This is the VOA SpecialEnglish SCIENCE REPORT.

Winter weather has arrived in northern areas of the world. Inmuch of the United States, winter means the return of snow. Lastmonth, during a severe storm, about two meters of snow fell on thecity of Buffalo, New York.

Snowstorm in Buffalo

Snow is a subject of greatinterest to weather experts. Experts sometimes have difficultyestimating where, when or how much snow will fall.

Snow is a form of frozen water. It contains many groups of tinyice particles, called snow crystals. These crystals grow from waterparticles 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 theair.

Snow crystals grow in one of two designs -- platelike andcolumnar. Platelike crystals are flat. They form when the airtemperature is about fifteen degrees below zero Celsius. Columnarsnow crystals look like sticks of ice. They form when thetemperature is about five degrees below zero Celsius.

The shape of a snow crystal may change from one form to anotheras the crystal passes through levels of air with differenttemperatures. When melting snow crystals or raindrops fall throughvery cold air, they freeze to form small particles of ice, calledsleet.

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

Each year, the continental United States has an average ofone-hundred snow storms. An average storm produces snow for two tofive days. Almost every part of the country has received snowfall atone time or another. Even parts of southern Florida have reported afew snowflakes.

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

This VOA Special English SCIENCE REPORT was written by GeorgeGrow.