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FRIDAY, JULY 18, began as another of those sunny days with high humidity, daytime temperatures nearing the 90s and overnight lows above 70, but it would mark the end of the heat wave.

It brought relief in a sudden and unexpected way. The Weather Channel predicted a break, and its early afternoon radar showed a mass of green and orange sagging down from Canada into the Niagara Frontier. On the accompanying map, a cold front sliced west to east along the south shore of Lake Ontario, its triangles pointed in our direction.

From our front lawn, we could see the storm coming. Giant thunderheads towered over the trees to our north. Their tops, high in the stratosphere, were big puffballs of cotton batting. Lower down, however, the cumulus white was mixed with the dark gray of nimbus, more threatening in character. My wife began to bring in her gardening tools.

As the clouds approached, we noticed something unusual. We're accustomed to seeing vertical light gray streaks pointing down from the bases of thunderheads that identify rain squalls, but these streaks were white. We would soon see why.

We barely got into the house before the storm line reached us, and hail began to fall. Little white ellipsoids rattled our roof and bounced off the driveway. Within minutes, lawns were covered, and the hail had increased to ice-cube size. The largest one we picked up later was a full inch in diameter. Tom Niziol and Steve McLaughlin of the Weather Bureau tell me that the golf-ball-sized hailstones of this storm were among the largest ever recorded in this area. But consider the record: one measuring 17.5 inches in diameter that fell in Coffeyville, Kan., in 1979.

Despite its intensity and the size of the hail, this storm appears to have done only minor damage in Amherst. Still, the fronds of Doris' big ostrich ferns were broken, and the larger leaves of perennials and shrubs were cut. Yards were strewn with leaves and small tree limbs.

Some recent regional hailstorms have been much more destructive. For example, the early morning storm of July 5, 1992, severely damaged crops across northern Niagara County and neighboring Canada. Grapes were ripped from their vines; apples cut and bruised. Some fields of vegetables were completely defoliated. The total cost to regional farmers was several hundred thousand dollars. McLaughlin was on duty at the Weather Bureau that night. The storm approached so rapidly that he had no opportunity to issue an advisory. How do you defend against a hailstorm anyway?

Hail develops when microscopic ice particles form in subfreezing areas high up in thunderheads. The tiny particles then collide with supercooled water droplets that freeze on their surface. (Supercooled water is water that, due to unusual atmospheric conditions, remains liquid at below freezing temperatures.) If those small ice pellets fall to earth, they melt on the way down, and we simply have cold rain. Occasionally, as they were this time, however, they are tossed about by air currents accumulating more and more ice until finally their weight can no longer be sustained by the updrafts.

Our Great Lakes affect the number of hailstorms we experience in this area. Through spring and summer, the lake temperatures are below that of the air above them, thus suppressing thunderhead activity and the associated possibility of hail. In late summer and fall, that situation usually reverses, and we get more hail than other regions.

A positive result of that July 18 storm was to drop the temperature in the space of a few minutes by 20 degrees, bringing welcome relief from the heat and humidity.

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