ii2 The Realm of Nature CHAP. 



so great as this, for the Sun has a certain heating- 

 effect. Several years of continuous observations on the 

 summit of Ben Nevis, and at sea-level at Fort William 

 have shown that the actual falling off of temperature with 

 height is i for every 270 feet of ascent. Thus, whatever 

 the temperature may be at sea- level, there is a certain 

 height where the air has an average temperature of 32 F., 

 no matter how much sun-heat passes through ; and snow 

 which falls above that height does not melt. This limit 

 is termed the snow-line. It is sea-level in the extreme arctic 

 regions, about 5000 feet at latitude 62 in Norway, about 

 9000 feet in latitude 46 in Switzerland, and ^bove 16,000 

 feet at the equator (see figure 63 and section in Plate VIII.) 



164. Heating and Cooling of Air. Near sea-level 

 the dense air is charged with water-vapour and dust which, 

 during the day, absorb solar radiant energy and pass on 

 the heat to the air. The ground also is rapidly heated, as 

 its specific heat is only about one quarter that of water, and 

 its temperature therefore rises four times as much as water 

 does for the same amount of heat. Once heated, the 

 ground is effectual in heating up the air in contact with 

 it. In the case of water, the Sun's rays penetrate to a 

 great depth, the temperature of the surface is very slightly 

 raised, and transfers little heat to the air over it. Hence 

 in sunshine a land surface heats air greatly, a sea surface 

 heats it only slightly. After sunset the hot land radiates 

 its heat through the atmosphere, and falls to a low tem- 

 perature, thereby chilling the air in contact with it, and 

 were it not for the dust-motes and condensed water catch- 

 ing and retaining most of this heat ( 160) the radiation 

 of a single clear night would chill down the land far more 

 than the solar energy received during the day could heat it 

 up. The temperature of the dust-motes is also lowered by 

 radiation from the particles at night, and this is not fully 

 compensated by the heat radiated from the earth, so that 

 the air temperature falls greatly. From a water surface 

 heat is radiated slowly at night, and the air over water is 

 not greatly chilled. 



165. Dew and Hoar Frost. On a 'clear night, when 



