Section III., 1906. [127] Trans. R. S. C. 



VIII. — Toil pc rature EecorcU of Nocturnal Radiation. 

 By Howard T. Barnes, D.Sc. 



Associate Professor of Physics, McGill University, Montreal. 

 (Read May 23rd, 1906). 



The problem of the nocturnal cooling of the atmosphere and the 

 earth is one of such importance in meteorology that it has attracted 

 rhe attention of many investigators. 



On clear nights the surface of the earth is cooled by emission 

 of heat into space. The temperature of the lower layers of air 

 fall, and being heavier remain on the ground. It has been shown by 

 Tyndall and others that pure dry air is almost completely diathermanous 

 to heat waves, and hence, during a clear, cold night in winter, very 

 little, if any, radiation takes place from the air to the sky. 



We know also that the character of the radiation from the earth 

 into space must be different to the forms of radiation usually 

 studied, such as that from a heated body to the face of a ther- 

 raophile. The radiation from a low temperature source, such as a sur- 

 face at 100°, is composed of a greater proportion of long heat rays than 

 the radiation from a high temperature source, such as an incandescent 

 mantle, or arc light. . Little or nothing is known of the character of 

 the radiation from a cold surface, such as the earth in winter, to the 

 absolute cold of space. The inadequacy of applying the results obtained 

 in a study of the radiation from hot bodies to the problems of nocturnal 

 radiation is at once apparent. 



So difficult is the treatment of the problem that very little has 

 at present been done. 



Dr. S. Tetsu Tamura has shown this very well in a paper on the 

 Mathematical Theory of the Nocturnal Cooling of the Atmosphere ^ 

 in which he gives a careful historical survey of the various experimental 

 results that have been obtained. 



Historical. 



In 1783, Patrick Wilson ^ of Glasgow, suspended a mercury ther- 

 mometer, at a height of al)out four feet above the surface of the snow 

 in an open field, under a clear sky and calm atmosphere, at night. A 

 second thermometer was placed on the snow surface. He observed on 

 one occasion a difference of six degrees Fahrenheit between the two 

 thermometers. The one on the snow read — 21-7° Fahr., and the one 



* Monthly Weather Review 33 p. 138 (1905). 

 ' Phil. Trans. Roy. Soc. Edin. I p. 153, 1783. 



