32 THE RELATION OF DESERT PLANTS TO 



ural transpiration; the air about the Desert Laboratory is seldom per- 

 fectly at rest and then only for short periods. 



Curves of temperatures and of evaporation rates were constructed 

 for the period during which the evaporimeter was observed, and these 

 two curves, when brought together upon the same sheet, bring out this 

 point very clearly. During dry gales the curve of evaporation lies 

 much higher in relation to the curve of temperature than when the air 

 was more nearly quiet. Several examples of such rises in rate of 

 evaporation are shown in the portions of these curves given in figs. 5 and 

 6 (pp. 30, 31) . The first of these is for the period from 3 p. m. , July 24 to 

 2 h 30 m p.m., July 25, the second from 8"30 m a.m., July 28 to 7 p.m., July 

 29. The evaporimeter curve is constructed by plotting rates per hour 

 as ordinates, with time intervals plotted as abscissas. The actual loss 

 for each period is divided by the number of hours, and the resulting 

 average rate per hour is plotted at the middle of the period. Thus 

 different abscissas represent, not the actual times of observations, but 

 the middles of the time periods. The temperature curve is constructed 

 in a similar way, the mean temperature between two readings being 

 plotted with the same abscissa as the rate of evaporation for the corre- 

 sponding period. The scales are merely chosen so as to bring the two 

 curves into proximity for the whole time of observation. In the figures 

 the broad line denotes evaporation, the narrow one temperature, and 

 the numbers placed near the points on the curves denote, in the one 

 case cubic centimeters per hour and in the other degrees Fahrenheit. 



Examination of the curves shows at once that, while in general 

 they both rise or fall at the same time, there are nevertheless many 

 periods during which the direction of change is in the opposite direc- 

 tion in the two, and even where they agree in direction the variations 

 in the two are often by no means quantitatively identical. While many 

 of the minor ones of these independent rises and falls in the curve of 

 evaporation are undoubtedly due to changes in the absolute amount of 

 water vapor in the air, all of the more pronounced ones are to be traced 

 to variations in wind. In figure 4 an extremely high evaporation rate 

 is shown during a violent dust storm which arose about 3 h 30"' p.m., 

 July 24, and continued until 6 p.m. A similar high wind arose the fol- 

 lowing morning about 7 l W a and gradually fell during the day. The 

 day was cloudy for the most part and a gentle, continuous rain began to 

 fall about 6 h 30 m p. m. and continued for an hour or more. In figure 5 

 the effect of a wind storm is shown between 12"30 m and 3"30 m p.m., 

 July 28, and a less violent one on the following day, rising about noon 

 and ending in the heaviest shower of the season, which lasted from 

 2 h 30 m to about 3 p. m. With this shower, as is quite usual, the wind 



