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How does Hail Form?
Hail forms when tiny clumps of ice, kept aloft by strong updrafts, get blown through freezing thunderclouds until they are heavy enough to fall to earth. Most large thunderstorms create some hail, but the proper conditions must be present for the hailstones to grow large, freeze solid and then survive until they reach the ground. The ideal conditions for hail are tall clouds that reach high into the atmosphere, many swirling updrafts such as in a tornado and cold temperatures within and beneath the storm.
Starts with a Nucleus
A hailstone begins to form as an ice nucleus, a small cluster of supercooled water droplets or clumps of snow. This center might continue to accumulate ice, melt in the thundercloud and turn to rain or be smashed apart by other clusters. If a bug, a piece of dirt, a seed or another small particle gets blown up into the storm cloud, it creates another possible nucleus for a hailstone.
Layers of Ice
If the thunderstorm is cold and windy enough, this ice cluster will accumulate layers of ice the way a dipped candle accumulates layers of wax, through a process called accretion. Opaque, whitish layers form when icy droplets trap air bubbles and stick to the cluster. Clear layers have accreted large drops of supercooled water that freeze when they encounter the hailstone. Of course, much larger hailstones can be made when two smaller ones freeze together.
Gaining Weight
Hail can accrete more layers when the hailstone blows up through layers of the thunderstorm. Even heavy hailstones can be kept aloft by strong enough updrafts. When the hail falls back through the storm because of gravity, it accretes even more layers, until it is so heavy that it falls as precipitation. Hailstones form in most tall, cumulonimbus storm clouds that reach the colder upper atmosphere, but not all hail survives after it is out of the thunderstorm. A few outer layers frequently melt when the hail mixes with other precipitation, such as snow and rain.
Sizes
The size of fully formed hailstones can vary from pinheads to softballs. There are official size categories for hail that are useful for gauging the damage that they can cause. Some hailstones have been measured at more than 6 inches (15.24 cm) across and more than 1 pound (0.45 kg) in weight. Most hailstones, however, are less than 0.5 inches (1.27 cm) across.
Frequently Asked Questions
What conditions are necessary for hail to form?
Hail formation requires the presence of strong updrafts in thunderstorms, which are common in cumulonimbus clouds. These updrafts carry water droplets high into the atmosphere where temperatures are below freezing. The droplets freeze and accumulate layers of ice as they are repeatedly lifted and dropped within the cloud, growing into hailstones before falling to the ground.
How do hailstones grow to different sizes?
The size of hailstones is determined by the strength of the updrafts within the thunderstorm. Stronger updrafts can support larger hailstones, allowing them to circulate longer and gather more layers of ice. According to the National Severe Storms Laboratory, hailstones can grow to 6 inches in diameter or more in extreme cases, although most hail is much smaller.
At what time of year is hail most common?
Hail is most common during the spring and summer months when thunderstorms are more frequent. This is due to the higher energy available in the atmosphere from the warmer temperatures, which contributes to the development of strong updrafts necessary for hail formation. Regions with a high frequency of thunderstorms, like the central United States, often experience more hail events.
Can hail form in any type of thunderstorm?
Not all thunderstorms produce hail. Hail formation is more likely in severe thunderstorms characterized by intense updrafts. These updrafts are crucial for carrying water droplets high enough to freeze and for sustaining the growth of hailstones. Ordinary thunderstorms with weaker updrafts may not have the necessary conditions for hail to develop.
Why does hail often fall in paths known as "swaths"?
Hail falls in patterns called "swaths" because of the directional movement of the thunderstorm. As the storm travels, it drops hail over a path that can be several miles long and several miles wide. The specific area affected by hail can vary greatly, with some places receiving large hailstones while nearby areas only experience rain.
Is there a way to predict the size of hail before it falls?
Meteorologists use radar technology and atmospheric data to estimate the potential size of hailstones in a developing storm. By analyzing the height of the storm cloud and the intensity of the updrafts, predictions can be made about the size of hail. However, these predictions are not always precise due to the complex nature of storm dynamics and hail formation processes.
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![Handful of hailstones.]()
By: Gavin AndersonHandful of hailstones. -
![Hail may form during severe thunderstorms.]()
By: valdezrlHail may form during severe thunderstorms. -
![Hail can occur during a thunderstorm, but not all the hail that forms will always remain intact.]()
By: Ivan MonteroHail can occur during a thunderstorm, but not all the hail that forms will always remain intact. -
![The swirling updrafts of a tornado are ideal for hail formation.]()
By: Minerva StudioThe swirling updrafts of a tornado are ideal for hail formation. -
![When the winds inside a storm cloud begin to circulate, a powerful updraft can pull dust particles, insects or tiny ice crystals up to the cloud's highest levels.]()
By: powerzillyWhen the winds inside a storm cloud begin to circulate, a powerful updraft can pull dust particles, insects or tiny ice crystals up to the cloud's highest levels.
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