44 THE WANDERINGS OF ANIMALS [OH. 



Broadly speaking the annual mean temperature 

 decreases towards the poles at an average of 1 F. 

 for each degree of latitude. With increasing height 

 the temperature falls nearly a thousand times as 

 rapidly, namely 0*5 centigrade for every 100 metres; 

 this is equal to l c F. for every 365 feet, an easy 

 number to remember. On mountains the rate is 

 somewhat greater, 1 F. = 300 feet. Given the tem- 

 perature and height of a lower station and the height 

 of the mountain its temperature at the summit can 

 easily be calculated 1 . In our latitude a mean annual 

 temperature of 32 F. or centigrade is reached at a 

 height of about 5000 feet, but under the equator this 

 'snowline' lies at about 15,000 feet, and near the 

 arctic circle it approaches the level of the sea. 



Atmospheric pressure also decreases, roughly by 

 1 inch with every 1000 feet of elevation, so that at 

 a level of 10,000 feet we experience only two-thirds 

 of the normal pressure at sea-level ; which implies of 



d 



1 The formula for centigrades and metres is Ut = Lt- 



200' 



d 



For degrees of Fahrenheit and feet the formula is Ut = Lt- 



300 

 [ft = Mean temperature of the upper station. 



/ = Mean temperature of the nearest known lower station. 

 d = difference in height of the two stations. 



Example : Highest mountain 5000 ft. 



Temp, at nearest sea-level 50 F. 



Temp, on top = 50 - ^ ^=50 - 17 = 33F. 



ouU 



