Making Ice
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Although your freezer may collect ice at the drop of a hat, it's not so easy to make ice in the atmosphere.

Hi, I'm Bryan Yeaton and this is the Weather Notebook.

In order for an ice crystal to grow, you need a nucleus--something on which the crystalline structure of ice can get a toehold. The inside of your freezer works just fine, unless you've got a frost-free unit.

In the atmosphere, you need some tiny airborne particle to get a snowflake started. Once a few ice crystals are blowing around, they often bump into unfrozen water droplets, and the snow machine is underway Nature often uses tiny bits of windblown dust to make ice, but not any particle will do. If you're sitting a little below zero Fahrenheit, then only about one in a million dust particles has the right chemical structure for making ice. The minerals in clay seem to be especially good at this. Silver iodide works great, too. Scientists found in the 1940s that they could sprinkle silver iodide into a cloud and speed up the production of ice crystals. But there's another natural substance that's still more effective as an ice nucleus. Believe it or not, it's cholesterol. Ice crystals start to form on a cholesterol molecule as soon as the temperature drops below 28 degrees Fahrenheit. The same is true for another natural substance, testosterone...although we don't know anyone who's eager to test this out in the real world.

Meteorologist and writer Bob Henson contributed to this story. The Weather Notebook is supported by Subaru and the National Science Foundation. It is produced by the Mount Washington Observatory. For more on ice, slide to our website at mountwashington.org. Thanks today to producer Margaret Landsman.