H. H. Clayton — Seven Day Weather Period. 11 



and the European stations Ekholm, Christiania, Lesina, and 

 St. Petersburg. The curves seem to indicate very clearly in 

 the average that twice in a period of 7 days 6 '43 hours in 

 America, and three times in Europe, waves of barometric 

 minima, or storms, tend to begin near the poles and sweep 

 across the continents. 



The day of the period in which occurred the maximum 5, 

 shown in diagram 1, was plotted on a. map for each station 

 in Europe and America. The heavy lines in diagram 2 show 

 the position of the maximum storm frequency for each suc- 



cessive day. It is seen that the first line in America passes 

 through Pt. Barrow on the extreme northern coast ; two days 

 later it passes through Cheyenne in the central United States 

 and two days later through Blue Hill on the Atlantic coast. 

 In Europe it first passes through Ekholm near the arctic circle 

 east of Greenland ; a day later it passes through Christiania 

 in southern ]N orway ; a day later, through St. Petersburg in 

 northern, and Lesina in southern Europe, and a day later over 

 Bucharest near the Black Sea. 



To determine whether these days of maximum storm fre- 

 quency remained the same at each station throughout the year 

 or changed with the seasons, the observations at each station 

 were divided into four seasons, namely : December to Febru- 

 ary, March to May, June to August, and September to 



