40 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 82 



waves. The beats are even more complicated when there are three or 

 more periods of nearly the same length. In such a case there may be 

 an apparent change of phase in one of the periods. 



Real changes in amplitude are brought about by the influence of 

 longer periods on shorter periods. An example of this is the influ- 

 ence of the annual period on shorter weather cycles. All weather 

 changes are most intense in winter, because then the contrasts in 

 temperature between Equator and pole, between ocean and continent, 

 and between adjacent bodies of land and water are at a maximum 

 intensity and the general atmospheric circulation is increased. 



Also all periodic changes in the atmosphere are more intense when 

 solar activity increases. The reason for this increased intensity will 

 be clear, first from the fact shown in the early part of this paper 

 that increased contrasts of temperature and pressure in the atmos- 

 phere result from increased solar activity, and second from the fact 

 that the amplitude of the solar cycles increases with increased solar 

 activity. 



An example of the increased amplitude of solar periods with 

 increased solar activity is shown in figure 22 where the amplitudes of 

 the 7.5-year sun-spot period is distinctly greater during the inter- 

 val 1865 to 1875, when the general level of solar activity was higher, 

 than during the interval 1885 to 1895, when it was lower. The 

 increase of amplitude during the first of these intervals and decrease 

 during the second was also evident in the sun-spot cycle and in its 

 harmonics of 5.65, 3.75, 2.82 years, etc. 



An example of increased amplitude of meteorological cycles with 

 increased solar activity is shown in figure 28 where a period of 7^ 

 months in pressure at Chicago shows a marked increase in ampli- 

 tude at the time of maximum of sun spots in 191 7 and a diminished 

 amplitude during the intervals of minima of sun spots in 191 3 and 

 1923-1924. The data for this curve are the means of 10 overlapping 

 periods of y^ months obtained in the manner indicated in table 6. 

 The dotted curves are sine values computed for each individual 

 period. 



That meteorological cycles change in phase as well as in intensity 

 is also evident. These changes of phase appear to arise from several 

 different causes. First, the solar periods themselves change phase. In 

 most cases this change occurs suddenly and appears to be about 180° 



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