106 EXPLANATION OF THE ISOTHERMAL CHARTS. 



sweeping the air which had been resting over the ocean across a great portion of 

 the country, thus impresses the chief character on the climate, viz. : a comparatively 

 high and uniformly distributed lointer temperature, which is even felt beyond the 

 Rocky Mountains in central Montana, to which latent heat is carried by the moist 

 winds, as clearly exhibited in my Rain Chart^ for the winter season. With the 

 high winter temperature, we associate the fact of comparatively great precipitation. 

 Secondly, we are impressed with the comparatively low sunimer temperature over 

 the Pacific States; in fact the coldest place in the whole United States, at this 

 season, excepting only the high mountain ranges and peaks, is just outside the 

 Golden Gate, Bay of San Francisco, where we encounter the isotheral of 56°, which 

 appears nowhere else during this season. To exhibit the contrast more forcibly, 

 we have in the corresponding season and latitude on the Atlantic side (near the 

 mouth of Chesapeake Bay, a temperature higher by as much as 18°. With this 

 low summer temperature we connect the fact of but little precipitation. 



In lointer this contrast between the two (opposite) coasts is of the opposite kind, 

 the isocheimal of 52°, ofi' the Golden Gate, corresponding to the isocheimal of 42°, 

 off the mouth of the Chesapeake, a temperature Imoer by 10°. Finally, we notice 

 the extraordinary difference in the range of the mean temperature at the extreme 

 seasons, this being nearly 4° on the Pacific, and nearly 33° on the Atlantic. 



We next notice the greater accumulation of heat in valleys than in the plains, 

 the most remarkable instance being that of the Joaquin Valley and its northern 

 prolongation, the Sacramento Valley. This feature is most apparent in the summer 

 season, when these valleys seem to become reservoirs of heat, and when their slop- 

 ing sides are most exposed to insolation. The mean summer temperature in the 

 central part of San Joaquin Valley rises above 84°, when on the sea-coast, close 

 by, it is below 60°. Other instances of this kind are presented on the chart for the 

 summer temperature, by the heated plains of the Columbia River, by the region 

 along the Colorado and Gila Rivers, and, to return to the eastern portion of the 

 country, by the lower valley of the Rio Grande, where the temperature reaches 84°, 

 by the Hudson Valley, and lastly by that of the St. Lawrence. 



The hottest region in the United States is along the lower course of the Colorado 

 and Gila Rivers, where we meet with the isotheral of 88°. 



It is needless to follow out, in further detail, the various features presented by 

 the charts, since they address themselves sufficiently to the eye, nor has it been 

 deemed necessary to construct isothermal charts for the intermediate seasons of 

 spring and autumn, which, being periods of transition, cannot present features as 

 striking as those exhibited by the extreme seasons. 



The total number of results from series plotted on the charts and from which 

 the isothermal curves were constructed are 1300 nearly for the year, 1450 nearly 

 for the winter, and 1500 nearly for the summer. For the base chart, the Smith- 

 sonian Institution is indebted to Prof. Francis A. Walker, Superintendent U. S. 

 Census. 



' Tables and Results of the Precipitation, in Rain and Snow, in the United States. Smithsonian 

 Contributions to Knowledge, No. 222 ; Washington, May, 1872. 



