Friday, November 9, 2007

Winter Questions

Nature Watch
November 9, 2007

By Susan Benson
Director of Education
Cable Natural History Museum

Well, we’ve got our first taste of it and more is around the corner: snow. Love it or hate it, snow is part of our lives here in the north, so let’s take a moment to think about and appreciate some of its unique properties.

Why is snow white?

Most natural materials absorb some wavelengths of sunlight and reflect others, which in turn gives these materials their color. Snow reflects most of the sunlight—and visible sunlight is white. The complex structure of snow crystals results in countless tiny surfaces from which visible light is reflected.

How big can snowflakes get?

Snowflakes are aggregations of many snow crystals. Most snowflakes are less than one-half inch across. Under certain conditions—usually involving near-freezing temperatures, light winds, and unstable atmospheric conditions—much larger and irregular flakes close to two inches across can form. No routine measure of snowflake dimensions are taken, so the exact answer is not known.

Is it ever too cold to snow?

It can snow even at incredibly cold temperatures as long as there is some source of moisture and some way to lift or cool the air. It is true, however, that most heavy snowfalls occur with relatively warm air temperatures near the ground—typically 15°F or warmer. Warmer air can hold more water vapor, and that vapor is what makes snow.

When is it too warm to snow?

Snow forms when the atmospheric temperature is at or below freezing and there is little moisture in the air. If the ground temperature is at or below freezing, of course the snow will reach the ground. However, snow can still reach the ground when the ground temperature is above freezing if the conditions are just right. In this case, snowflakes will begin to melt as they reach this warmer temperature layer; the melting creates evaporative cooling which cools the air immediately around the snow flake. This cooling retards melting. As a general rule, though, snow will not form if the groud temperature is 41°F or warmer.

Is it true that there is one inch of water in every ten inches of snow that falls?

The water content of snow is quite variable. While many snows that fall at temperatures close to 32°F and snows accompanied by strong winds do contain approximately one inch of water per 10 inches of snowfall, that ratio is not always accurate. Ten inches of fresh snow can contain as little as one-tenth of an inch of water or as much as four inches of water, depending on the snow’s crystal structure, the wind speed, temperature, and other factors.

Why is snow a good insulator?

Fresh, undisturbed snow is composed of a high percentage of air trapped among the lattice structure of the accumulated snow crystals. Since the air can barely move, heat transfer is greatly reduced. Fresh, uncompacted snow typically is 90-95 percent trapped air.

Why do weather forecasters have so much trouble forecasting snow?

Snow forecasting remains one of the more difficult challenges for meteorologists. One reason is that for many of the more intense snows, the heaviest snow amounts fall in surprisingly narrow bands that are on a smaller scale than observing networks and forecast zones. Also, extremely small temperature differences that define the boundary line between rain and snow make huge differences in snow forecasts.

Why does snow crunch when you step on it?

A layer of snow is composed of ice grains with air in between the ice grains. Because the snow layer is mostly empty air space, when you step on a layer of snow you compress that layer—a little or a lot, depending on how old the snow is. As the snow compresses, the ice grains rub against each other. This creates friction or resistance; the colder the temperature, the greater the friction between the grains of ice. The sudden squashing of the snow at lower temperatures produces the familiar creaking or crunching sound. At warmer temperatures—closer to melting—this friction is reduced to the point where the sliding of the grains against each other produces little or no noise. It’s difficult to say at what temperature the snow starts to crunch, but the colder the snow, the louder the crunch.

Nature Watch is brought to you by the Cable Natural History Museum. For 40 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility in Cable at 43570 Kavanaugh Street or on the web at to learn more about exhibits and programs.

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