Hail can vary in size due to different atmospheric conditions it passes through during its formation, such as temperature variations, humidity, and upward movement in thunderclouds.
Hail forms mainly in thunderstorm clouds called cumulonimbus. At high altitude, where it is very cold, liquid water droplets freeze into tiny pieces of ice. Then, due to powerful updrafts, these pieces move rapidly through the clouds. By rising and falling multiple times, they grow by accumulating successive layers of ice. The longer these hailstones spend going back and forth in the cold heights of the cloud, the larger they become, until they fall under their own weight. The variation in size therefore mainly depends on the number of trips back and forth and the time spent at altitude before the hailstone falls to the ground.
Temperature plays a key role in the formation of hail. A very cold layer of air at high altitude promotes the rapid freezing of water droplets, leading to the creation of larger hailstones. Conversely, a less frigid temperature limits the maximum size, as the hailstones fall before reaching a large dimension. Humidity also matters: a lot of humidity means more water available to accumulate around the ice core, thus forming larger hailstones. Powerful updrafts keep the hailstones in the clouds longer, multiplying the layers of ice that further increase their size. In contrast, atmospheric conditions low in humidity or updrafts result in significantly smaller hailstones.
Hailstones are born in updrafts, those columns of warm, moist air that rise upwards. The stronger the updraft, the longer it can hold the hailstone in the cloud, on a back-and-forth journey between the very cold top and the milder bottom. During these multiple trips, hailstones gain new layers of ice, grow, and become heavier. As long as the updraft remains strong enough to carry them, they stay in the air, accumulating layer upon layer and growing even larger. When their weight eventually exceeds the strength of the updraft, they tumble down to the ground. That’s why during storms with very powerful updrafts, like those from large thunderstorm cells, the hailstones produced can be particularly enormous.
Several elements play an important role in the growth of hailstones. The amount of available moisture particularly influences their size: the more water present in the cloud, the faster the hailstones will grow. The ambient temperature around the cloud is also essential. If the surrounding air is cooler, the ice melts less quickly during the falling phases, allowing the hailstones to continue to grow. Let’s not forget the speed and strength of the updrafts, as a strong updraft keeps the hailstone in the cloud longer and facilitates the accumulation of additional layers of ice. Finally, the presence of fine particles such as dust or pollen can serve as initial nuclei on which water droplets will freeze and grow. All of this mixed together explains why sometimes we get hailstones as small as peas or as large as golf balls.
Hailstones generally have several internal layers: each cycle of ascent and descent within a cloud adds a new layer of ice, similar to the growth rings of a tree.
Hailstorms cost billions of euros in damages to agricultural crops, buildings, and vehicles worldwide every year.
Contrary to popular belief, even in hot summer weather, hail can reach the ground while still frozen, as it falls too quickly to completely melt before hitting the surface.
The speed and strength of the updrafts primarily determine the size of hailstones: the stronger these updrafts are, the larger the hailstones can grow, sometimes reaching the size of a golf ball or even larger!
Some studies indicate that climate change could lead to an intensification of extreme weather events, including violent phenomena such as thunderstorms and potentially hail episodes. However, research on the specific increase in hailstone size due to climate change is still ongoing.
Thunderstorms create favorable conditions for the formation of hailstones because they have powerful updrafts capable of lifting small water droplets well above the freezing point. The frozen water droplets then grow larger as they are carried up and down within the cloud, resulting in hailstones that vary in size depending on the strength of the updraft and atmospheric conditions.
Yes, some regions around the world are known for the frequency or exceptional size of their hailstones. This is the case, for example, in the American Midwest, certain areas of Australia, and parts of Central Europe, where atmospheric conditions (warmth at ground level and strong instability at altitude) promote the formation of particularly large hailstones.
Damage caused by hail can be minimized by proactively protecting sensitive items: for instance, by parking vehicles in shelters, installing hail nets to protect crops, or choosing roofs that are resistant to severe impacts. Following weather alerts also allows for better preparedness in case of a hailstorm.
Small hailstones (less than 1 cm) generally cause little damage. However, larger hailstones (over 2 cm in diameter) can severely damage cars, break windows, and significantly impact agricultural crops. Very large hailstones (greater than 5 cm) can even cause significant damage to roofs or injure people.
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