Snow Science

December 27, 2017
S.H. LeeSnow science helps researchers and outdoor enthusiasts understand the ingredients of a white winter.

Want to know if an upcoming storm will dump heavy, wet snow or soft powder perfect for skiing? You’ll probably need to check a meteorologist’s forecast.

But to understand why the white stuff falls, and which atmospheric conditions create different kinds of snow, you can learn the basics yourself. Here’s a primer from Mike Carmon, a weather observer and senior meteorologist at the summit-top Mount Washington Observatory in New Hampshire.

Mount Washington, the highest peak in the northeastern United States at 6,288 feet, is notorious for its extreme weather. The summit usually sees its first measurable snowfall in September and its last in June. From his perch on high, Carmon explains how cold temperatures combine with other factors to bring the snow that makes skiing, snowshoeing, and sledding possible.

Snow is precipitation falling as ice crystals. “You need three ingredients for snow: cold temperatures, moisture, and a source of lift of the air to allow clouds to form,” Carmon says.

The process starts in the clouds some 30,000 to 40,000 feet up, where crystals form and grow larger by absorbing surrounding water droplets. “Eventually they’ll grow to the point where they get very heavy and gravity takes over,” Carmon says. If the air is cold enough, that falling precipitation will make it to the ground as snowflakes.

Air cools as it lifts, so water condenses, creating clouds. Air can be forced upward by several different factors: low-pressure systems, cold fronts, warm fronts, and the mountain effect (air pushed upward when it collides with a mountain’s slope). The mountain effect sometimes causes storms to linger at summits after they have left surrounding valleys.

Mountains also get more snowfall than the valleys since the summits are colder and higher, meaning there’s less chance of the snow melting or evaporating before it lands.

Heavy, wet snow occurs when a warm layer of air is sandwiched between two cooler layers. Ice crystals falling from the cold clouds above melt in the warm layer then refreeze in the colder air below, Carmon says. “You have more liquid content mixed in that snow.”

Nor’easters—New England storms in which the winds blow from the northeast—often create conditions in which there is cold air at ground level; warmer, moister air at midlevel; and colder air above. If a storm is closer to the coast, where ocean waters warm the air, then heavier, wetter snow and perhaps even some rain will fall. “If you’re out hiking, trudging through heavier, wetter snow can be more difficult,” Carmon says. “It can affect whether you should wear snowshoes, whether you should pack crampons.”

Fortunately, inland areas tend to have drier snow, which is good news for outdoor enthusiasts. Plus, as you go higher up in elevation, snow tends to be more powdery because the air temperatures tend to be colder, Carmon says.

When forecasting the weather for the region’s summits, Mount Washington Observatory staff always emphasize that conditions change quickly, and backcountry visitors should be prepared for extremes. As an outdoor enthusiast himself, Carmon understands that people get used to venturing out in the cold and snow. But he says: “You should never get comfortable with it. You shouldn’t let your guard down.”


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Heather Stephenson

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