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What is Hail and How Does it Form?
March 7, 2023 at 05:47 PM EST
By WeatherBug Sr. Meteorologist, Julie Gaddy
When you hear the term “hail," you may visualize large balls of compacted ice falling out of the sky during thunderstorms. The official definition for hail as provided by the American Meteorological Society in its Glossary of Meteorology is that hail is “precipitation in the form of balls or irregular lumps of ice, always produced by convective clouds, nearly always cumulonimbus”. Hail need not be large, however. Hail diameters can be as small as 5 mm (about 0.2 inch), similar in size to a typical pencil-top eraser.
How is hail measured? Hail size is commonly estimated by comparing it to similarly sized objects such as coins and sporting equipment. An exact ruler measurement can be obtained, but the hailstones can be dangerous to obtain and need to be measured promptly before melting.
Hail forms when towering clouds contain strong updrafts, or upward moving air, lifts moisture high into the atmosphere. When high levels of moisture are present, large raindrops will be present within the clouds. When those large drops encounter very cold air in the mid-levels of the atmosphere, they freeze. Upon freezing, the newly formed small hailstone will descend toward the surface, but it can collide with other raindrops and other hailstones. Thus, the shape of a hailstone can turn irregular rather than being spherical.
If sufficiently strong winds are present within a thunderstorm, hailstones get lofted back up into the updraft before reaching the ground. This cycle leads to the hailstone gathering another layer of ice which leads to the stone increasing in size. Multiple trips can occur within the strongest thunderstorms, leading to the formation of large to very large hailstones. Ultimately, the hailstone becomes too heavy for the updraft and it falls to the ground.
Large hail poses the highest hazard to people and property. Hailstones with diameters of an inch or more are classified as severe, and can damage crops, cars, roofs, siding and windows. If hail is very large, it can be deadly to people and animals. Hailstone sizes ranging from baseballs to softballs and grapefruits (3.0-4.5 inches diameter) are big enough to create serious to fatal head injuries. Such enormous hail is rare and occurs only within thunderstorms having the strongest updrafts where very high levels of atmospheric instability are present.
Large volumes of smaller hail can create hazards as well. High intensity hailfalls can lead to accumulations upon the ground, even to the point of needing snowplows to remove them. Such hail drifts on the roads create hazardous travel conditions while also stripping off leaves from trees or damaging light vegetation such as flowers. At times, the leaf debris can be enough to clog storm drains leading to ponding of water on local streets.
Hail most commonly occurs at mid-latitudes but can occur in lower latitudes where mountain ranges are present. For example, northern India and Bangladesh have had occasional large hail events. In North America, hail is most common in the high Plains where Wyoming, Colorado and Nebraska border each other. This part of the U.S. has been dubbed “Hail Alley”, with Cheyenne, Wyo., experiencing nine to ten hailstorms per year.
Hail is a fascinating phenomenon to watch and is best viewed from a safe indoor location.
How is hail measured? Hail size is commonly estimated by comparing it to similarly sized objects such as coins and sporting equipment. An exact ruler measurement can be obtained, but the hailstones can be dangerous to obtain and need to be measured promptly before melting.
Hail forms when towering clouds contain strong updrafts, or upward moving air, lifts moisture high into the atmosphere. When high levels of moisture are present, large raindrops will be present within the clouds. When those large drops encounter very cold air in the mid-levels of the atmosphere, they freeze. Upon freezing, the newly formed small hailstone will descend toward the surface, but it can collide with other raindrops and other hailstones. Thus, the shape of a hailstone can turn irregular rather than being spherical.
If sufficiently strong winds are present within a thunderstorm, hailstones get lofted back up into the updraft before reaching the ground. This cycle leads to the hailstone gathering another layer of ice which leads to the stone increasing in size. Multiple trips can occur within the strongest thunderstorms, leading to the formation of large to very large hailstones. Ultimately, the hailstone becomes too heavy for the updraft and it falls to the ground.
Large hail poses the highest hazard to people and property. Hailstones with diameters of an inch or more are classified as severe, and can damage crops, cars, roofs, siding and windows. If hail is very large, it can be deadly to people and animals. Hailstone sizes ranging from baseballs to softballs and grapefruits (3.0-4.5 inches diameter) are big enough to create serious to fatal head injuries. Such enormous hail is rare and occurs only within thunderstorms having the strongest updrafts where very high levels of atmospheric instability are present.
Large volumes of smaller hail can create hazards as well. High intensity hailfalls can lead to accumulations upon the ground, even to the point of needing snowplows to remove them. Such hail drifts on the roads create hazardous travel conditions while also stripping off leaves from trees or damaging light vegetation such as flowers. At times, the leaf debris can be enough to clog storm drains leading to ponding of water on local streets.
Hail most commonly occurs at mid-latitudes but can occur in lower latitudes where mountain ranges are present. For example, northern India and Bangladesh have had occasional large hail events. In North America, hail is most common in the high Plains where Wyoming, Colorado and Nebraska border each other. This part of the U.S. has been dubbed “Hail Alley”, with Cheyenne, Wyo., experiencing nine to ten hailstorms per year.
Hail is a fascinating phenomenon to watch and is best viewed from a safe indoor location.