CLIMATE 321 



of residence, and compare the climate of the place where they once 

 lived, perhaps New York, with that of the place where they now 

 live, perhaps Iowa. 



Variations in rainfall, temperature, etc., do occur in short periods. 

 Thus there seems to be a faintly marked weather cycle of about 

 eleven years, corresponding to the sun-spot cycle. A longer cycle 

 of about thirty-five years is indicated for Europe, where records 

 have been kept longer than in our own country. Within this 

 cycle there may be said to be two focal periods of a few years each, 

 one when the rainfall is above the average, and the temperature 

 below, and the other when the rainfall is below the average and 

 the temperature above. The reason for this cycle is not yet known. 



Variations of this sort affect the movements of glaciers. This 

 has been observed especially in connection with the glaciers of the 

 Alps. They advance after (commonly some years after) periods 

 of years of heavy precipitation and low temperature, and retreat 

 after periods of years of light precipitation and high temper- 

 ature. 



Certain historic facts have been thought to show changes of 

 climate in some places since the beginning of the historic period. 

 Thus regions once populous are now too arid to support a large 

 population. This is the case in southwestern Asia and northern 

 Africa, where there are ruins of aqueducts and irrigating canals in 

 places where there are now no adequate sources of water. 



In pre-historic time. There is abundant evidence of great 

 changes of climate in the course of the earth's history. There have 

 been at least three (probably more) periods, widely separated in 

 time, when there were glaciers where glaciers do not now exist. 

 During some of these periods there were extensive glaciers in low 

 latitudes, even in regions which now have tropical and subtropical 

 climates (India, Australia, South Africa, and South America). 



Warm climates, on the other hand, have persisted for long 

 periods in polar regions, even down to relatively recent times. Thus 

 Greenland enjoyed a warm climate not long (geologically) before 

 the development of its present ice-sheet, as shown by the remains 

 of plants, such as magnolias, which once grew there. It seems 

 probable that the climate of the present time is cooler, and has a 



