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BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



evaporation from the radio-atmometer and from the white porous 

 atmometer of the same size and form is practically identical, by day 

 the black cup, on account of its higher temperature, shows a greater 

 fate of evaporation. Therefore the difference in the evaporation 

 from the radio or black atmometer and the white atmometer affords 

 a rough estimate of varying intensities of sunlight for the different 

 periods and type stations. The actual variation intensity, of course, 

 bears an important relation to transpiration and photosynthesis. 



Only the duration of sunshine is measured by the Marvin sunshine 

 instrument of the Weather Bureau pattern. Unless the investigator 

 is working in the vicinity of an experiment station or similar base, 

 the Marvin sunshine recorder can not, of course, be operated. The 

 black and white porous cup atmometers, on the other hand, may be 

 set up wherever desired. Accordingly, it is worth while to compare 

 the sunshine records obtained from these two instruments. The data 

 are given in Table 8 and are shown graphically in figure 10. 



TABLE 8. Comparison of sunshine records obtained from a sunshine recorder 

 of the Weather Bureau pattern, and the difference in evaporation between 

 atmometers with black and white spheres. Readings taken in August, 1916. 



In following the slope of the curves as shown in figure 10, an agree- 

 ment of 67 per cent (20 periods out of 30) is found, and the disa- 

 greements in practically all cases are relatively slight. The most 

 conspicuous differences in ordinate values occur for days when the 

 sky is partly cloudy and the sunshine more or less intermittent. In 

 such instances there is almost invariably a much greater fluctuation 

 in the values of the atmometric sunlight index than in the values of 

 sunshine duration as recorded by the Marvin recorder. This may be 

 accounted for by the fact that the evaporation from the black surface 

 atmometer responds more quickly to fluctuation in sunshine than 



