By Jan Null
Special to the Mercury News

On June 22, the small town of Aurora, Neb., experienced the full fury of Mother Nature as severe thunderstorms and tornadoes raked the area. But it was the monster-size hail that made this a very special event.

An irregularly shaped hailstone with a diameter of 7 inches and a circumference of 18.75 inches was recovered, making it the largest hailstone in U.S. history. The previous record was set in Coffeyville, Kan., in 1970, where a hailstone ``only'' 5.7 inches in diameter was found. It had a circumference of 17.5 inches and a weight of 1.67 pounds.

The Aurora hailstone, however, had partially melted, and at 1.33 pounds did not exceed the record for the heaviest hail -- but craters were left in lawns, and holes the size of grapefruit were punched into roofs.

Hailstones of this size are created inside monster thunderstorms that have extremely strong updrafts that keep them aloft, where they accumulate many layers of ice. A hailstone the size of a golf ball is kept aloft by updrafts and downdrafts within the storms for four to 10 minutes when as many 10 billion cloud droplets accumulate in concentric rings around the stone's nucleus. When a hailstone finally becomes too heavy to be held aloft by the thunderstorm's updrafts, it plummets to the earth with velocities that can exceed 100 mph.

The size of hail is often related to common objects of similar size, as most people caught in a hailstorm don't carry rulers. For example, hail with a diameter of 1/4 inch is ``pea-size'' and 1-inch diameter hail is ``quarter-size.'' Golf ball-size hail has a diameter of 1 3/4 inches, baseball-size is 2 3/4 inches, and at the top of the scale is softball-size hail with a diameter of 4 1/2 inches.

Reports of the record-breaking hail in Nebraska described it as ``volleyball-size.'' You can view pictures of it, as well as the craters the hail left behind, on the Internet at www.crh.noaa.gov/gid/svrwx/events03/22jun03/22June2003_megastorms.htm

In the United States, hailstorms rarely kill people. Just two fatalities were reported in the 20th century, the most recent being in Fort Collins, Colo., in 1979. In July 2002, a hailstorm in China's Henan province killed 22 people and injured 200, while a 1986 storm in China reportedly had a hailstone that weighed as much as 11 pounds and was responsible for more than 100 deaths, 900 injuries and 35,000 homes damaged.

The United States sustains an average of $1.4 billion in hailstorm damage to crops and property each year -- the equivalent of 1 percent to 2 percent of the nation's annual crop yield. The Midwest and southern Plains are hit especially hard. These are the areas where the atmospheric conditions breed the largest thunderstorms, unlike the relatively infrequent and small thunderstorms in California.

Even when ``hail'' is reported in California, it's often ice pellets, which form differently. Hail forms within a cumulonimbus cloud when a frozen particle gets caught up in the cloud's updraft and downdraft cycle and has layering, which is a unique characteristic of hail. Conversely, ice pellets form when a raindrop or melted snowflake refreezes into a tiny BB pellet-size ball. Ice pellets often bounce when striking hard surfaces and make a fairly distinctive sound.

Q I've lived in other parts of the country, and lightning seems to be much more frequent there than here in the Bay Area. Is this true, and if so, why? Where is there the most lightning? Mike Holstrom - San Jose

A Your observation is correct, as the Bay Area gets only a handful of thunderstorms per year and lightning is indeed a byproduct of thunderstorms. Florida tops the United States in thunderstorms, with an average of about 90 days per year, while most of the Gulf Coast and portions of the Colorado Rockies see 70 to 80 days of thunderstorms per year. This discrepancy is caused by the differences in the air masses that affect each region. Here on the West Coast, the air masses that move inland from the Pacific Ocean are relatively cool compared to the warm humid air from the Gulf of Mexico that affects the eastern two-thirds of the nation.