Wind Basics Hurricane Tornado Wind / Hail Thunderstorm History / U.S. Hurricane Preparation Call Center Certification

[ Wind / 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. The ice rises through the clouds until the updraft can no longer sustain the weight of the ice at which time it falls to the ground as hail. Hail may take on many different sizes and shapes, such as a tiny pebble or up to 4.5 inches in diameter, roughly the size of a grapefruit or softball. Hail can be very dangerous causing damage to exterior objects, such as motor vehicles, home/roofs, outdoor structures, exterior furniture, gardens and trees. Bodily injuries and even deaths can result if people are caught outdoors when extreme hail occurs. As of January 2010, the severe warning criteria for hail size will increase to one inch (the size of a quarter) or larger.

Because updrafts within a storm are somewhat tilted, hailstones can be blown several miles from the cloud where they originated. Hailstones can fall to the ground with enough force to pierce and damage roofs. However, it is not uncommon with the mixture of high winds and force, to have hail stones propelled at an angle, sending them through windows normally out of the line of fire.

There are only estimates about the speed hail falls. One determination 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, lisions 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 of a perfect sphere. Hail is generally not a perfect sphere.


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