Hail can vary in size within the same storm due to differences in atmospheric conditions encountered by suspended water droplets. Conditions such as temperature, humidity, and updrafts can influence the growth of hailstones, resulting in different sizes.
When a storm forms, all the water droplets or ice crystals at the outset are not identical. Some are already larger or smaller than others depending on how they condense around the suspended particles in the air (these are called condensation nuclei). These particles can be dust, pollen, or even volcanic ash. The more the initial droplets and crystals vary in size or composition, the greater the differences in size of the hailstones formed in the storm will be in the end. It's a bit like rolling a snowball: the larger it is at the start, the faster it will grow and create a significant difference by the end.
Hailstones grow by being carried by updrafts, which keep them suspended long enough to accumulate additional layers of ice. These updrafts, also called updrafts, vary in intensity: within the same storm, some areas have powerful updrafts capable of holding hailstones aloft for a long time. Elsewhere in the same cloud, weaker updrafts will allow hailstones to fall quickly, limiting their growth. Descending air currents, known as subsidence, play the opposite role by rapidly pushing some hailstones downwards in the cloud, reducing their time spent at altitude and therefore their size. Because not all hailstones travel exactly in the same air currents, you will find a significant difference in size among your hailstones, even while remaining under a single storm.
The size of hailstones in the same storm can change simply because conditions vary slightly from one place to another within the cloud. Some areas are a bit more humid, providing more available water, while others may be a little colder, facilitating the rapid freezing of water droplets. Where the air is very humid, ice crystals grow quickly by capturing many water droplets. Conversely, in less humid regions of the cloud, ice develops more slowly, resulting in smaller hailstones. Similarly, if a part of the cloud is significantly colder, droplets freeze immediately and quickly form large, solid hailstones. These small local changes explain why, in the same storm, one can find both tiny barely noticeable pieces of ice and impressive hailstones capable of damaging a car.
The size of hailstones often depends on the time spent circulating in the cloud. The longer a hailstone is caught in a powerful updraft, the more it can make back-and-forth trips at altitude. During each cycle, it accumulates a new layer of ice. Therefore, multiple cycles result in larger and sturdier hailstones, with a layered structure somewhat like an onion. In contrast, hailstones that are ejected rapidly to the ground after only a few cycles will be smaller. Within the same storm, hailstones often follow very different paths, leading to the significant size variations observed at a specific location.
Hailstones often display layers similar to the rings of trees: each layer corresponds to a new upward-downward cycle within the cloud.
A hailstone typically reaches the ground at a speed ranging from 30 to 160 km/h, depending on its size and density, potentially causing significant damage.
The different colors observed in some hailstones (opaque white, transparent, or sometimes bluish) reflect variations in their formation: microbubbles of air create an opaque layer, while clear ice is associated with a slower freezing process.
The formation of hail requires specific conditions of temperature and humidity, which is why hail is often associated with severe spring or summer storms, rather than during the colder winter months.
The record size of hail known is approximately 20 cm in diameter, observed in South Dakota in 2010. These exceptionally large hailstones result from very violent and specific atmospheric conditions, involving extremely powerful updrafts and high humidity.
Not necessarily. The size of hailstones depends more on the internal cycles that the ice pellets undergo within the cloud than on the precise moment of their fall during the storm. Thus, large hail can be observed at the beginning, in the middle, or at the end of the weather event.
The regions frequently affected tend to combine specific climatic factors such as strong updrafts, high humidity, and rapid temperature changes at altitude. Mountainous areas or those located in corridors of unstable atmospheric currents are generally the ones that present an increased risk of hail.
No, once the hailstones leave the cloud, they can no longer grow because they no longer encounter conditions that allow the accumulation of frozen water around them. On the contrary, they generally start to partially or completely melt before reaching the ground.
Although it is possible to predict the conditions favorable for the formation of hail, it is very difficult to accurately forecast the size of the hailstones. The final size heavily depends on local conditions such as the intensity of updrafts, humidity, and temperature variations within the cloud, which are highly variable both spatially and temporally.
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