What is it?

Hail is a solid precipitation in the form of an ice chunk falling from a cumulonimbus cloud. Most hail fall from the central region of a cloud in a severe storm forming in advance of cold fronts. The size of hailstones can vary from tiny pea-sized hailstones to huge hailstones the size of a tennis ball. The largest hailstone ever measured fell in South Dakota, USA, three years ago and measured 8.0” in diameter and weighed almost two pounds. Large hailstones are an indication of powerful updraft and downdraft winds of approximately 100 miles per hour within a thunderstorm! They can cause significant damage to exposed structures and vehicles. Documented cases reveal that people have been killed as a result of a heavy hailstorm.

How does it form?

For hail to form there needs to be several meteorological conditions. It forms within strong thunderstorm clouds with intense updrafts, high water content, large water droplets and a good portion of the cloud layer below freezing. Hail begins as water droplets and freezes on contact with condensation nuclei (also known as cloud seed). It will keep rising in the thunderstorm until its mass can no longer be supported by the updrafts. It then falls toward the ground. 

Hail in Rock Garden

Photos : Hail bouncing off the pavement by State Farm, http://flic.kr/p/ekgrjZ, (CC BY 2.0)

               Hail by Peter, http://flic.kr/p/51u822, (CC BY 2.0)