“Come on, it’s only water !”. May 4 in Oakwood. A shopper is urging her children towards the car at a big box store. Rain is intensifying in thunderstorms over Hall County. Soon, it isn’t only water. Lightning flashes straight above the Sam’s Club parking lot, with an immediate thunderclap announcing that it’s too close for comfort. If it took 5 seconds for the thunder to arrive, it would be a mile away. But in this storm, it’s only half a second between flash and boom. A thousand feet isn’t a safe distance. The metal cage of the car provides more protection than a drenching sprint back to the store, so I pull out. On Mundy Mill Road, steady pounding of rain on the roof changes into a bright clickety-clack. Outside, hail grains are now dancing on the water covering the road. But they aren’t the dreaded golf ball size. The rain slows down a bit, and visibility improves, so it seems reasonably safe to move on.
Thunderstorms with hail are more common in spring and summer than in winter. This may seem odd, considering that the air is much colder in February than in May. But it’s not a function of temperature. It’s controlled by speed and height of uplift. In a storm, air is rising rapidly inside individual cells as if it were in a group of big chimneys. It contains a lot of moisture in the form of water vapor, an invisible gas. As it shoots upward some 20,000 or 30,000 feet, it cools fast because the temperature up there is minus 40 or even colder. The rapid cooling makes its moisture drop out, and rain begins to fall. Along with this condensation comes heat that the water vapor was holding. This provides for an additional upward bounce. Little rain drops that get kicked up the highest will freeze at the top of the thunderstorm, and finally hit the ground after traveling several miles back down. In the heaviest storms they can get recycled upward and accumulate more ice until they come down several inches in diameter. But it’s uncommon in the South, and didn’t happen last week in Oakwood. “We rarely see golf ball-sized hail on the patio,” said a colleague to me once. “Living next to a par-70 resort, most often we get hail-sized golf balls.”
Rudi Kiefer, Ph.D., is a professor at Brenau University, teaching physical and health sciences on Brenau’s Georgia campuses and in China. His column appears Sundays and at gainesvilletimes.com.