HEAT. 13 



proceeding north. The cHmate thus produced is frequently 

 called the cieof/rajyliieal or solar climate. It exists practically 

 nowhere on the earth, because it is modified and converted into 

 the pJiysical or local climate by a series of influences, amongst 

 which the following deserve attention : — 



a. Elevation above the Level of lite Sea. 



The temperature falls with elevation above the sea. In 

 the Alps the fall is one degree for every 300 to 400 feet of 

 elevation ; it is about 900 times as rapid as the fall caused by 

 increasing latitude. The effect of elevation upon temperature 

 is subject to modifications. High plateaux of considerable 

 extent show a milder climate than that calculated for their 

 elevation, because the sun's rays are more intense than at the 

 level of the sea. On the other hand, wind currents exercise a 

 considerable effect, so that isolated peaks have, as a rule, a 

 comparatively rough climate. 



Cold air, owing to its weight, glides down slopes and may 

 become stationary in valleys and low land generally, producing 

 locally and temporarily a low^er temperature than that which 

 corresponds to the elevation of the locality. Hence in such 

 localities late and early frosts are more common than in 

 localities which are under the influence of a free circulation of 

 the air. 



h. Presence of Extenm'e Sheets of Water. 



Owing to a difference of temperature, and the consequent 

 exchange of air between dry land and sheets of water, the 

 latter cause the climate of the former to be more equable, the 

 temperature being lower during the day and higher during 

 the night. It is chiefly for this reason that the longitude of 

 a locality, by affecting its distance from the ocean or other 

 extensive sheets of water, influences the local climate. 



c. Aspect and Gradient. 

 The angle at which the sun's rays strike the soil depends on 

 the aspect of the locality ; hence, in the northern hemisphere, 

 aspects between south-east and south-west are the warmest, 



