HYDROLOGY OF NEW YORK 83 



Rainfall 



Cause of rainfall. The cause of rainfall lias been discussed by 

 Mr Velschow in the transactions of the American Society of Civil 

 Engineers. 1 This paper may be referred to for a very good dis- 

 cussion of the subject. 



The subject is also very ably discussed by Alfred J. Henry in 

 one of the Weather Bureau Reports. 2 Mr Henry remarks that the 

 theories of rainfall given in books of twenty or thirty years ago 

 are not now wholly accepted. Still there is one simple principle 

 upon which no disagreement exists — that in order to produce rain 

 the temperature of the air must be suddenly cooled below the 

 dewpoint. When the air is thus cooled a portion of the vapor is 

 changed to the liquid and the particles thus formed may float 

 away with the wind or they may increase in size and fall to the 

 ground by virtue of gravity. Whether the condensation results 

 simply in cloud, or whether rain falls, depends on the magnitude 

 of the temperature changes taking place in the air mass. 



The precise manner in which air is cooled to produce rain, 

 whether by contact or by mixing, is not clearly apprehended. 

 Cooling by expansion, as air ascends, is one of the most effective 

 causes of rainfall. The ascensional movement is brought about 

 in several ways, probably the most important of which is circula- 

 tion of air in cyclonic storms, by a radial inflow from all sides and 

 an ascensional movement in the center. A very large percentage 

 of the rain of the United States is precipitated in connection with 

 the passage of storms of this class. 3 



Mr Henry discusses the precipitation of the United States under 

 the following topics: (1) The statistics used and their accuracy; 

 (2) geographic distribution and annual allowance; (3) monthly 

 distribution by districts and types; (4) the precipitation of the 

 crop-gTowing season; (5) secular variations; (6) details of the 



l The Cause of Rain and the Structure of the Atmosphere, by Franz A. 

 Velschow : Trans. Am. Soc. Civil EDg., Vol. XXXIII, 1890, p. 303. 



2 Rainfall of the United States, by Alfred J. Henry, chief of division: 

 Ann. Rept. Weather Bureau, 1896-97, p. 317. 



3 Abstracted from Mr Henry's paper. 



