Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.
By: Lee Grenci , 8:38 PM GMT on December 18, 2012
georgevandenberghe broached the issue of frost in his reply to my blog yesterday (many thanks, George). Any mention of frost, as it relates to gardening, is almost always perceived in a pejorative context. Indeed, there are routinely a spate of frost advisories in spring and autumn around the beginning and end of the growing season (respectively). Portraying frost as a killer of tender plants always bothers me because frost is actually Nature's way of trying to protect them. Yes, frost gets an undeserved rap, in my opinion.
By definition, frost typically forms on clear nights with light winds and relatively low dew points as ice crystals deposit on the ground and other objects (tender plants, car wind shields, etc.). Frost can form when two-meter temperatures are as high as the mid 30's. That's because temperatures at ground level on such nights are sufficiently far below the melting point of ice (in other words, there's a nocturnal inversion extending from the ground upward).
In case my reference to the "melting point of ice" (instead of "freezing") caught you off guard, I confess that I stubbornly refuse to call 32 degrees Fahrenheit (0 degrees Celsius) "freezing." For the record, water almost always freezes at temperatures lower than 32 degrees Fahrenheit because impurities in the water introduce a "disorder" that interferes with the formation of the highly ordered lattice of ice. Only pure water freezes at 32 degrees Fahrenheit, and distilled water doesn't freely exist in Nature. Forgive me, but I regularly shake my head when I hear or read 32 degrees Fahrenheit being described as "freezing." I'm afraid it's hopelessly ingrained in our culture (more on freezing and my disdain in just a moment).
Let's get back to the issue of frost. When water vapor deposits as frost on a tender plant, latent heat of deposition acts to keep the plant warmer (a release of 680 calories per gram; see the energy staircase below). As long as deposition occurs, the temperature of the plant will not precipitously fall into the 20's. At such readings, ice crystals can form inside plants...if plant cells freeze, they can rupture and then there's real damage.
The energy staircase for water. When water vapor deposits onto objects as ice, 680 calories per gram are released (latent heat of deposition). Larger image Courtesy of A World of Weather: Fundamentals of Meteorology.
Are the owners of orchards crazy to spray their tender plants in order to avoid such damage? Facing the loss of their crop, would they willingly exacerbate matters? Orchard owners don't have to know squat about science. All they need to know is that spraying water on plants offers them some protection against dangerously low temperatures. And so it is with frost, which is Nature's way of trying to protect plants (via the release of latent heat of deposition). Killing plants is just not even in frost's purview.
Granted, frost advisories can alert gardeners that temperatures might become threateningly low (I have no problem with this kind of interpretation). Unfortunately, I suspect that most people interpret frost advisories literally...it's the frost that will kill. If gardeners perceive frost as a killer of plants (the appearance of ice crystals on the exterior of plants), please rid your brain of this notion.
Which leads me back to the issue of 32 degrees Fahrenheit being called "freezing." Honestly, meteorologists should know better. Almost all precipitation that falls over the middle latitudes begins as snow at cold, high altitudes. Here, ice crystals grow at the expense of surrounding tiny water drops. Yes, tiny water drops can resist freezing down to minus 40 degrees Fahrenheit (which equals minus 40 degrees Celsius). So I take issue with meteorologists who, on one hand, accept the Bergeron-Findeisen process (and its underlying assumption that tiny water drops resist freezing down to extraordinarily low temperatures), but who, on the other hand, describe 32 degrees Fahrenheit as "freezing." Like it or not, 32 degrees Fahrenheit is the melting point of ice. Indeed, all ice begins to melt at 32 degrees (actually, ever so slightly above 32 degrees), but water does not freeze at 32 degrees Fahrenheit unless it's pure (not present in Nature).
There, I feel a lot better.
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