NO. 7 THE ATMOSPHERE AND THE SUN CLAYTON 23 



in the polar basin and spread southward toward the Equator. Fig- 

 ure 15 is derived from a plot made by Dr. Weickmann. The plot is 

 made to show the wave at successive dates along the meridian of 

 45° E. longitude. The dotted curve No. i in figure 15 shows that on 

 December 10 there was a minimum of pressure in the arctic basin 

 north of Spitzbergen and a high pressure over Central Asia about 

 60° N. Six days later, on December 16, as shown by the broken 

 curve No. 2, the low pressure was about 70° N. and a high pres- 

 sure about 45° N. Twelve days later on December 22, as shown 

 by the continuous curve No. 3, the period was in opposite phase and 



10 

 -J 1 45''E. 



DEC. 10 DEC. 16 DEC.22,I9Z5. 



Fig. 15. — Pressure departures in 24-day period (data derived from diagram 

 by Weickmann). 



the low pressure is found at 60° N. with a high pressure south 

 of 30° latitude and also' in the polar basin. Eighteen days later, on 

 December 28, as shown by the dotted curve. No. 4, the low pres- 

 sure is at 45° and a high pressure is advancing southward. 



The plot brings out clearly the decrease in amplitude of the oscil- 

 lations with decreasing latitude. Owing to the decrease of ampli- 

 tude with latitude the velocity of progress of the wave is best 

 obtained from the points where the curve crosses the zero line. 

 The first zero point is at 72° latitude and the second about 30° 

 latitude. This is the distance traversed by the wave in 12 days, a 

 rate which would carry it from pole to Equator in about one period 

 of 24 days. 



In Mr. Clough's ' study of a period of about 2^ years in pres- 

 sure he says : " The epochs of the short ])eriod for St. Paul, St. Louis, 



'Monthly Weather Review. Vol. 52, No. q, p. 436, Sept., 1924. 



