66 ANNUAL REPORTS OF DEPARTMENT OF AGRICULTURE, 



SOLAR RADIATION INVESTIGATIONS. 



Continuous records of the total radiation received on a horizontal 

 surface from the sun and sky are obtained at Washington, D. C, 

 Madison, Wis., and Lincoln, Nebr., and the daily totals are pub- 

 lished month by month in the Monthly Weather Eeview. For the 

 two first-named stations, which now have records covering periods 

 of 5 years and 6 years, respectively, the daily departures of radia- 

 tion from the normal, and the accumulated excess or deficiency of 

 radiation for the month and since the first of the year, are also 

 published. During the crop-growing season of 1915 the accumu- 

 lated departures were quite insignificant at Washington, but showed 

 a marked deficiency at Madison. Between May 1 and August 10 this 

 deficiency amounted to 14 per cent of the normal radiation, and be- 

 tween May 1 and September 30 it amounted to 11 per cent. Between 

 May 1 and August 31 the mean daily temperature over the State of 

 Wisconsin averaged 4.5° F. below the normal, and the development 

 of corn and some other crops was greatly retarded. During the first 

 half of 1916 Madison recorded about the normal amount of radiation, 

 but Washington showed a deficiency of about 7 per cent. There is 

 evidence that this deficiency was even greater in New England and 

 the North Atlantic States, where the development of certain crops 

 was markedly retarded. The exact relation between plant develop- 

 ment and the amount of the incoming radiation, and whether the 

 relation is a direct one, or is a secondary effect of the resulting 

 lower temperatures, are questions that can not be answered definitely 

 until the radiation measurements have extended over a longer period 

 of time. 



Measurements of the intensity of direct solar radiation on a surface 

 normal to the incident solar rays have been continued at Washington, 

 D. C, Madison, Wis., Santa Fe, N. Mex., and Lincoln, Nebr. At 

 each of these stations except the last named the series of readings 

 extends over a sufficient number of years to give reasonably accurate 

 monthly normals. Between July 1, 1915, and June 30, 1916, the 

 monthly means of the intensities measured were generally above the 

 normal at Madison and Santa Fe, and below the normal at Wash- 

 ington. At the two first-named stations the monthly maxima have 

 generally exceeded those of 1914-15, and are the highest that have 

 been measured since the depression in radiation intensities that fol- 

 lowed the eruption of Katmai Volcano in Alaska in June, 1912. 



