COMPOSITION OF THE ATMOSPHERE 



27 



Place 



Mean 



annual 



relative 



humidity 



Mean 

 monthly 

 minimum 

 humidity 



Month 



Yuma, Arizona 



Santa Fe, New Mexico 

 Pueblo, Colorado 



42.9% 

 44-8% 

 46.2% 



34-7% 

 28.7% 

 37-6% 



June 

 June 

 April 



Death Valley, California, a typical American desert, showed a mean 

 relative humidity of 23 per cent, for the five months from May to 

 September in 1891. In regions of summer rain the relative humidity 

 is high, even far inland. Thus at Dorpat, Baltic Provinces of Rus- 

 sia, the mean relative humidity in summer is 'j'i^ per cent. ; at Yeni- 

 seisk, Central Siberia, it is 70 per cent. 



Source of Water Vapor. Evaporation is the chief source of the 

 water vapor of the air. All exposed moist surfaces furnish water 

 vapor to the atmosphere, the conversion of water into vapor con- 

 stituting evaporation. Evaporation is, of course, most marked over 

 large water surfaces, such as the oceans, the ultimate source of the 

 water vapor, and over lakes, ponds, etc. It is estimated that the 

 average amount of evaporation from the surface of the earth 

 amounts to a layer 30 or 40 inches deep each year. (Some esti- 

 mates make it 60 inches.) At this rate, if the water were not re- 

 turned to the sea, the oceans would be dried up in from 3,000 to 

 4,000 years, while the lakes of the earth would probably be ex- 

 hausted in a single year. The water evaporation from the surface 

 of the Mediterranean and Black Seas is estimated as more than 

 226 cubic miles per annum. Evaporation may be going on from an 

 approximately dry land surface, the moisture being drawn from 

 beneath the surface. By such processes mineral matter held in 

 Ablution within the pores of the rock is drawn to the surface, and 

 there left on evaporation. Such is the Origin of the brown desert 

 varnish on rock surfaces, and the efflorescence of salt and alum 

 found on many rocks. Snow and ice also evaporate, at a tempera- 

 ture below the melting point, for snow and ice slowly disappear, 

 even in temperatures below ^2° F. 



Other sources of water vapor are animal and plant respiration, 

 and volcanic ejections, the latter often adding an appreciable 

 amount. 



Impurities. The impurities of the atmosphere are in large 

 part inorganic particles or dust. Organic matter, such as spores of 

 plants, bacteria, etc., also abounds, but tiie dust is by far the most 



