80 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 65 



the effective radiation has a maximum at an altitude of between 

 1,000 and 4,000 meters. Between 2,500 and 4,400 meters the mean 

 gradient is generally negative; between 1,200 and 2,500 meters it 

 generally has a positive sign. From the general discussion and the 

 curves that represent ideal cases it is probable that the effective radia- 

 tion always decreases with an increase in altitude, when about 3,000 

 meters is exceeded. Up to that altitude we shall generally find an 

 increase of the effective radiation with the height. The latter condi- 

 tions are demonstrated by my simultaneous observations at Indio 

 and Mount San Gorgonio (table XIIIb), as well as by Pernter's 1 . 

 observations at Rauris and on the top of Sonnblick. 



B. INFLUENCE OF HAZE AND ATMOSPHERIC' DUST UPON THE 

 NOCTURNAL RADIATION 



From the observations made in Algeria, the conclusion was drawn 2 

 that a slight haziness, indicated by a decrease in the transmission by 

 the atmosphere of visible rays (clouds not formed), had no appre- 

 ciable influence upon the radiation of the atmosphere. In fact it was 

 found from pyrheliometric measurements during the day that the 

 transmission of the atmosphere generally kept a high or low or 

 average value during periods of several days, the changes being slow 

 and continuous from one extreme to the other. The assumption 

 being made that the nights falling between days of a certain value of 

 transmission can be classified as showing the same character as the 

 days, it was found that -the nocturnal mean radiation during nights 

 belonging to a period of high transmission only differed within the 

 limits of probable error from the mean value obtained during low 

 transmission periods. 3 



The observations at Bassour, Algeria, were taken at a time when 

 the volcanic dust from the eruption of Mt. Katmai at Alaska caused 

 a considerable decrease in the sun radiation transmitted to the sur- 

 face of the earth. Several observers, such as Hellmann, 4 Abbot and 

 FOwle, 5 Kimball," Jensen, 7 and others, all agree as regards the prob- 



1 Pernter, loc. cit. 



2 A. Angstrom : Studies in Nocturnal Radiation, I. Astroph. Journ., June, 

 1913. 



3 Abbot and Fowle : Volcanoes and Climate, 1. c, p. 13. 



4 Zeitschrift f iir Meteorologie, Januari, 1913. 



5 Volcanoes and Climate. Smithsonian Misc. Collections, Vol. 60, No. 29. 



6 Bulletin of the Mount Weather Observatory, Vol. 3, Part 2. 



7 S. A. Mitt. d. Vereinigung von Freunden d. Astronomie und kosm. 

 Physik, 1913. 



