What is Hail?

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What is Hail?


Definition of hail

Hail is a form of precipitation that occurs when updrafts in thunderstorms carry raindrops upward into extremely cold areas of the atmosphere where they freeze into ice.

How does hail forms?

There are two ideas about hail formation. In the past, the prevailing thought was that hailstones grow by colliding with super cooled water drops. Super cooled water will freeze on contact with ice crystals, frozen rain drops, dust or some other nuclei.
Thunderstorms that have a strong updraft keep lifting the hailstones up to the top of the cloud where they encounter more super cooled water and continue to grow.The hail falls when the thunderstorm’s updraft can no longer support the weight of the ice or the updraft weakens. The stronger the updraft the larger the hailstone can grow.

Recent studies suggest that super cooled water may accumulate on frozen particles near the back-side of the storm as they are pushed forward across and above the updraft by the prevailing winds near the top of the storm. Eventually, the hailstones encounter downdraft air and fall to the ground.



Hailstones grow two ways: by wet growth or dry growth processes. In wet growth, a tiny piece of ice is in an area where the air temperature is below freezing, but not super cold. When the tiny piece of ice collides with a super cooled drop, the water does not freeze on the ice immediately. Instead, liquid water spreads across tumbling hailstones and slowly freezes. Since the process is slow, air bubbles can escape resulting in a layer of clear ice.
Dry growth hailstones grow when the air temperature is well below freezing and the water droplet freezes immediately as it collides with the ice particle. The air bubbles are “frozen” in place, leaving cloudy ice.
Hailstones can have layers like an onion if they travel up and down in an updraft, or they can have few or no layers if they are “balanced” in an updraft. One can tell how many times a hailstone travelled to the top of the storm by counting the layers. Hailstones can begin to melt and then re-freeze together – forming large and very irregularly shaped hail.

What is the difference between hail, sleet, and graupel?

The different ways precipitation is formed determines what type of precipitation it becomes. Hail is larger than sleet, and forms only in thunderstorms. Hail formation requires air moving up (thunderstorm updraft) that keep the pieces of ice from falling. Drops of super cooled water hit the ice and freeze on it, causing it to grow. When the hailstone becomes too heavy for the updraft to keep it aloft, ot it encounters downdraft air, it falls. Sleet forms from raindrops that freeze on their way down through a cloud. Snow forms mainly when water vapour turns to ice without going through the liquid stage. There is no thunderstorm updraft involved in either of these processes.
How does hail fall to the ground?
Hail falls when it becomes heavy enough to overcome the strength of the updraft and is pulled by gravity towards the earth. How it falls is dependent on what is going on inside the thunderstorm. Hailstones bump into other raindrops and other hailstones inside the thunderstorm, and this bumping slows down their fall. Drag and friction also slow their fall, so it is a complicated question! If the winds are strong enough, they can even blow hail so that it falls at an angle. This would explain why the screens on one side of a house can be shredded by hail and the rest are unharmed!

How fast does hail fall?

We really only have estimates about the speed hail falls. One estimate is that a 1cm hailstone falls at 9 m/s, and an 8cm stone, weighing .7kg falls at 48 m/s (171 km/h). However, the hailstone is not likely to reach terminal velocity due to friction, collisions with other hailstones or raindrops, wind, the viscosity of the wind, and melting. Also, the formula to calculate terminal velocity is based on the assumption that you are dealing with a perfect sphere. Hail is generally not a perfect sphere!

How does NSSL contribute?

Much of the research on hail is trying to figure out how hailstones grow and in what environments. One ingredient necessary for the development of large hail is a strong updraft – one that can support the weight of a hailstone long enough for it to grow to a large size. Other ingredients scientists are looking into are storm-scale wind structures, and the effect of melting as hailstones fall through the freezing level to the surface.
SHAVE – The Severe Hail Verification Experiment, conducted over the summer of 2006, gathered data in support of correlating changes in the hail size distribution with storm evolution.

hail informations from the National Severe Storms Laboratory- www.nssl.noaa.gov


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