Arctowshl — Pleionian Cycle of Climatic Fluctuations. 27 



Art. IV. — The Pleionian Cycle of Climatic Fluctuations',* 

 by Henkyk Arctowski. 



As we observe changes of weather from one day to another, 

 so we observe climatic fluctuations from one season to another, 

 from one year to the following year. Persistency of given 

 weather conditions may frequently be observed. In the case 

 of climatic fluctuations, also, there may be a series of years 

 abnormally dry or abnormally rainy, or we may have groups 

 of years offering some other particularities such as a late spring 

 for example, or an unusually warm winter, and such excep- 

 tional conditions, recurring for a succession of years, give the 

 impression of a radical change of climate. 



In reality, therefore, we may consider the study of these 

 changes or fluctuations just as important and as having a far 

 more practical value than the study of the so-called normal cli- 

 matic conditions. 



Considering ten-yearly means of atmospheric temperature as 

 representing quasi-normal values, I inscribed the annual depar- 

 tures from these means on maps. For each year so far taken 

 into consideration the departures are never positive all over 

 the world, or negative; in each case some regions are charac- 

 terized by an excess of heat, whereas in other regions tempera- 

 ture is in deficiency. The areas of positive departures have 

 been called thermo-pleions and those of negative departures 

 anti-pleions. The anti-pleions do not necessarily compensate 

 the thermo-pleions. The year 1900, for example, was a year of 

 an excess of pleions and the year 1893 was a year of deficiency 

 of pleions. The difference of the world's temperature, for such 

 exceptional pleionian and anti-pleionian years, may reach 0'5 o 

 C or perhaps even more. 



Taking barometric measurements into consideration, one also 

 finds that for each year some centers of abnormally high and 

 abnormally low atmospheric pressure are conspicuous. f These 

 baro-pleions and anti-baros displace themselves from year to 

 year, and evidently influence atmospheric circulation very 

 greatly. 



These changes must have an effect on the distribution of 

 storm frequency:): and on rainfall. Of rainfall data, I have 

 studied extensively the ombro-pleions observed in Europe dur- 

 ing the years 1851-1905, but the results of these researches 

 have not been published. 



* An address before Section II & of the Second Pan-American Scientific 

 Congress, on December 29th, 1915. 



t Bull. Amer. Geogr. Soc, v. xlii, p. 270, 1910. 

 t Month. Weath. Eev., v. xliii, p. 379, 1915. 



