Abstract
DR. J. SATTERLY, of the Physics Department, University of Toronto, writes to ask whether it is a fact that in England x degrees of frost means a temperature x degrees below 30° F., not x degrees below 32° F., the reason being that English meteorologists consider that the “freezing of plants” or the killing of tender plants by frost does not begin at 32° F. but two degrees lower. The answer to this question is, of course, that both for the public and professional meteorologist x degrees of frost ordinarily means a temperature x degrees below 32° F., but the method of reckoning ground frosts adopted by the Meteorological Office is to account as an occasion of ground frost every night on which a thermometer freely exposed to the sky, with its bulb resting on the top of short grass, indicates a temperature of 30° F. or lower. Such a thermometer is peculiarly well placed for recording low temperatures; the underlying turf protects it from heat conducted from the soil; it experiences very little wind, the action of wind being to prevent it from cooling much below the temperature of the surrounding air. The result is that a lower temperature is generally indicated than is reached by plants. This alone would make it improbable that vegetation would suffer frost damage every time the exposed thermometer fell slightly below 32° F., but in addition there is the fact that the freezing point of sap would normally be below 32° F. This official practice is a very old one, and it is difficult to know whether both these considerations were borne in mind by those responsible for it. It is a matter of common observation that there are many occasions when readings substantially lower even than 30° F. are obtained without vegetation suffering; much depends, no doubt, upon the length of time during which the temperature has been below 30° F., and whether any plants exceptionally liable to frost damage are present and also bearing sensitive new growth such as might appear during a sudden spell of unseasonable warmth and moisture.
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Degrees of Frost. Nature 137, 180 (1936). https://doi.org/10.1038/137180a0
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DOI: https://doi.org/10.1038/137180a0