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HAIL FAQs

Hail is precipitation that is formed when updrafts in thunderstorms carry raindrops upward into extremely cold areas of the atmosphere and freezes into ice. Hailstones grow by colliding with supercooled water drops. Thunderstorms that have a strong updraft keep lifting the hailstones up to the top of the cloud so they continue to grow. The hail falls when the thunderstorm's updraft can no longer support the weight of the ice. The stronger the updraft, the larger the hailstone can grow.

The largest hailstone recovered in the U.S. fell in Aurora, NE on June 22, 2003 with a diameter of 7 inches and a circumference of 18.75 inches. Since its weight could not be determined, the hailstone that fell on Coffeyville KS in 1970 remains the largest on record at 5.7 inches and 1.67 pounds, with a circumference of 17.5 inches

Scientists don't agree on why the sky may rarely appear green during severe weather, although there is plenty of anecdotal evidence that it can, and that hail may be nearby. One theory is that large amounts of water and ice in a severe thunderstorm will scatter green light during the strong updrafts that occur in severe storms. Some people report the sky seems "brighter".

Not always, but possibly. Since large hail often appears near the area within a thunderstorm where tornadoes are most likely to form, you should assume a tornado could be nearby and seek appropriate shelter.

The type of precipitation is defined by the process by which it was formed. Hail only comes from thunderstorms because it is the thunderstorm updraft that keeps the pieces of ice from falling. Drops of supercooled water hit the ice and freeze, causing it to grow. Sleet forms from raindrops that freeze on their way down through a cloud. There is no thunderstorm updraft involved in the formation of sleet or snow.

How hail falls is dependent on what is going on inside the thunderstorm. Hail falls when it becomes heavy enough to overcome the strength of the updraft and is pulled by gravity towards the earth. Hailstones bump into other raindrops and other hailstones inside the thunderstorm. This bumping slows down their fall. Drag and friction are also factors. We really only have estimates about at what 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). A 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.

There have been reports of hail being thrown from a thunderstorm, but it is rare and it usually melts before it hits the ground. Most hail falls in the main part of the thunderstorm, and either straight down or at a bit of an angle if it gets blown by the wind.

Hail Basics

What is 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.

Large hail covers this deserted street.

How does hail form?

There are two ideas about hail formation. In the past, the prevailing thought was that hailstones grow by colliding with supercooled water drops. Supercooled 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 supercooled 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 supercooled 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 supercooled 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 traveled 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 supercooled 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 vapor 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.

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