WRITINGS OF JOSEPH HENRY. [1855 



ON THE RAIN-FALL AT DIFFERENT HEIGHTS. 

 (From the Smithsonian Annual Keport for 1855, pp. 213, 214.)* 

 December, 1855. 



The subject of the difference of rain at different elevations 

 has received much attention in this country and in Europe; 

 though more investigations are required to settle definitely 

 all the principles on which it depends. It would appear 

 that the greater part of the observed difference is due to 

 eddies of wind, which carry the air containing the falling 

 drops more rapidly over the mouth of the upper gauge 

 than over an equal portion of the unobstructed surface 

 of the ground. Professor Bache found, from a series of 

 observations on the top and at the bottom of a shot-tower in 

 Philadelphia, that not only was there a difference due to 

 elevation, but also to the position of the upper gauge, whether 

 it was placed on the windward or leeward side of the tower. 

 It would also appear, that when the air is saturated with 

 moisture down to the surface of the earth, the descending 

 drop would collect at least a portion of the water it meets 

 with in its passage to the ground, but the amount thus col- 

 lected would not be sufficient to account for the difference 

 observed. Besides this, the condition does not always exist; 

 the air near the earth is frequently under-saturated during 

 rain, and in this case a portion of the drop would be evapo- 

 rated, and its size on reaching the earth less than it was 

 above. If the drop is increased by the deposition of new 

 vapor in its descent, then the rain at the bottom ought to 

 be warmer than at the top, on account of the latent heat 

 evolved in the condensation; on the other hand, if the drop 

 be diminished by evaporization during its fall, then the tem- 

 perature of the rain caught at the greater elevation ought to 

 be in excess. That evaporization does sometimes take place 

 during the fall of rain, would appear from the fact that 



* [Remarks appended to an article on the subject, by Prof. 0. W. Morris, 

 of New York.] 



