First-grader: How does snow fall from the sky?
William Hernandez, 6, a first-grader at MacArthur Elementary in Hoffman Estates, asked, "How does snow fall from the sky?"
Two things make snow - water and temperature. There has to be water or moisture in the air to create snow, and the temperature has to be just cold enough so ice crystals form that become snowflakes.
"In order for snow to form, a significant layer of the atmosphere needs to be below freezing," said Blake Hamilton, a senior meteorology student at Iowa State University.
If the temperature is cold enough, clouds containing moisture become snow factories. Moisture droplets become ice crystals as the temperature drops. These ice crystals collide with molecules of moisture and grow bigger. As they become heavier, they fall out of the cloud and down to the Earth.
Hamilton said snow can form high up in the atmosphere, but if the temperatures closer to the Earth's surface are much warmer, the snow that formed up in the clouds could melt on the way down.
Sometimes the opposite happens. Snow formed in clouds could melt, creating liquid precipitation, or rain. "When it gets closer to the ground, the temperature goes back below freezing," Hamilton said. "The melted snow, now in the form of freezing rain, doesn't have time to cool down and become snow again. So the precipitation falls in the form of rain. It freezes as it hits the ground, sticking to roads, trees or power lines.
Forecasting snow means identifying the temperatures at different levels of the atmosphere. "Meteorologists look at temperatures and the thickness of the levels of the atmosphere. Thickness is a measure of distance between two different levels of the atmosphere."
At what temperature should we expect to see snow? "You may have noticed that snow sometimes falls when it is above 32 degrees outside. This is because the atmosphere above this location is still below 32, and the snowflakes haven't had the time to melt yet," Hamilton said. "However, snow rarely falls when a temperature of 40 degrees or greater is reached at or near the surface."
It can be too cold to snow. Super cold temperatures in the atmosphere means the air can't hold on to moisture. Without water, snow can't form.
Check these out
The Schaumburg Township Public Library suggests these titles on snow:
•"Snow Amazing: Cool Facts and Warm Tales," by Jane Drake and Ann Love
• "Snowflakes," by Joan Sugarman
• "Snowflake Bentley," by Jacqueline Briggs Martin
• "It's Snowing! It's Snowing!," by Jack Prelutsky
• "The Long Winter," by Laura Ingalls Wilder
On the Web:
Weather Wiz Kids www.weatherwizkids.com
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