4 GENERAL CONSIDERATIONS 



show that the same zone is not always to be found 

 at the same altitude. The plants of the cold thickets 

 up near the timber line will be found straggling 

 downward for a considerable distance in shaded ra- 

 vines on the northern slopes, while on the side exposed 

 to the sun the plants of the hot plains at the base 

 may extend upward for many hundred feet. 



In a level country like the great Mississippi Valley 

 there is nothing to interrupt the free movement of 

 the air, and the cold northwest winds of winter rush 

 unchecked throughout its whole extent. If a great 

 chain of mountains extended from east to west 

 across the middle of it, these northers would to a 

 great extent be deflected and the southern portion 

 would have a much warmer, more equable climate. 

 On the other hand the warm winds from the Gulf of 

 Mexico would be prevented from reaching the upper 

 portion and its climate would be correspondingly 

 colder. Italy owes its warm, genial climate to the 

 protection of the Alps. Small local variations in 

 altitude also have a pronounced effect on climate. 

 The cool air at night settles into the low places, 

 while the warmer air rises about the tops of the hills. 

 In driving through the country at night every one 

 has noticed the chill on descending into some damp 

 ravine and the pleasant warmth encountered on the 

 higher ridges. Fruit growers take advantage of this 

 fact and locate peach orchards and strawberry fields 

 on high ground, where they will be more likely to 

 escape damage from spring frosts. 



Topography is thus seen to have its effect on 

 temperature, but it has an equally great effect in con- 



