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[ Friday, Jan. 15, 1999 ]
Falling snow: Tiny, unique flakes embark on swirling journey
By JENNIFER NEJMAN
Collegian Staff Writer
The mounds of dirty snow now clumped on the sidewalks and alleyways of State College began as minute structures. Before snowflakes embark on their swirling, chaotic journey to earth, they start as ice crystals in clouds. "Several miles above our heads, ice crystals co-exist with small water droplets," said Lee Grenci, Penn State meteorology instructor. "Water droplets, if they are tiny, can resist freezing temperatures down to near-minus 40 degrees Fahrenheit in the atmosphere." A water droplet may be on the order of 10 micrometers in diameter. Ice crystals are a little bigger, about 20 to 30 micrometers. A micrometer is one-millionth of meter, Grenci said. The height of a lowercase letter in a printed book is usually about 200 micrometers, while the diameter of a water droplet is about one-twentieth of that measurement. In a cold cloud, water and ice crystals reside in the same area. Tiny ice crystals draw liquid from water droplets in the cloud, growing larger.
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| 1993-94 | 109.0 inches | ||||||||||||||
| 1994-95 | 19.4 inches | ||||||||||||||
| 1995-96 | 95.7 inches | ||||||||||||||
| 1996-97 | 42.2 inches | ||||||||||||||
| 1997-98 | 47.0 inches | ||||||||||||||
| 1998-99 | 13.9 inches | ||||||||||||||
| SOURCE: Penn State Weather Observatory | |||||||||||||||
"The ice crystals continue to grow and pretty soon get weighty enough to descend. They bump into each other. When they bump together, sometimes they splinter," Grenci said. "It's helter-skelter in the sky. It's serendipitous collisions and coalescence."
A snowflake's haphazard journey provides one explanation for each snowflake's individuality. But all snowflakes share a hexagonal shape and have six points.
Most precipitation around the middle latitudes, whether it is summer or winter, begins high in cold clouds as snowflakes, Grenci said. As they fall, the temperature of the air determines if it will snow, sleet or rain.
If the snowflake falls when the temperature is above freezing (32 degrees Fahrenheit), then the snowflake will melt and descend to earth as a raindrop, Grenci said. Sleet occurs when the atmosphere forms a "warm-air sandwich." As the snowflake falls to the ground, it melts into a raindrop, then freezes again as it falls through a thick layer of cold air.
Snowfalls that produce the largest amount of accumulation usually occur when the temperature is near 32 degrees Fahrenheit, Grenci said.
"It maximizes the water vapor in the air and ensures that the snowflakes won't melt," he added.
Temperatures during the past two winters have been warm, creating an environment in which snow will land and disappear quickly, said Bill Syrett, manager of the Penn State Weather Observatory. Last winter, he said, 47 inches of snow was measured for State College, which is slightly above average.
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