Hail is one of the more spectacular phenomena associated with strong thunderstorms. Many locations throughout the Twin Tiers experienced hail this past weekend when severe thunderstorms rolled through. Some places even whitnessed graupel (soft hail).
Unlike lightning which is present in every thunderstorm, hail only reaches the ground in a small fraction of storms. The impacts of hail can be dramatic. Hail causes more than $1 billion in crop losses annually in the U.S. Hail also results in significant property damage, primarily to automobiles, roofs, and windows.
Hail develops in thunderstorms with very strong updrafts that extend well above the freezing level. As air rises in a developing thunderstorm updraft, tiny water droplets are swept from the cloud base to high altitudes and cold temperatures by the rising updraft. Once these cloud droplets are carried above the 0 C level, they are supercooled and can remain liquid at temperatures colder than -20 C. The droplets can remain as liquid through these cold temperatures because of the absence of ice nuclei and because they travel through these altitudes so rapidly.
When a developing thunderstorm rises to levels where the ambient air temperature is below -15 C, ice particles begin to form in the storm. Some are carried into the anvil (top of the storm) in the strong updraft, but others remain near the periphery of the updraft and begin to fall due to gravity. The falling ice crystals grow by collecting supercooled cloud droplets and are called graupel once they grow to sizes 1-5 mm in diameter. Graupel are soft ice particles that have the consitency of snowballs. These are the initial souce of particles that grow to become hailstones.
As the storm continues to grow, some of the graupel particles fall to the ground and some of them are swept back into the updraft. Once in the updraft, the graupel particles grow rapidly to hail size by collecting supercooled droplets. As the growing hailstone moves into the core of the updraft it will float and spiral while continuing to grow more rapidly to a large size... sometimes exceptional sizes like a grapefruit or softball! The larger hail is eventually carried aloft and ejected to the Northeast side of the storm. A hail cascade can form in this area. The bottom line is that hail can cause a lot of damage. If you live in an area where severe thunderstorms can or are occuring, seek shelter. For more information on hail go to: http://www.erh.noaa.gov/er/cae/svrwx/hail.htm
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