E. Loomis — Contributions to Meteorology. 249 



center, but generally such a correspondence was not very dis- 

 tinctly marked ; the changes of temperature being often due 

 to causes which had but little influence upon the barometer. 



9. A single rain-area seldom occurs alone. In 90 per cent of 

 the 106 cases of great rain-fall under examination, there was 

 more than one rain-area east of the Rocky Mountains with at 

 least a half inch rain-fall, and if we include smaller rain areas, 

 the percentage is still greater. In 36 per cent of the whole 

 number of cases, there were at least four rain-areas with not 

 less than a half inch rain-fall ; in 9 per cent of the cases there 

 were at least six rain-areas with not less than a half inch rain- 

 fall ; and in one case (Sept. 11.3, 1S72), there were eight rain- 

 areas all of which exceeded a half inch. These facts suggest 

 the idea that those conditions which are favorable to rain-fall 

 at one locality, are generally favorable to rain-fall over a much 

 larger district. Extensive rains frequency result from an un- 

 stable condition of the atmosphere, in consequence of which 

 the stratum of air near the earth's surface tends to ascend. 

 This unstable condition may result from a temperature above 

 the mean for the given time and place, and it may also result 

 from the presence of an unusual amount of aqueous vapor. 

 These conditions of unusual heat and unusual humidity often 

 prevail simultaneously over an area several hundred miles, in 

 diameter. Over such a region the entire stratum of air near 

 the earth's surface tends to ascend. A general ascent of the 

 air over a large area is impossible, but some local cause may 

 determine an upward movement at some point in this area, 

 and the surrounding air will be drawn in to supply the place 

 of the air which ascends. The vapor of the ascending air 

 will be cooled by elevation, and be precipitated, and thus may 

 commence a shower which under favorable conditions will 

 increase and continue for several hours. When this unstable 

 condition of the air prevails over a large area, there may be 

 more than one point where such an ascending current is 

 formed, and thus we may have several rain-areas prevailing 

 simultaneously within a few hundred miles of each other. 



Rain-areas, with a total rain-fall of 6 or 7 inches in eight 

 hours for all the stations east of the Rocky Mountains, seldom 

 continue for more than 21 hours ; only five such cases having 

 been found in a period of 11 months, and there were only six 

 cases in which the rain-center continued at the same station 

 for a period of 16 hours. These facts seem to indicate that 

 the causes which produce rain do not derive increased force 

 from the rain-fall for an indefinite period of time, but after a 

 few hours they expend themselves and become exhausted. 



10. The preceding examination of great rain-storms seems 

 to warrant some generalizations respecting the conditions 

 favorable for rain-fall. 



