II.— MEAN MONTHLY AND ANNUAL RELATIVE 

 HUMIDITY CHARTS OF THE UNITED STATES. 



By Kenneth S. Johnson, Harvard University. 



(Received through Professor R. de C. Ward, Harvard University, 

 and communicated by J. R. Sutton). 



[Abstract.] 



Loomis in 1880 * published a chart with relative humidity lines 

 for a few stations east of the Rocky Mountains. There were but four 

 such lines, and these were based on only one month's record — 

 January, 1875 — but the purpose which Loomis had in mind was 

 accomplished, namely, to show that on the east side of the Rocky 

 Mountains " there is a narrow belt of territory where the mean 

 relative humidity is less than one-half." 



In 1902 the U.S. Weather Bureau published t three charts of 

 relative humidity, one for the year, one for January, and one for 

 Julv. The charts are incomplete in that they do not cover all the 

 months. Moreover, they are based on records of varying length, 

 ranging from 4 to 14 years. The present charts for all the months 

 and for the year are based on data for the uniform period of 14 

 years, from 1 888-1 901 H In a few parts of the country, where the 

 stations are scattered, records of slightly shorter periods have been 

 used in determining the position of some of the lines, but in no 

 case have the short period records been given equal weight with those 

 of the 14-year period. 



The curves are drawn for differences of 10 per cent, only ; for 

 the sake of clearness in presenting the main facts. Furthermore, the 

 length of the records is not sufficient to warrant greater detail. 



Among the most striking features shown on the charts is the 

 uniform high relative humidity along the coasts, in contrast with 

 which is the extremely low relative humidity in the S.W. interior, 

 especially in Arizona, Nevada, S.E.California, and the adjacent 

 districts. While the high relative humidity on the coast remains 

 fairly constant throughout the year, the low in the interior basin (or, 

 as it used to be called, the Great American Desert) becomes more 

 marked as summer approaches, thereby increasing the already strong 

 relative humidity-gradient between the Pacific coast and the interior. 



Another interesting fact is the annual movement of the lines in 

 the northern part of the interior basin. These travel north in sum- 

 mer, reaching the northern limit in June or Julv, returning south or 

 perhaps entirely disappearing in winter. 



Among other features worthv of note is the distorting effect of 

 the Great Lakes, where the relative humiditv tends to remain at or 

 near 80%. an effect quite similar to that produced by the oceans. 

 Also — a thing which rarely occurs — there is a prevailing north and 

 south trend of the lines over the Great Plains in the vicinity of the 

 looth meridian, showing that the relative humidity there does not 

 vary with the latitude. 



* American Jounml of Science, Third Series, Vol. XX., p. 22. 

 t Report of Chief of Weather Bureau, 1901-2, p. 320. 

 H Report of Chief of Weather Bureau, 1901-1902, p. 318. 



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