When no snow falls at ski resorts, they make some

As weather patterns change and snowstorms and cold temperatures become less predictable, winter resorts depend more and more on making snow, rather than waiting for it to fall from the sky. Skiers and snowboarders may not realize that much of the snow under their boots comes not from the clouds, but from carefully monitored snow guns.

Michael Holden is an expert in snowmaking. He is a ski instructor as well as a rocket scientist. He has studied the science of snowmaking and advises resorts on how to do it best.

Artificial snow is made by forcing a fine mist of water through a nozzle at very high pressure, Dr. Holden explains. Water molecules freeze when they meet the cold air and create what we think of as snow.

Although it's called "snow," the result is actually sleet. Snow is frozen water vapor; sleet is frozen water. This sleet is so fine and dry, though, that most people cannot tell the difference. Sometimes people ask, "Where is the artificial snow?" not realizing that they are standing on it.

The snowmaking process begins when workers add a fungus solution to enormous tanks of water before the water is sent to the snow guns.

The additive is a carefully processed solution made from rotting leaves and water. Tests have shown that the solution is perfectly safe. However, It does look disgusting in the water tank – thick and gloppy.

The additive provides a nucleus, a center, to which water molecules attach. This mimics what occurs in nature where dust in the air is necessary for a snow crystal to form.

This mixture of water and fungus travels through pipes and then through hoses to what resembles a nozzle at the end of a garden hose. Here, a second pipe provides compressed air that forces the mixture out into the cold air and down onto the slope.

Dr. Holden recommends that snow guns be put up on poles about 12 feet above the ground. This allows for maximum "hang time," the time the mixture hangs in the cold air before hitting the ground.

When the molecules leave the snow guns, they're "ice balls with liquid centers," he says. If they don't have enough hang time, they will shatter when they hit the ground, making slush instead of snow.

Although we commonly think that water freezes at 32 degrees F., the science isn't that simple. Both the purity of the water, as well as the humidity of the air, will alter the exact freezing temperature.

Pure water freezes at 32 degrees F. In snowmaking, the leafy additive means that the water isn't pure. This causes what is called "heterogeneous nucleation." It can occur at a degree or two higher than pure water freezes.

If the air is dry (low humidity), freezing can also occur if the temperature is a bit higher.

These conditions allow ski resorts to make snow at 34 and even 35 degrees F. The best snow is made right before dawn when the air is driest and probably coldest, says Dr. Holden.

Winter resorts start making snow early in the season. They churn out as much as possible so they will be ready for skiers and snowboarders.

"Quantity is more important than quality at that point," says Tim Woods, general manager at Windham Mountain, a resort in New York's Catskill Mountains. "The snow is usually moist and heavy because the air is warmer, but that's fine. It makes a solid base that will be durable through the season."

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