Welcome Jack Frost

Frost coating blades of green grass
Frost coating blades of green grass after sub-zero night time temperatures, close up background view Photo courtesy of: http://www.freeimageslive.co.uk/

This morning I woke up to frost on my bird feeders and deck railing, the first of the season. Poetically, frost is called “rime,” as in Coleridge’s poem “The Rime of the Ancient Mariner” (also a neat play on the word rhyme). But what is frost? Why does it show up almost exclusively in fall and winter?

Colorado’s Front Range has a semi-arid climate. It’s not a desert, but it only gets 14 inches of rain a year. In the summer, we talk about a “dry heat” and “low humidity.” This is in contrast to where I grew up in Virginia, where the average amount of rain in a year is about 3 times that and there’s a picture of the State in the dictionary under the word “muggy.”

Despite Colorado being so dry, there’s still some “humidity” (a measure of the amount of water vapor in the air) here. The higher the humidity, the more water. When it’s warm, water molecules are very energetic. You can see this when you boil water. Eventually, the water becomes so energized it escapes the bonds that hold one water molecule to the next and changes into steam. When air cools down, water molecules slow down and the steam turns back to liquid. This is called condensation.

Water boiling in a pan. The bubbles form as the molecules in the water speed up and eventually escape to form steam. Photo by Jamie Simo.

At night, when the temperature falls, the air can’t hold as much moisture as during the day because some of the water molecules in the air have slowed down enough to change back into liquid. That water then collects on surfaces. Have you ever noticed the grass is wet in the morning, but it hasn’t rained? This is called “dew.”

Frost is actually frozen dew. Remember that when you add heat, water molecules speed up? When you cool water down enough, the molecules that form it actually stop moving altogether and form a solid: ice.

So, while you’re scraping that frost off your car’s windshield this morning or some morning a month or two from now, pause to marvel at how amazing it is that it came to be there while you’re waiting for the defroster to really kick in.

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