. hance alone and suggests that some climatic or ex- 

 iraordiiiary factor may be effective, either directly or 

 111 controlling the time schedule at which the in- 

 trinsic factors function. 



There have been various attempts to show cycles 

 in the weather. Bruckner (1890) examined records 

 since 1700 of temperature, precipitation, and other 

 factors and suggested a cycle of around 35 years. 

 Scarcities of ducks reported in the 1820's, 1860's, 

 1890's, and 1930's apjiear correlated with drought 

 and may possibly represent the intervals of the 

 Bruckner cycle (Rowan 1954). 



Weather is affected by variations in solar radi- 

 ation (Abbott 1931, Clayton 1943). Sunspots, chro- 

 mospheric eruptions, solar coronal disturbances, and 

 ionospheric and geomagnetic disturbances are indices 

 of solar activity, but not direct measurements of it. 

 The agent i)resumably effecting changes in the at- 

 mosphere and in living organisms may be short- 

 wa\elength ultraviolet radiations or emitted charged 

 particles. Emissions from the sun are constantly un- 

 dergoing great fluctuations, with ma.ximum intensi- 

 ties for short periods being a hundred or a thousand 

 times the minimum intensities. 



Sunspnats are the only e.xpression of solar activity 

 that has been measured over a long period of time 

 (\\'illett 1953). The intervals between peaks in the 

 mean daily number of sunspots per year fluctuate 

 from 7 to 17 years, average 1 1.2 years. A correlation 

 between number of suns])ots and temperature, rain- 

 fall, and cloudiness is sometimes indicated but needs 

 more complete substantiation before it can be fully 

 accepted (Thomson 1936). For instance, an anal- 

 ysis of records spanning 109 years for the period 

 May through October in southern Wisconsin indi- 

 cates lower temperatures, greater precipitation, and 

 less sunshine in years when sunspots were increasing 

 or at a ma.ximum than when they were decreasing 

 or at a minimum (Morris 1947). A similar correla- 

 tion between summer rainfall and the sunspot cycle 

 has been demonstrated for the Toronto, Canada, area 

 (Clayton 1943). 



The sunspot cycle is as variable in length as are 

 the cycles of grouse and snowshoe rabbit, and there 

 have been repeated attempts to correlate these cycles 

 (MacLagan 1940, Huntington 1945). It is doubtful, 

 however, if such a correlation is real. Since 1750, the 

 sunspot cycle has coincided with the snowshoe rabbit 

 and lyn.x cycles part of the time, but goes out of 

 phase until one becomes the inverse of the other. If 

 solar radiation is responsible for population cycles, 

 it is clear that the number of sunspots is not a reli- 

 able index for judging the intensity of radiation. 



Since the growth of trees and the width of the 

 annual rings they form is largely dependent on rain- 

 fall, one may conjecture the weather record back 

 3,000 years by measuring the width of annual rings 



in the giant sequoias. This analysis of tree rings in- 

 dicates the possibility of a variety of weather cycles, 

 some imixjrtant ones JH-ing in the neighborhood of 

 9-10 years (Douglass 1928). Weather cycles of 3-4 

 years are difficult to demonstrate, but an analysis of 

 changes in barometric pressure and other charac- 

 teristics of the annual atmospheric circulation over 

 the British Isles indicates that they may exist (Goldie 

 1936). Cycles or outbreaks resulting from climatic 

 factors are not necessarily absolutely synchronous 

 over large areas. There is a limit to the size of the 

 area over which a change in weather produces a 

 single common effect. Outbreaks of spruce budworm 

 progressed eastward in Canada between 1945 and 

 1949. These outbreaks were probably less a result of 

 spontaneous dispersal of the moth, although this was 

 a contributing factor, than of tiie progressive east- 

 ward circulation of favorable polar air masses 

 (Greenbank 1957). 



Solar radiation may affect animals and plants in 

 other ways than through the weather. The atmos- 

 phere above the earth's surface is divisible into the 

 troposphere, which extends to a height of 3-4 km 

 (5-6 miles) ; the stratosphere, which rises to about 

 30 km (50 miles) ; and the ionosphere, which ex- 

 tends beyond. The concentration of oxygen dimin- 

 ishes with height above the earth, and becomes very 

 low in the stratosphere. However, at heights of about 

 10 to 20 km (15 to 30 miles), there is a thin hut 

 concentrated layer of ozone (O3). Oxygen and ozone 

 are responsible for absorbing most of the ultraviolet 

 radiation (below 3200 A) emanating from the sun 

 before it reaches the earth's surface. Atmospheric 

 gases, especially carbon dioxide and water vapor, ab- 

 sorb most of the infrared wavelengths (over 20,000 

 A) (Shaw 1953). A little ozone in the atmosphere 

 at the earth's surface is stimulating to animals, but 

 a high amount is harmful. There is some experi- 

 mental evidence that ionization of the air, i.e., the 

 conversion of neutral gas molecules into electrically 

 charged ions, may affect the health and the vigor of 

 animals. The height of the ionosphere and ozone 

 layers above the earth is controlled by the intensity 

 of solar radiation, and it is possible that cyclic vari- 

 ations in the height of these layers may affect organ- 

 isms in ways that are little understood at the present 

 time (Reiser 1937, Huntington 1941). 



Fluctuations in solar ultraviolet radiation vary 

 the extent of ionization of air and the rate of ozone 

 formation. The ozone layer serves as a protective 

 blanket which prevents ultraviolet rays from destroy- 

 ing all life on the earth. In small doses, ultraviolet is 

 anti-rachitic, germicidal, and erythemal ; in some spe- 

 cies ultraviolet also affects skin pigmentation (Luck- 

 iesh 1946). Animals obtain vitamin D either as a 

 product of direct radiation of the skin or in the food 

 that they consume. Wavelengths other than ultra- 



Irruptions, catastrophes, and cycles 243 



