The winter of 1987-88 is two weeks old today. One of the first things you probably think of when you hear the word "winter" is "Snow!" Here in the Washington area we receive at least some snow every winter: There are an average of five storms that leave at least an inch of snow behind in our region each year. Sometimes -- like last winter -- we have real big storms.

When snow falls in Washington, schools close, traffic snarls the roads and kids all over the city and suburbs go out to play. When you're building a snow sculpture or sledding down a hill in Rock Creek Park, you may wonder, "Is is true that no two snowflakes are alike?"

Take a minute to watch the snow as it lands on your sleeve. If your jacket is a dark color, it will be easier to see the individual flakes. Some of them may be those beautiful, lacy, six-sided crystals you see printed on Christmas cards. Others will probably be little globs of ice or pellets of snow without the lacy pattern. You'll have to look fast, because the snowflakes will melt quickly from the warmth of your body.

Snow is simply tiny bits of ice that formed in a cloud and grew large enough -- and heavy enough -- to fall to the ground. Much of the moisture that freezes into snow and falls in our area came from the ocean. So it's just possible that the snow you're playing in is made of some of the same water molecules that you swam in last summer at the beach -- only in a different form.

An ice crystal is the first stage of a snowflake. When water freezes, or becomes solid, it forms a hexagonal, or six-sided, pyramid. A snowflake usually has several crystals stuck together. The crystals may be shaped like six-pointed stars, six-sided columns, or skinny bullets -- depending on the temperature of the cloud.

Snow crystals start to grow when the temperature inside clouds reaches freezing -- 0 degrees Celsius or 32 degrees Fahrenheit. Measurements made inside clouds show that the crystals grow the biggest at 0 degrees Celsius. In addition, the farther a snowflake falls, the larger it gets, as it bumps into other flakes and they join together.

Different kinds of crystals create different kinds of snow cover. Needle-shaped snowflakes, for example, which form when temperatures inside clouds are just below freezing, leave a dense snow cover, too.

The fluffiest snow is made of crystals called dendrites, the delicate, star-like snow shapes that form when temperatures inside a cloud are very cold (around minus 15 degrees Celsius). When this kind of snow reaches the ground, the arms that stick out of the individual snowflakes lock together, forming a light, feathery blanket with a lot of air in it. Even five inches of this kind of snow is easy to sweep away.

Each snowflake may contain anywhere from two to several hundred snow crystals. This means there's a limitless variety of possible crystal formations that can make a snowflake. There are also many other variables, or chances for change in the life of a snowflake -- from the moment it starts off as a couple of ice crystals that stick together up in a cold cloud. The snowflake may be whipped by high winds. It may melt and refreeze. It may fall through warmer air or cooler air than it formed in. All these variables mean that it's practically impossible for two snowflakes to look just alike. No one has ever reported seeing two identical snowflakes -- and scientists have been studying the crystals for a long time.

Try This

If you want to make a study of snowflakes, get yourself a low-power hand lens with 5 to 10X magnification. You can get these at hobby shops. Then get a square piece of black velvet, and wait for a good snowstorm. Bundle up well and go out to catch flakes on your "snowflake catcher." How many shapes do you find? If you have a sketch pad along, make drawings of the flakes. You may catch similar kinds -- but it's extremely unlikely that you'll have two that look just alike. You can also use your hand lens to study the crystals in other substances such as sugar, salt or rocks.

Catherine O'Neill is a free-lance writer in Baltimore.