THE CLIMATOLOGY OF THE UNITED STATES. 17 



Boston, these are veiy nearly alike and yet the climatesof 

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

 mometerin the heated spells of sammer often marl?:s one 

 hundred desfrees in the shade, while in the coldest winter 

 weathör the mercury often flills to zero and sometimes 

 below, shovving a Variation diiring 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 sameliability 

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

 know what figures we need and how to juclge 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 chansfes 

 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 betvveen the Rocky mountains 



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



^J' zona. It has a wet anda dry season, the former occurring 



q^ 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. XVIII. 2 



