Chill factor

Science News,  Feb 23, 2008  by Gene Barth,  Susan Gaidos,  Dietrich Marcuse

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"Supercool, and Strange" (SN: 1/26/08, p. 58) keeps the reader on track with accurate, entertaining metaphors. It ends with a riveting observation from the White Mountains of New Hampshire: The tree line occurs where windchill temperatures reach 220 kelvins, the temperature at which supercooled water "undergoes a phase transition." Windchill temperatures are not physical temperatures--neither the trees nor the air is at a temperature of 220 kelvins. Windchill temperatures answer the question, "At what lower temperatures would air have to be to achieve the same rates of cooling observed at the actual, higher temperatures in the absence of winds?"

GENE BARTH, CHICAGO, ILL.

Windchill does not equate to actual temperature, so the phrasing in the story was misleading. However, high winds also play a role in limiting tree growth. On Mount Washington, winds can exceed 50 miles per hour and temperatures can plunge as low as -47[degrees]F. Also, as moist air is forced up the mountain, supercooled water droplets form around 4,500 feet. When those droplets contact a tree, they freeze into a deadly coat of ice.

Your article "Supercool, and Strange" reminded me of the excitement about "polywater" in the late 1960s. Just as in the new report, polywater was produced by studying water confined in capillary tubes. Could we be seeing the same phenomenon all over again?

DIETRICH MARCUSE, LAKEWOOD, N.J.

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