J. P. Hall— Short Cycle in Weather. 227 



Aet. XXVIII. — A Short Cycle in Weather ; by 

 James P. Hall. 



If a diagram is drawn exhibiting the changes of daily mean 

 temperature in New York City for a few months it will be 

 discovered that these fluctnations occur every three or four 

 days, on an average, but that some have much greater ampli- 

 tude than others. In the course of four weeks, perhaps, there 

 will be only two or three conspicuous rises and falls. Upon 

 further scrutiny, there will be observed a tendency in these 

 more prominent features of the curve to repeat themselves at 

 intervals of about 27 days. Diagrams marked " Series I," 

 " Series II " and " Series III " are submitted herewith to illus- 

 trate that tendency. The left hand vertical marks the true 

 beginning of each cycle ; and the right hand one, which is 

 placed at a distance of exactly 27 days, the end ; although for 

 greater distinctness the curves are extended three days in each 

 direction beyond the strict limits. In each of the three series, 

 which cover different years, the point at which the second 

 curve is intersected by the left hand vertical is exactly 27 days 

 later than the corresponding point in the curve preceding, and 

 so on through that entire set. 



Considering only the general features, and not the minor 

 details of these traces, one finds a sort of repetition of both 

 warm and cold waves in nearly the whole succession. Thus, 

 in Series I (beginning with Aug. 18, 1889, and covering eight 

 cycles in immediate succession) there is a marked rise, A, 

 which in the first trace culminates on Aug. 21, and in the 

 second on Sept. 17. Recurrences, more or less distinct, appear 

 on October 12, November 9, December 9, January 3, January 

 30, February 26 and March 26. In the third, fourth, fifth, 

 sixth, seventh and eighth periods another warm wave, C, 

 closely follows A at an interval of from four to eight days, 

 the dates of climax being October 20, November 14, Decem- 

 ber 13, January 6, February 5, a,nd March 5. After C has 

 become well developed, the cold wave E that followed A in 

 the first three cycles becomes inconspicuous for a time. A and 

 C may be regarded either separately or combined ; but one or 

 the other, or both, recur with tolerable regularity. In the 

 first curve of this first series, we find also a rather sustained 

 warm spell, designated " B," having maxima on Aug. 31 and 

 Sept. 6 (central point, Sept. 3). The rises of Sept. 26 and of 

 Sept. 30-Oct. 1 (central point Sept. 28) imitate it, although 

 the warmth is not well sustained. Recurrences appear on 

 Oct. 27, Nov. 19-23, Dec. 19, Jan. 13-16, Feb. 12-14 and 



