10 



MASS. EXPERIMENT STATION BULLETIN 402 



on a clear day is least about December 21 and from then increases to a maximum 

 on June 21. Perfectly clear days, however, are infrequent; haze in the atmosphere 

 and cloudiness cause the light intensity to vary between wide extremes, values 

 as low as 3 or 4 per cent of the June 21 mean maximum occurring when clouds 

 are very dense. 



Some measurements of light intensity under ice on winter-flooded bogs were 

 made by means of a photoelectric cell of the barrier layer type which was con- 

 nected with a quadruple range portable microammeter having a range of to 4000 

 microamperes. The photoelectric cell was enclosed in a watertight brass case 

 'provided with a glass window in the top. All readings of light intensity were 

 taken with the active surface of the photoelectric cell in a horizontal position 

 corresponding to the angle of exposure of the ice surface. The same instruments 

 were used to measure the intensity of the incident light but these measurements 

 were compared with those of the sun and sky radiation received on a horizontal 

 surface made with an Eppley pyrheliometer and an Engelhard recording micro- 

 ammeter as described by Hand.* Although the latter instruments measure the 

 heat energy received on a horizontal surface, it has been shown^ that measure- 

 ments thus made may be used to determine the intensity of daylight illumina- 

 tion. The percentage of incident light which was found to pass through ice or to 

 reach the cranberry vines under stated conditions is given in Table 1. 



Table 1. — The Percentage of Incident Light Reaching Cranberry Vines 



Submerged to Different Depths, in June and in Winter 



Under Ice of Different Thickness. 



Radiation Percentage of Incident Light 



Intensity Penetrating — 



as Percent Ice 



Date Time of Day of Mean Thick- Ice of thickness given 



Maximum ness Ice and water to depth of — 



Intensity (o) Only 



on June 21 12 in. 15 in. 18 in. 



Percent Inches Percent Percent Percent Percent 



Junes, 1938. . . 12:45-1:15 p.m. 55-60 90-93 86-88 

 Ditto 9:10-9:45 a.m. 35-38 85-89 79-81 



Jan. 14, 1941... 11:50 a.m.-12 m. 52-55 4-4.5 86-88 70 65 

 Ditto 2:00-2.05 p.m. 40 4 74-83 52-55 



Jan. 15. 1941... 10:00-10:15 a.m. 7-9 4.5 65-70 50-55 45-58 

 Ditto 9:00-9:20 a.m. 5-7 5.5 57-62 45-50 



Feb. 2, 1942.... 2-40-2:45 p.m. 40 5.5 65-70 53-55 



Ditto 2:30-2:35 p.m. 32-34 6 60-67 40-45 



Ditto 2:45-2:50 p.m. 28-30 6 61-64 35-40 



Feb. 11. 1942... 1:00-1:05 p.m. 11-13 7.5 40-45 33-35 

 Ditto 2:40-2:45 p.m. 30-32 7.5 34-39 27-30 



Jan. 24. 1940. . . 12:10-12:25 p.m. 6-86 7.5-8c 21-25 

 Ditto 2:45-3:00 p.m. 2-46 7.5-8<; 10-17 



Apr. 30, 1941 . . 9:20-9:25 a.m.. 60 9.5a 90-96 



Jan. 25, 1942... 9:00-9:15 a.m. 11.5 47-51 



a Snow included in the ice in all instances except on April 30, 1941. 

 b Densely cloudy, snowing. 



c No snow on the ice where measurements of light intensity were made, but near by were 

 patches of snow 3^ -1 inch deep. 



''Hand, Irving F. Monthly Weather Review 65:415-441. 1937. 

 ^Kimball, H. H. Monthly Weather Review 52:473-479. 1924. 



