ADD YOUR PHOTOS: Bizarre Hail Storm Hits Gilroy

If you were outside just before 1 p.m. Thursday, you experienced a hailstorm that lasted for about 20 minutes, or three times longer than the average hail storm, according to the American Meteorological Society.

The median length of time for hailstorms is six minutes, and a 15-minute storm is considered long, so our weather Thursday was notable even for professional meteorologists.

What Causes Hail?

Hail begins in cumulonimbus clouds, the same clouds present during thunderstorms. A large portion of the cloud has to be below freezing, 32 degrees or colder. The precipitation in the cloud is supercooled, which means raindrops are cold enough to freeze, but haven't frozen because of particle movement. As the precipitation falls, wind moving up from the earth, called an updraft, blows the liquid back into the cloud where it binds with ice. Each time the wind blows a hailstone back up into the cloud, that hailstone gets another coating of ice. The more the wind bounces a drop back into the clouds, the bigger the pellet of hail becomes. So higher wind means bigger hailstones.

What Was the Biggest Hailstone in U.S. History?

A hailstone eight inches in diameter and 1.93 pounds fell in Vivian, SD on July 23, 2010.

By comparison, most of the hailstones that hit Gilroy on Thursday were .25 inches in diameter (the size of a pea) or smaller. Gilroyans in their cars during the storm noticed hailstones of different consistency. Some hailstones were slushy when they hit the windshield, whereas other hailstones were frozen solid.

Enough hail fell to coat patio furniture, windshields and lawns, leaving them with a sprinkling of white. One enterprising Gilroyan even created artwork from the hail on her family's deck.