The winter storm of Feb. 8 surprised many with its quick accumulation of snow. The question came up about why it wasn’t hail, or sleet, or an ice storm.
Hail, like snow, consists of frozen water falling from the sky. But in order to form, layers of ice have to build up very quickly around frozen droplets of water, high up in the atmosphere. This requires a great deal of moisture, and rapid lifting of the air. That kind of fast ride to a height of 5 miles and beyond is made possible when a mass of cool air bumps into much hotter, humid air near the ground. Typically, we get this condition in spring and summer, when North Georgia is sweltering in heat and humidity. Hail is usually an indicator of violent weather, possibly with tornadoes to follow.
Snow is more peaceful in most cases. It develops relatively slowly in the cold conditions high in the clouds. Tiny ice crystals attract water vapor (water in gaseous form) more readily than liquid water does. This makes the crystals grow, and once they are heavy enough, they float down to the ground. The physical mechanisms are quite complex, commonly known as the Bergeron Process. In essence, water vapor is scavenged by ice crystals, which permits them to grow, or “accrete”.
Sometimes temperatures near the ground are much warmer than up above, and the ice crystals begin to melt again before they reach the bottom. In that case, it seems like a mixture of rain and snow coming down, but what gives a sleet storm its character are soft, partially melted snow flakes and ice pellets.
Ice storms are the most dangerous type of winter events. No ice is actually falling from the sky. Instead, rain is dropping from the clouds but encounters ground conditions well below 32 degrees. The water freezes quickly on road surfaces, rooftops and tree limbs. Power lines collapse, rooftops cave in, and roads acquire a treacherous layer of ice that leads to multi-vehicle car crashes and fatalities. As a longtime Georgia resident, I’ll always remember Christmas Eve of 1983, when freezing rain turned the roads from Atlanta to Athens, and also in the Gainesville region, into perilous skating rinks. This current winter season, 37 years later, looks like we may be spared a repeat of those conditions.
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.