INVERSIONS OF TEMPERATURE. 



45 



although it is well known that species of plants and animals 

 often form isolated colonies outside the natural boundaries of 

 their zones because of special or highly local conditions of 

 dissemination agencies or food supply. 



Incidentally, it is believed that these meteorological factors 

 may have some connection with the variations in the depth and 

 tint of the autumnal colorings of foliage leaves, though the 

 apparent evidence may be entirely coincidence. 



Aside from these phenomena which deflect the zonal bound- 

 aries of plants and animals, I now wish to call your attention 

 to certain movements of the air which may be said to have a 

 purely local selective effect. 



Among these are the chinook or foehn winds, which are 

 prevalent in certain localities over the world, being especially 

 well developed in western North America, and the moist ascend- 

 ing currents which exercise their greatest influence in regions 

 with low relative humidity. 



Chinooks are formed to the leeward of nearly all high moun- 

 tain ranges, and are heaviest near those which stretch their 

 barriers directly across the course of the prevailing winds. The 

 factors necessary to produce a chinook consist chiefly in the 

 difference in barometric pressure on the opposite sides of the 

 mountain ridge, a high-pressure center to windward and a low- 

 pressure center to leeward. The ascending current of air 

 which flows over the crest to restore the equilibrium loses much 

 of its moisture in the elevating, expanding, and cooling processes, 

 and then as it descends the opposite slope, undergoing compres- 

 sion, its temperature increases at the normal adiabatic rate of 

 1.6° F. for every loo meters of descent, and reaches the lower 

 slopes as a warm, dry wind, comparatively speaking. The tem- 

 perature of such winds is rarely above 50° F., but they occur 

 at a time when the general temperature is near or below the 

 freezing point. 



Under the influence of this high temperature many plants 

 are induced to start into activity, and as these chinooks last 

 only a few hours, the restoration of the general average tem- 

 perature results in the destruction of the partially opened buds 

 and active growing plants, while the direct desiccating action 



