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This is the VOA Special English SCIENCE REPORT.

Winter weather has arrived in northern areas of the world. In much of the United States, winter means the return of snow. Last month, during a severe storm, about two meters of snow fell on the city of Buffalo, New York.

Snow storm in Buffalo

Snow is a subject of great interest to weather experts. Experts sometimes have difficulty estimating where, when or how much snow will fall.

Snow is a form of frozen water. It contains many groups of tiny ice particles, called snow crystals. These crystals grow from water particles in cold clouds. They usually grow around a piece of dust. All snow crystals have six sides, but they grow in different shapes. The shape depends mainly on the temperature and water levels in the air.

Snow crystals grow in one of two designs -- platelike and columnar. Platelike crystals are flat. They form when the air temperature is about fifteen degrees below zero Celsius. Columnar snow crystals look like sticks of ice. They form when the temperature is about five degrees below zero Celsius.

The shape of a snow crystal may change from one form to another as the crystal passes through levels of air with different temperatures. When melting snow crystals or raindrops fall through very cold air, they freeze to form small particles of ice, called sleet.

When snow crystals stick together, they produce snowflakes. Snowflakes come in different sizes. As many as one-hundred crystals may join together to form a snowflake larger than two-and-one-half centimeters. Snow contains much less water than rain. About fifteen centimeters of wet snow has as much water as two-and-one-half centimeters of rain. About seventy-six centimeters of dry snow equals the water in two-and-one-half centimeters of rain.

Each year, the continental United States has an average of one-hundred snow storms. An average storm produces snow for two to five days. Almost every part of the country has received snowfall at one time or another. Even parts of southern Florida have reported a few snowflakes.

Snow creates many problems for people traveling. But it also is important. Much of the water we use comes from snow. Melting snow provides water for rivers, electric power centers and agricultural crops. In the western United States, mountain snow provides up to seventy-five percent of all surface water supplies.

This VOA Special English SCIENCE REPORT was written by George Grow.