How does hail form?

FARMERSBURG, Ind. (WTWO?WAWV) –

Hail. It can be destructive, even dangerous. Look at all this hail in downtown Terre Haute from a couple of years ago. While hail can dent your car and lead to colorful language on occasion, is hail an indicator of something worse about to happen?

Does a hailstorm mean there could also be a tornado? 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.

What is hail? Hail is a form of precipitation consisting of solid ice that forms inside thunderstorm updrafts. Hail can damage aircraft, homes and cars, and can be deadly to livestock and people.

How does hail form? Lets go into the storm.

Frozen water droplets are swept up by updrafts. Additional layers of ice form on the droplets each time the droplets are swept up and past the freezing line. Eventually the weight of the hailstones exceeds the force of the updrafts and the hailstones fall.

The speed of the updrafts determines the size of the stone. 24 miles-an-hour and you get pea size hail. Quarter size, 49 mph. Baseball, 81. Softball, 103 miles-an-hour. Quarter size, along with 58 mph winds is the minimum criteria for a severe thunderstorm warning to be issued.

The fall speed of hail primarily depends on the size of the hailstone, the friction between the hailstone and surrounding air, the local wind conditions (both horizontal and vertical), and the degree of melting of the hailstone. Early research assumed that hailstones fell like solid ice spheres and showed very high fall speeds, even for very small hailstones. However, recent research outside of NSSL (National Severe Storms Laboratory) using 3-D printed casts of real hailstones suspended in a vertical wind tunnel has repeatedly shown that natural hailstones fall more slowly than solid ice spheres. For small hailstones (<1-inch in diameter), the expected fall speed is between 9 and 25 mph. For hailstones that one would typically see in a severe thunderstorm (1-inch to 1.75-inch in diameter), the expected fall speed is between 25 and 40 mph. In the strongest supercells that produce some of the largest hail one might expect to see (2-inches to 4-inches in diameter), the expected fall speed is between 44 and 72 mph. However, there is much uncertainty in these estimates due to variability in the hailstone’s shape, degree of melting, fall orientation, and the environmental conditions. However, it is possible for very large hailstones (diameters exceeding 4-inches) to fall at over 100 mph.
What areas have the most hail?

Although Florida has the most thunderstorms, Nebraska, Colorado, and Wyoming usually have the most hailstorms. The area where these three states meet – “hail alley” – averages seven to nine hail days per year. Other parts of the world that have damaging hailstorms include China, Russia, India and northern Italy.

Hail

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