CHAPTER VIII 



ATMOSPHERIC PHENOMENA 



163. Solar Energy in the Atmosphere. All the 



changes in the atmosphere are directly or indirectly due to 

 the radiant energy received from the Sun ( 119-125), the 

 whole of which must pass through the air before reaching 

 the Earth's surface. Thermometers placed in specially 

 contrived screens are employed to measure the temperature 

 of air. On lofty mountains, where the atmosphere contains 

 little water-vapour and few dust-motes, the air is heated 

 so slightly by the Sun's rays passing through, that it 

 remains bitterly cold, although the Sun's direct heat- 

 blisters the traveller's face and hands. At an eleva- 

 tion of 11,000 feet, water has even been boiled by 

 exposing it in a blackened bottle to the sunshine. On 

 account of the low pressure of the air at great heights, air 

 from sea-level rising as a heated current expands greatly, 

 as explained by Boyle's law ( 148). But the work of 

 expansion against the attraction of gravity consumes heat, 

 and the temperature of the expanded air, if unsaturated, 

 falls i for every 180 feet of ascent. When cold 

 air from a great altitude is carried toward the Earth's 

 surface by a descending current, the pressure upon it is 

 continually increasing, and its volume is being reduced. 

 The work thus done on the air by gravity is changed into 

 heat, and the temperature of the air rises i for each 

 1 80 feet it descends. The actual rate of change 

 of temperature in the air near the Earth's surface is not 



