692 



ZONES AND REGIONS [Pt. Ill, Sect. IV 



In climates with cold winters an annual periodic change exists in the 

 variation of temperature that is due to altitude. 



REDUCTION OF TEMPERATURE PER 100 METERS IN 

 CENTRAL EUROPE. 



(After Harm, Bd. I, p. 243.) 



A further consequence of the rarefaction of the air, which is also directly 

 noticeable with increasing altitude, just as is the reduced atmospheric tem- 

 perature, is the increased intensity of heat-radiation. Objects exposed to 

 radiation become more heated than in lowlands, but also cool more rapidly 

 and to a greater extent. 



The atmosphere is well known to possess the property of easily allowing the 

 passage through itself of rays emitted by a luminous source of heat, such as the sun, 

 but with difficulty allows the passage of those from a dark body. Consequently in 

 the lowlands the soil gets strongly heated by the solar rays, but only slightly cooled 

 by its own nocturnal radiation. The more rarefied the atmosphere, the greater is the 

 heating by day, but also the cooling by night. The rarefaction of the air, at high 

 altitudes, is reinforced in its action by another factor, the diminution in aqueous 

 vapour. Atmospheric vapour, according to Violle, when compared with dry air, 

 absorbs five times as much heat as does dry air. 



The absorptive action of the atmosphere on solar radiation is well characterized 

 by Hann, as follows : 'When the sun stands nearly vertically over India, the amount 

 of direct sunlight falling on the valleys of Tibet, where wheat is still cultivated, is 

 nearly one and a half times greater than the amount of light falling on the plains of 

 Hindustan, and even when the sun is at an angle of 45 , its chemical action on the 

 highland is more than twice as great as in the plains.' 



The following data (from Peschel, Hann, and Junghuhn) give an account 

 of the inequality of the temperatures in the sun and in the shade : — 



Hooker observed $£ C. in the sun and —5*6° in snow in the shade at 3,000 

 meters altitude in the Himalayas with a black-bulb quicksilver thermometer. 

 Przhevalsky found simultaneously i6'3° C. on the sunny side of his tent 

 and — 8-o° C. on its shady side on the Tibetan plateau (altitude not given) 

 on the 27th of October, 1879. ' Cayley observed the thermometer in the 

 sun standing at 57-8° C, while the temperature in the shade was only 

 23-9°, on the nth of August, 1867, at Leh ; a black-bulb thermometer in 

 a vacuum glass-cover (solar thermometer) rose even to 101-7° C., i.e. almost 



