250 E. Loomis — Contributions to Meteorology. 



A. One of the most common causes of rain is an unstable 

 condition of the atmosphere resulting from an unusually high 

 temperature combined with unusual humidity. This condition 

 of the atmosphere is most frequently found where the baro- 

 metric pressure is somewhat below the mean, although it 

 sometimes extends beyond the isobar of 30 inches. It is most 

 frequently found in the eastern segment of the low area, and 

 is generally accompanied by easterly or southerly winds. 



B. Another very common cause of rain, and one which is 

 frequently associated with the former, is a cold northerly or 

 westerly wind in the western segment of the low area. This 

 cold wind pushes under the warm and humid wind which 

 prevails in the eastern segment of the low area, and lifts it up 

 from the earth's surface to such a height, that a considerable 

 portion of its vapor is condensed. Frequently there is direct 

 evidence that the westerly winds in the rear of a storm are 

 merely surface winds, and that the southeast winds which pre- 

 vailed in front of the storm extended to the rear occupying a 

 stratum of considerable elevation. It is generally difficult to 

 obtain evidence of the direction of the upper stratum of air, 

 while a rain-storm is prevailing at the surface of the earth ; but 

 occasionally there are breaks in the lower stratum of clouds 

 which enable us to observe the movement of the upper clouds. 

 The observations on Mt. Washington afford us at all times the 

 means of comparing the winds at low stations with the winds 

 at the height of 6000 feet. We frequently find that the latter 

 winds are from the south or southeast, while the surface winds 

 are from the north or west. 



C. Proximity to the ocean or to a large inland sea is favor- 

 able to rain-fall. We have seen that heavy rains are more 

 frequent near the coast of the Atlantic ocean and the Gulf of 

 Mexico, than they are at interior stations. 



The following facts seem well established : 



D. ]STo great barometric depression with steep gradients ever 

 occurs without considerable rain. This is true not only for the 

 United States, but also for the cyclones of the West Indies, for 

 those of the China sea, of India and the Bay of Bengal. 



E. In great rain-storms the barometric pressure generally 

 diminishes, while the rain-fall increases. 



F. The greatest depression of the barometer generally occurs 

 about twelve hours after the greatest rain-fall. 



G. A great fall of rain is favorable to a rapid progress of the 

 center of least pressure, while a small rain-fall is generally 

 attended by a less rapid progress. It is, however, plain that the 

 rate of progress of a low center depends partly upon other 

 causes than the amount of rain-fall. 



11. In my 7th paper I have shown that considerable depres- 



