. Annual report of the New York State College of Agriculture at Cornell University and the Agricultural Experiment Station. New York State College of Agriculture; Cornell University. Agricultural Experiment Station; Agriculture -- New York (State). Frosts in New York 521 during the night. For example, a temperature of 40° at about 6 p. m., with a clear sky and light wind, is considered critical; particularly is this the case if the rate of fall approximates 1° for each two hours, which, if continuous — as would be likely with a clear sky and light wind — would bring the temperature close to th
. Annual report of the New York State College of Agriculture at Cornell University and the Agricultural Experiment Station. New York State College of Agriculture; Cornell University. Agricultural Experiment Station; Agriculture -- New York (State). Frosts in New York 521 during the night. For example, a temperature of 40° at about 6 p. m., with a clear sky and light wind, is considered critical; particularly is this the case if the rate of fall approximates 1° for each two hours, which, if continuous — as would be likely with a clear sky and light wind — would bring the temperature close to the freezing point by early morning. A fall in temperature of 2° per hour would indicate frost, even with the temperature considerably above 40° in the late afternoon. Atmospheric pressure The rate of change in the pressure of the atmosphere as indicated by the barometer is of some assistance in forecasting frost. The actual stage of the pressure, whether above or below normal, is not important, except that a warm period with unusually low pressure is likely to be followed by the opposite extreme. The important factor is the rate of change. If the pressure is increasing rapidly, as indicated by a rapid rise in the baipmeter, it is a good indication that the cold period or cold wave is approaching rapidly. The change in pressure usually precedes by a short interval the change in direction of the wind, because the wind depends on the pressure; but one will have to watch the barometer very closely to gain much advantage over the indications afforded by the direction and force of the wind. THE EVENING DEW-POINT AS A GUIDE TO THE MINIMUM TEMPERATURE OF THE FOLLOWING NIGHT Recently there has been considerable discussion as to the value of the evening dew-point as indicating the probable fall in temperatiire during tmrn^L^^^^^^i^^^^. Fig. 141.—-Psychrometer for obtaining the dew point and the relative humidity {percentage of moisture in the atmosphere). The bulb of one thermom
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