THE CLIMATOLOGY OF THE UNITED STATES. 17 



Boston, these are very nearly alike and yet the climates of 

 the two places are very dissimilar. In Boston, the ther- 

 mometer in the heated spells of summer often marks one 

 hundred degrees in the shade, while in the coldest winter 

 weather the mercury often falls to zero and sometimes 

 below, showing a variation during the year of over one 

 hundred degrees ; while in San Francisco, the variation 

 from winter to summer is not much over forty degrees 

 and the changes are much less abrupt. The same liability 

 to error exists in judging of the rainfall, so we must 

 know what figures we need and how to judge by the va- 

 rious statistics at our command. In order to explain the 

 character of and the laws governing the various phases 

 of the weather, I shall first describe our own climate and 

 the operation of the United States Signal Service, and 

 then the climate of the Pacific slope, supplementing the 

 whole with a short consideration of the climatic changes 

 which have been taking place throughout the world dur- 

 ing the last few years. 



Within the limits of the United States there exist three 

 distinct meteorological regions. The first including that 

 part of the country lying east of the Rocky mountains. 

 This region has a precipitation of rain or snow at frequent 

 intervals throughout the year; the greater part of the 

 region has cold winters and hot summers ; it is subject 

 to variable winds at all seasons. The second region em- 

 braces the country lying between the Rocky mountains 

 and the Pacific coast and north of New Mexico and Ari- 

 zona. It has a wet and a dry season, the former occurring 

 in the winter months ; the precipitation is almost entirely 

 in the form of rain, except in the mountains and is about 

 one-half of that in the above-named region. The winters 

 are mild and the summers cool on the coast and hot in the 

 interior. The winds are variable in winter and westerly 



ESSEX INST. BULLETIN, VOL. XVIH. 2 



