Saturday, February 6, 2010

The Science of Snow

Two years ago, I had the pleasure of interviewing Dr. Joseph M. Moran, of the American Meteorological Society, for an article about snow that appeared in Washington Parent magazine. Given that snowy weather has taken over much of the United States, I thought you might enjoy this reprint of our conversation.


The Science of Snow: Ask the Expert!

Dr. Joseph M. Moran, of the American Meteorological Society, answers your questions about snow:

1. Is it true that no two snowflakes are alike?

Dr. Moran says, "‘No two snowflakes are alike' is a widely held assumption. Although all snowflakes are composed of ice crystals having six-sided ... symmetry, they may occur in billions of different forms." Even so, a researcher discovered two identical snowflakes while sampling clouds over Wisconsin in 1988.

2. Are there different kinds of snow?

"Yes," says Dr. Moran. He explains, "Snow is an agglomeration of ice crystals in the form of flakes that develop in clouds and fall to the Earth's surface. Snowflakes vary in shape and size depending on air temperature and humidity. With decreasing cloud temperature, snowflake crystals occur as needles, dendrites, plates, and columns."

A dendrite snowflake below; a plate snowflake on the right. Both of these images were taken by Wilson "Snowflake" Bentley. For more information, has nice descriptions of different types of snowflakes.

"Snowflake size depends in part on how humid (moist) the air is. At very low temperatures, the humidity is relatively low, and snowflakes tend to be small. Snowflake size also depends on the efficiency with which they collide with one another as they fall through the atmosphere to the ground. At air temperatures near freezing, snowflakes more readily stick together after colliding, and their diameters sometimes reach 2 to 4 inches (5 to 10 centimeters). Snow pellets and snow grains are similar to snowflakes but much smaller. Snow pellets are soft conical or spherical white particles of ice with diameters of 2 to 5 millimeters. Snow grains are flat or elongated opaque white particles of ice, usually less than 1 millimeter in diameter."

3. How do meteorologists measure snow? My measurements at home never seem to match what the weathercaster says.

Meteorologists measure three things, says Dr. Moran: "The depth of snow that falls between successive observations, the meltwater equivalent of that snowfall, and the depth of snow on the ground at observation time."

Dr. Moran tells us how to measure a storm's snowfall like the experts. "Prior to an anticipated snowfall, place a simple wooden board on the ground ... New snowfall accumulates on the board, and at observation time a ruler is used to measure the snow depth to the board. Record the snowfall ... and sweep the snowboard clean so that it is ready to receive new snowfall. Repeat this process throughout the snow event, and then compare your total snowfall with that reported by the local television or broadcast meteorologist. Note that snowfall is notoriously variable from one place to another ..." depending upon things like temperature, the distance you are from the ocean, and the track of the storm.

Photo credits: Wilson Bentley, from NOAA's National Weather Service Collection.

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