250 ENVIRONMENTAL CONDITIONS. 



power of the surroundings throughout the period in question. A 

 usable index of this is obtained by dividing the quantity P or tt by the 

 number of days in the average frostless season. This procedure is 

 logically no better here than the average temperature for the frostless 

 season would be in the case of temperature relations, but, as has been 

 said, lack of knowledge prevents as logical a treatment of the moisture- 

 relation as is now possible in this aspect of the other case. 



Rainfall is universally measured in terms of depth units, which 

 denote volume or weight units per unit of horizontal surface. The 

 position of the horizontal surface of reference is assumed to be at the 

 level of the soil surface. Raising this surface a few meters above the 

 soil has no considerable influence upon the readings in most regions, 

 though it would be undesirable to place the rain-gage funnels at anj'- 

 very great distance above the ground in an arid country. In such a 

 country a considerable amount of rain might frequently be recorded 

 on a gage supported a few hundred meters above the ground, while a 

 gage directly beneath, at the ground-level, might remain quite dry; 

 the rain-drops often evaporate as they descend. Of course, the opposite 

 is sometimes true in a very moist region, where the drops may increase 

 in size as they fall. Our precipitation data are in terms of inches of 

 depth, since inches are still employed in the tables of the United States 

 Weather Bureau publications, from which we derive our original values. 

 All such data may of course be readily converted into metric values, 

 where more universally comparable numbers are desired. 



As a basis for our computations we have again had recourse to Bige- 

 low's tables of normal daily values (Bulletin R of the U. S. Weather 

 Bureau). These tables present the results of an elaborate treatment 

 of the observation data in the United States, resulting in a precipita- 

 tion value for each day in the year for each station considered. Thus 

 the normal precipitation is given for each day of the year and for each 

 station in the list. If a ''normal" year, in this sense, ever occurred, 

 then the actual precipitation for each day in the year, for any station, 

 would be the value given in Bigelow's table. We have treated these 

 normal daily precipitation values in somewhat the same manner as 

 was followed in handling the normal daily means of temperature given 

 by Bigelow in the same pubUcation. By the use of these tables it is 

 possible to study the comparative lengths of what may be called normal 

 drought periods and normal rainy periods, as will be brought out below. 

 All of our computations involve both duration and intensity factors, 

 as will also appear in the discussions that follow. 



(2) Normal Mean Daily Precipitation for Period of Average Frostless 

 Season ( -^ V (Table 11, Plate 46, and Fig. 2.) 



The mean daily rainfall for the period of the frostless season should 

 be a general measure of aridity in a certain sense, and we have obtained 



