CLIMATE 11 



2. Altitude. — The temperature falls as we rise above 

 the sea-level just as it falls as we recede from the Tropics. 

 In consequence of this, the succession of vegetation 

 up a mountain is much the same as that which we observe 

 as we proceed along the earth's surface from the Equator 

 to the Poles. 



II. Humidity. 



Water has a high capacity for heat ; in other words, 

 it takes a relatively large amount of heat to raise its 

 temperature one degree ; it is slow to get warm and slow 

 to cool. This fact has an important bearing upon climate, 

 for where there is a great deal of water-vapour present 

 in the air, the variations and extremes of temperature 

 are not so marked as in the case of a dry atmosphere. 

 The climate is more equable, the summers being cooler 

 and the winters milder than in drier countries ; nor does 

 the night-temperature differ so greatly from the day. 



1. Rainfall. — This is by far the most important factor 

 in climate, and as we are here concerned with the 

 effects of rain on vegetation, the following considerations 

 with respect to the rainfall become very important : 



(a) The actual number of inches of rain falling in a 

 year. The effect of this varies with the temperature. 

 In a hot country a much larger rainfall is necessary to 

 serve the needs of the vegetation than in a cold country ; 

 for example, the minimum amount of rain necessary 

 to maintain forest will not be the same in aU latitudes. 

 In dealing with climatic effects, heat and moisture must 

 always be considered together. 



(6) The frequency of rainy days. This is more impor- 

 tant than the actual quantity of rain which falls in a 

 year. Occasional torrents, however heavy, influence 

 chmate, and therefore vegetation, far less than precipita- 

 tions, which, though less in quantity, are more evenly 

 distributed throughout the season. 



(c) • The season of greatest rainfall. Rain falling during 

 the season of vegetative inactivity is largely lost to 

 vegetation. Most rain should fall when the vegetation 

 needs it most — that is, in summer. 



2. Proximity to the Sea. — Countries near the sea 

 generally enjoy a mild and moist climate, especially 

 when the winds that reach them come laden witli moisture 



