J9I3.] OF THE UNITED STATES. 269 



Bigelow^" has given us the daily normal temperatures throughout 

 the year for 177 stations well distributed over the country. This 

 excellent piece of work has laid the foundation for many kinds of 

 climatological study that would otherwise be impossible. The data 

 are generally based on an observation period of about thirty years 

 and may be regarded as quite as reliable as any other data that we 

 now have. In summing the daily normal temepratures for the days 

 within the average frostless season, for each one of the numerous 

 stations, some temperature must be assumed as a starting point. I 

 have taken 32° F. The results of such summations may be termed 

 average or normal temperature summations, above 32° F.. for the 

 frostless season at the various stations. 



The method here used is somewhat similar to that employed by 

 Merriam^^ in his well-known study of the temperature relation in 

 the United States. This author did not use the average length of 

 the frostless season, however, and his manipulations differed from 

 my own in other details. The general method of summations is not 

 at all new, having been long employed by phenologists. 



When we plot the temperature summation indices upon a map 

 and draw iso climatic li nes in the usual way, there results a chart 

 which presents the country divided into zones or bands. Such a 

 chart is shown by the dotted lines of plates IX., X., and XI. Without 

 entering into details, it is at once seen that the temperature summa- 

 tion zones cross the continent in a generally west-east direction, being 

 southwardly displaced in the regions of the two mountain systems 

 and also to some extent along the Pacific seaboard. Practically all 

 of the area of the United States is characterized, according to this 

 chart, by normal temperature summation indices ranging from 3,000 

 to 13,000. The southern half of the Florida peninsula exhibits still 

 higher indices. 



Integrations of the inoisture relation, i. General considera- 

 tions. — While temperature furnishes us a single means of studying 



" Bigelow, F. H., " The Daily Normal Temperature and Daily Normal 

 Precipitation of the United States," U. S. Dept. Agric. Weather Bureau 

 Bulletin R, 1908. 



" Merriam, C. H., " Laws of Temperature Control of the Geographic 

 Distribution of Animals and Plants," Nat. Geog. Mag., 6: 229-38. 1894. 



