1917] METEOEOLOQY. 315 



following heads : Source, amount, and distribution of atmospheric water vapor ; 

 influence of atmospheric water vapor on climate ; terms " absolute " and " rela- 

 tive " humidity ; moist and dry climates ; measurements of atmospheric mois- 

 ture; statistical data available; diurnal changes, annual march, and seasonal 

 variations in relative humidity ; absolute humidity ; and saturation deficit. Data 

 from recording hair hygrometers for a period of about five years are reported 

 and discussed. 



Average rainfall, May and June ( U. S. Dept. Agr., Nat. Weather and Crop 

 Bui. 7 (1917), pp. 2, 3, 7, fig. 1). — A chart is given which shows the average 

 rainfall for the period from May 1 to June 30, inclusive, for the 20 years 

 1895 to 1914. This shows that " the heaviest rainfall during these two months 

 is over the Florida Peninsula, where it reaches between 13 and 14 in. In the 

 lower Mississippi Valley the rainfall is over 10 in. at a number of points, and 

 the chart indicates that there are quite large areas in the Great Plains States 

 where the fall during May and June averages over 9 in., and that this amount 

 is exceeded at a few points in the east-central part of the country. The least 

 rainfall is in southern California and southwestern Arizona, where it is less 

 than 0.5 in. Over most of the region west of the Rocky Mountains the fall 

 averages less than 3 inches. 



" In most of the eastern part of the country, except in southern Florida, the 

 annual rainfall is well distributed throughout each month of the twelve. Over 

 the Great Plains region there is much more precipitation during the summer 

 months than falls in winter time. In eastern North Dakota and central South 

 Dakota 80 per cent of the annual precipitation falls during the period from 

 April 1 to September 30, and over a large part of the Great Plains States from 

 western Texas northward to Montana and western Wisconsin, 70 per cent of 

 the annual fall comes between the above-mentioned dates. On the Pacific coast, 

 on the other hand, the rainy season is in the winter time, and very little 

 precipitation occurs from June to October, during most seasons." 



A safe method proposed for estimating minimum rainfall, J. P. Wells 

 (Engin. News, 77 (1917), No. IS, pp. 502-504, fig. J).— The author presents the 

 results of his studies of all the long-time rainfall records of the U. S. Weather 

 Bureau up to 1909 and tabulates the mean, maximum, and minimum annual 

 precipitations. The relations between these records, as calculated, are called 

 maximum-minimum, maximum-mean, and minimum-mean rainfall factors. It 

 is considered essential to have some safe method of determining the minimum 

 rainfall that will occur during a long period of years in order to estimate con- 

 servatively minimum run-oft conditions. From these studies, it is concluded 

 that the minimum-mean factors afford a safe basis for estimating minimum 

 rainfall. 



" In hydraulic investigations it is often the case that only .short records of 



precipitation are available and it is desirable to know what the minimum 



rainfall for a long period of years would be. If the minimum-mean factor for 



the locality is known, at least an approximate estimate of the minimum rainfall 



(J-iS) 

 can be made." The following formula is proposed: "X=M-jy-> in which 



X=required minimum rainfall, M= minimum-mean factor for the district, 

 L=mean rainfall for entire period at adjacent station which has a long record, 

 L'=mean rainfall at adjacent station during the period in which the records 

 were taken at the station which has a short record, and S=the mean rainfall 

 at the station which has a short record. By averaging the results obtained 

 in the above way from a number of adjacent stations with long records, a safe 

 estimate of the minimum rainfall can be made. 



