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Monday, February 27, 2012
Snow, and Sleet, and Hail - Oh My!
Graupel and virga, too! During the winter, lots of different kinds of frozen precipitation can fall down from the clouds. They aren't the same thing:
Snow consists of clumps of tiny ice crystals that stick together as they fall. Freezing rain starts falling as snow. However, on the way to the ground, it passes through warm air, which melts the snowflakes. Then they move through a layer of cold air, which re-freezes them into tiny drops of ice. Sleet is frozen raindrops that bounce on the ground. Hail is large ice balls that form in the presence of both rain and ice. The rain coats the ice, making the drops larger. As the ice drops fall, the winds push them back up again, where they get an even heavier ice coating. This can happen several times. Eventually, the hail stones become too heavy to be blown around, and they fall to the ground. They can even get to 6-8 inches (15-20 cm) in diameter! Graupel, or "soft hail" are little balls of snow surrounded by ice. Virga is rain or ice that never reaches the ground because it evaporates first. You often see virga streaks below clouds; they are particularly obvious at sunset.