The Weather Notebook: Freezing Water

Freezing Water
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"I wanted to know why when water freezes it expands instead of contracts like everything else in the universe?"

Derek Noonis, seventh grader from Rye, New Hampshire.

This is Bryan Yeaton for The Weather Notebook. Interesting thing about water. Most substances expand when warmed, but contract when cooled. Water is the same-- at least to a point. That point is 39 degrees F (39.16, actually). Below that magical temperature, water actually expands as it cools. Why? When it freezes it forms these molecular "bridges," which actually take up more space than the bonds in the liquid state. One interesting upshot of this: since water is densest at 39 degrees, this dense water will sink, leaving a layer of warmer water behind until that cools to 39, and so on. In order for ice to form on the surface, the entire body of water (fresh water, of course) needs to be 39 before the surface can cool further. So, if there is a layer of ice on the pond's surface, water at the bottom is 39 degrees.

More detail on this and other cool physics can be found in Robert L. Wolke's book "What Einstein Didn't Know," from Birch Lane Press.

Do you have a question about a weather issue? We love questions! Please write to us at questions@weathernotebook.org, or call our toll-free number: 888-RAIN-001, that's 888-RAIN-001. If you didn't have pencil and paper handy for those numbers, just go to our website at www.weathernotebook.org.

Thanks today to Derek Noonis at Rye Jr. High School, and his teacher, Sheila Adams. The Weather Notebook to brought to you from the Mount Washington Observatory, with support from Subaru, and the National Science Foundation.