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 



t mrn^L^^^^^^i^^^^ 



Fig. 141. — -Psychrometer for obtaining the dew point and the relative 

 humidity {percentage of moisture in the atmosphere). The bulb of 

 one thermometer {wet bidh) is covered with thin cloth, which is satu- 

 rated with water when observation is made. The instrument is then 

 whirled for a minute or two and a reading of both thermometers 

 made. The greater the difference in the readings, the drier the air 

 and the lower the dew point. The relative humidity and dew point are 

 found by reference to tables that accompany the instrument. 



Price, $4 to $6ci 



the ensuing night, and in order to gain a clear understanding of this question 

 it seems necessary to review briefly the subject of atmospheric moisture 

 and the principles involved. 

 24 



