218 BIOLOGICAL EFFECTS OF RADIATION 



THE VERTICAL COMPONENT OF THE TOTAL SOLAR RADIATION (DIRECT 

 + DIFFUSE) RECEIVED AT THE SURFACE OF THE EARTH^ 



As already noted under the previous section, a considerable percent- 

 age of the radiation received from the sun is scattered, or diffused, by- 

 atmospheric gas molecules, and by dust and other foreign particles that 

 are always present in the atmosphere. The measurement of the vertical 

 component of the total radiation (direct + diffuse) is easily accom- 

 plished, since the sensitive element exposed to radiation is fixed in a 

 horizontal position, and the mechanism, or the labor necessary to keep 

 instruments for measuring the intensity of the direct solar rays pointed 

 accurately toward the sun, is avoided. The measurements of the 

 intensity of the vertical component of solar radiation are, therefore, 

 usually continuously recorded by automatic instruments, thus giving 

 very complete records of this, the most important component of solar 

 radiation, at least to vegetation, and thus indirectly, if not directly, to 

 animal life, including man. 



A considerable collection of data of this character has already been 

 summarized by one of us, and published in the Monthly Weather Review 

 (8, 9) . Table 3 gives the coordinates of stations in the United States at 

 which this component of the intensity of solar radiation is continuously 

 recorded, and in Table 4 are given the average weekly means of daily 

 totals. In the footings of this table are given the average annual totals 

 of radiation for each station. Table 4, therefore, for stations in the 

 United States, shows variations with latitude, altitude, and the season of 

 the year in the daily amount of radiation received on a horizontal sur- 

 face. It also shows that smoky cities like Pittsburgh, New York, and 

 Chicago, receive about one-third less than stations like Riverside, Fresno, 

 and Twin Falls, and one-fourth less than Blue Hill and Washington, 

 which latter are on about the same latitude as the first-named cities. 



In Table 5 are given the hourly averages of the vertical component 

 of solar radiation for weeks that include the dates for which the radiation 

 at normal incidence is given in Table 1. It therefore resembles Table 1 

 in that it shows variations in the intensity of solar radiation with latitude, 

 altitude, the season of the year, and the time of day, but for the vertical 

 component, including that received diffusely from the sky, instead of the 

 intensity at normal incidence. 



SOLAR-RADIATION INTENSITY IN TROPICAL, SUBTROPICAL, AND ARCTIC 



REGIONS 



Table 6 gives the coordinates of stations in the above-named regions 

 for which solar-radiation data are available for considerable periods of 



2 For a reproduction of a record of the two components of the total radiation 

 received on a horizontal surface, see Fig. 18 of the paper by Brackett, p. 172. 



