. Compendium of meteorology. Meteorology. NIGHT-SKY RADIATIONS 343 small, rarely as much as a factor of 2. During the full nighttime hours there appear to be many small irregular and complex intensity fluctuations which vary with the > 4 I- t 3 I I I I I I I M I I I I I I I M I I I I M I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I 1 3000 4000 5000 WAVE LENGTH (A) 6000 Fig. 2.âSpectral energy distributions of night-sky (After Babcock and Johnson [3].) light. place in the sky and which are different for the various wave lengths. There are strong intensity changes d
. Compendium of meteorology. Meteorology. NIGHT-SKY RADIATIONS 343 small, rarely as much as a factor of 2. During the full nighttime hours there appear to be many small irregular and complex intensity fluctuations which vary with the > 4 I- t 3 I I I I I I I M I I I I I I I M I I I I M I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I 1 3000 4000 5000 WAVE LENGTH (A) 6000 Fig. 2.âSpectral energy distributions of night-sky (After Babcock and Johnson [3].) light. place in the sky and which are different for the various wave lengths. There are strong intensity changes during twihght. Diurnal Variations. In France, Dufay and Tcheng Mao-lin [10] obtained 588 spectrograms during 189 nights from October 1940 to January 1944. From these spectrograms systematic measurements were made of the intensity variations of the oxygen lines 5577 and 6300 A and of the sodium pair 5893. The oxygen line 5577 A usually showed a weak maximum around mid- night (not more than 40 per cent) often obscured by fluctuations; 6300 and 5893 A merely weakened slowly during the night, the enfeeblement for 6300 being greater than that for 5893 A, which amounted only to 10 or 15 per cent. In Russia [23] photocell measurements of radiation in the infrared region 850 to 11000 A indi- cated that the intensity was at a maximum around midnight, being about twice as great then as at 10 and 2 Twilight Ejfects. The green line 5577 was observed to exhibit no twilight enhancement. On the other hand, the sodium yellow emission 5893 dropped suddenly in intensity to one per cent of its former value at about ten minutes after sunset [7] with a corresponding recrudes- cence near dawn. Likewise the red oxygen lines 6300 showed a similar but slower change during twilight [13]. The strong Nt flash at twilight discovered by Slipher is intense in aurorae but absent from the night sky. Annual Variations. In temperate latitudes 5577 has slight maxima in October and February, and a 27-da
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