The Science of Snowmaking: A Lesson on the Slopes

Skiway director Douglas Holler says technology is improving as the climate changes.  

As students return for winter term, many are toting skis and boots, ready to make the most of the Skiway—one of a handful of College-owned ski areas in the nation, and a training ground for generations of Olympians. For the past few weeks, Skiway director Douglas Holler and his crew have been making huge batches of snow and moving it around to sculpt the terrain. Holler says snowmaking has been changing, along with the climate: Snow guns are more efficient than they used to be, and Dartmouth has added 27 new ones, bringing the number to 300. 

On most days, at the crack of dawn, you can see Holler barreling up and down Skiway trails in a snowcat, making sure that the slopes are well groomed. On a recent frigid morning, he took visitors along, and explained the snow-making process. The snow cat occasionally skidded sideways or slid backwards as he maneuvered it through drifts and ice patches up a steep slope where races are held.

Where does the snowmaking process start?   

It starts in our spring-fed pond, which has three pumps that send water to a pumphouse. From there it gets pumped up the hill at 700 pounds per square inch through a series of pipes—about 9 miles of pipe. At the base, near the snowmaking building, there are also four air compressors, and that air gets pumped up the hill, too.

So you have two sets of pipes, one for water and one for air. At what point do the water and air mix?

We’re looking right now at a sled-mounted gun, which is mobile. Air and water each go through the gun, and they mix at the head. First, in a small nozzle, the air combines with a very small amount of water to create condensation nuclei—particles of ice that act as agents to promote freezing. You can take water to well below zero and it still won’t freeze, but when compressed air exits the gun, it expands, and you get super-cooling. When those little ice particles meet the bulk water that comes from larger nozzles, you get snow—not exactly the way it falls from the sky, though. What comes out of snow guns is more spherical and more durable than fresh, natural flakes.

OK, so this stuff gets sprayed, making piles all over the mountain. When can you start grooming, moving the snow where it needs to be, and creating varied terrains?  

Fifty percent of snowmaking actually takes place after the particle hits the ground. If possible, you want to let it sit there for 24 hours before you do any grooming, so it has time to “cure.” If you push it around right away, it clumps, and you end up with what skiers call “death cookies”—tough to ski on.

Why are snow guns a lot taller than they used to be?

Our tall guns are permanently mounted and they drop snow in a smaller area, so you do have to move it more, but they have advantages, in terms of efficient use of resources. Once you get 18 feet off the ground, you increase snowmaking by 50 percent because of what we call “hang time” in the air—time when the snow can form before it hits the ground. The old ground guns used between 300 and 600 cubic feet of air per minute. The new guns use 5 to 10 cubic feet of air per minute. Air is the most expensive part of snow-making. We have to rent the compressors, and fuel them—each uses between 18 and 20 gallons of diesel per hour—and we have four of them, working at least 400 hours per season.

There’s an iPad screen between the driver’s and passenger’s seats, displaying changing wave forms as you drive. What’s it telling you?

That is our new snow depth indicator, which gives readings in real time. It tells me we are riding on 18 inches of snow, underneath this cat. Measuring depth helps us move snow around to create terrain that shapes or maintains natural contours of the trails. Good skiers don’t want the surface to be a like a billiard table.

You’ve been making snow for almost four decades. How has the technology changed over that time?  

We have not always had the technology to make proper-sized particles through the nozzles. That’s fairly recent. You still need the air-water mix initially to get condensation nuclei, but you can now control bulk water particles through the nozzles. You can choose the size spray pattern, and you can change out the nozzles. And you can add nozzles.

Would you say that there is an art as well as a science to making snow?

There is still a lot of human judgment involved. And as the temperature drops or rises, we still send guys out. They stand under the guns, look at their sleeves and say, yeah, it could be wetter. Or drier. We control that air and water mix from the snowmaking shed, where computers help us program the pumps, depending on temperatures and humidity. Once you get a firm base, you can make a more powdery snow. But our racers actually want ice. We groom those hills at closing time so it stiffens up overnight.

Is climate change affecting the science and art of snowmaking?

Well, I’m not a scientist, but my anecdotal sense is that despite cold snaps here and there, winters do seem to be getting warmer, and that gives us fewer windows for snowmaking. We are investing in more snow guns—27 new ones this year—so we can put them closer together and start making snow sooner in the season, and let people ski on it sooner after we make it.