with too weak or short-lived updrafts typically produces only rain.
The damage caused by hail varies significantly. Small hailstones usually result in minor damage to crops and vehicles. However, large hailstones can cause substantial damage, such as deterioration of roofs, windshields, or even lead to physical injuries if necessary precautions are not taken.
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