1896.] PARKER CLIMATOLOGY OK ROCHESTER. 229 ' 



only from 9 to 8. Generally the spring and summer rainfalls are the 

 highest ; other things being equal, the rainfalls of spring and, next to 

 that, of summer are the most useful for agricultural operations. At 

 Rochester this statement becomes absolute. 



With the depth given it is not difificult to get the average total 

 rainfall for the entire United States, excluding Alaska. For this 

 purpose we may take the average for each state and multiply it by the 

 area of the state including water surfaces. Adding these together we 

 get 1,407 cubic miles as the average annual total of water which 

 descends as rain or snow in the United States. The annual depth of 

 rainfall thus calculated is 29 inches, or less than that given by the 

 other method. This is to be expected since the other method gave 

 equal weight to each political division and these divisions are generally 

 smaller in the regions of greater rainfall. 



To get some conception of this enormous mass of water we may 

 compare it with the contents of the great lakes and an approximate 

 comparison is near enough. Lake Ontario contains about 636 cubic 

 miles of water. The annual rainfall would fill it two times and leave 

 something over for a third time. 



Source of Moisture. 



It is probable that the greatest source of water vapor supply for 

 this section is the Gulf of Mexico, notwithstanding its great distance ; 

 next in importance the Atlantic ocean, and lastly the lakes. The 

 evaporation from the land is very largely carried out of the country by 

 the easterly movement of the atmosphere during spells of dry weather. 

 Large bodies of water produce vapor in great quantities continually and 

 are producing as rapidly at the end of a dry spell when the land is 

 pretty well exhausted. The machinery for bringing the vapor in from 

 the oceans is found in the great storms which continually cross the 

 country from west to east. In the eastern half of every storm the wind is 

 from the south to the north and if the storm happens to be sufificiendy 

 large to reach well down over the Gulf, vast volumes of vapor are 

 engaged and brought north to be precipitated over the country, 

 sometimes far north of the border. 



Coming from the Gulf of Mexico, the moisture-laden air is com- 

 pelled to ascend to a much greater elevation and is exposed to mixture 

 with bodies of cold air encountered in the journey and to loss of heat 

 by radiation from the tops of clouds after they are formed. When a 

 storm center passes south of this section masses of vapor from over 

 the Atlantic are engaged in the northerly and westerly movement of 



