CLIMATIC CONDITIONS OF THE UNITED STATES. 237 



to centigrade intervals of 10°, between the extremes of 0° and —40°. 

 Thus, there appear 5 Unes, representing 32° F. (0° C), 14° F. (-10° 

 C), -4° F. (-20° C), -22° F. (-30° C.) and -40° F. (-40° C). 

 No attempt has been made to smooth the Unes; they represent, as 

 nearly as possible, the actual data given in the Summary by Sections, 

 the topography being also taken into account, as usual, in the placing 

 of the lines. The five different zones are shown by different patterns 

 in plate 41. There is only one station with a value of over 32° F. 

 (Key West, Florida), so that no temperature province for values 

 above 32° is shown. 



Plate 41 shows that the isoclimatic lines based on this criterion have 

 generally the usual east-and-west trend of temperature lines. Here 

 they are markedly displaced to the northward in the vicinity of either 

 ocean. Southward displacement by the mountains is also more or less 

 pronounced. The regions with absolute minima above 14° F. contain 

 most of the popular winter resorts. 



. The United States Weather Bureau chart showing the lowest tem- 

 perature ever observed^ is here reproduced as plate 42, for comparison 

 with our plate 41. The increments represented by the lines are each 

 10° F. It is seen that this chart agrees with ours in its main points, 

 but that its lines have been subjected to an effective smoothing, so 

 that they are much more regular than those of plate 41. 



(H) AVERAGE DAILY NORMAL TEMPERATURE FOR COLDEST 14 DAYS OF YEAR. 



(TABLE 8, PLATE 43.) 



Since the absolute minima of temperature do not furnish an indica- 

 tion of the intensity of cold usually encountered at the various stations, 

 it seems desirable to employ some normal temperature mean that may 

 represent this. We have chosen for this purpose the average of the 

 normal daily means for the 14 days having the lowest normal daily 

 means, as given in Bulletin R of the United States Weather Bureau. 

 It is to be noted that this 14-day period does not include the same days 

 for the different stations, so that this climatic feature may be expected 

 to be somewhat different from the mean temperature of some uniform 

 period, such as the first two weeks of January, etc. The nature of the 

 normal daily means of Bulletin R is such that it is impossible always to 

 select 14 days as representing the lowest values. Thus, for Anniston, 

 Alabama, the normal daily mean is 42° F. for all days from December 

 27 to January 24, and the average of any 14 of these 29 days remains 

 42°. In table 8, which gives these averages, the first and last dates of 

 the period considered are given for each station. Wherever the period 

 includes more than 14 days the normal daily mean is constant (and the 

 same as the average given) for the entire period. When a 14-day 

 period includes several values of the normal daily mean, the day repre- 



HJ. S. Weather Bureau, Chart of lowest temperatures ever observed. (To and including 

 1914. — Letter from Professor C. F. Marvin). 



