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."

It may seem odd, but when resorts pump out snow before the ground freezes, it acts like a blanket, and the ground will not freeze all winter, no matter how cold it gets.

Mr. Woods pays a great deal of attention to snowmaking. The process requires huge amounts of water and electricity, and he doesn't want to waste either resource. He watches weather reports to see if the conditions will be right for snowmaking. He keeps track of every gallon of water the resort uses and files monthly reports with the state.

Like many resorts, Windham is located within an environmentally sensitive area. To save water, the snowmaking team collects the water from melted snow into ponds to reuse. To save electricity, the heat generated by the air compressors is used to heat the lodge.

Making snow is only half the job. Winter resorts also "groom" snow. At night, when guests are finishing their dinners or climbing into bed, the snow groomers begin getting the mountain ready for the next day.

Using what are called "snow cats" – large tractors made just for this job – they move piles of snow, break up ice that may have formed on the surface, and even out tracks left by skis.

Snow cats are big – 16 feet wide and more than 20 feet long. Instead of a steering wheel, many have joysticks like those used with video games.

The machines travel on belts driven by five wheels on each side. Each belt is about three feet wide and is fitted with cleats that resemble the blade of an ice skate. These cleats grip the snow as the 12-ton tractor travels up and down the slopes.

To keep from slipping on really steep slopes, the snow cat uses a winch, a cable attached to a large tree or another snow cat that stays at the top of the slope.

Snow cats look like giant, fat scorpions as they come out of the garage at night. Guided by bright headlights that resemble big, round eyes, they have metal grates that look like pincers moving in and out, collecting snow and depositing it in areas scraped down to the snow base.

The driver uses the front part of the snow cat to level and sweep the snow or to lift a large pile of snow.

As the snow cat travels over the surface, it packs the snow. In the back, there's a rake 16 feet wide with teeth three inches long that drags across the surface cutting a pattern that is referred to as "corduroy."

Nelson Armlin is the person in charge of the snowmaking and grooming crews at Windham. Since he was a boy, he has loved big machines and has known that he wanted a job where he could drive them.

He enjoys heading out at night in a snow cat to groom the slopes. He appreciates the stillness of the mountain at night, very different from the activity during the day.

Settling into the driver's seat, Mr. Armlin starts the engine, switches on the bright lights, and heads up a hill. As he guides it around a curve, the headlights shine on a fox at the edge of the forest, silently watching him work.

Mr. Armlin sees the fox many nights when he works, but it is always gone before the skiers arrive in the morning. By then, the groomers have gotten the snow ready for another day of skiing.