326 Prof. Frankland on some Winter L^ a 7 ?> 



different. On the other hand, the absence of suspended watery particles 

 in the air has, no doubt, very considerable influence in preventing the 

 chilling of the skin. JSTot only are such liquid particles present when 

 there is visible fog, but they often exist in great numbers when the air 

 possesses its usual transparent appearance. Another very important in- 

 fluence upon the sun-temperature is the reflection of solar rays from the 

 snow. The valley of Davos is about one mile wide, and has precipitous 

 sides and a flat sole. The villages of Davos-Dorfli and Davos-Platz are 

 situated on the north-west slope of the valley, and consequently receive 

 the scattered solar rays reflected from a large area of snow. I have no 

 doubt that the sun-temperature at the opposite side of the valley is 

 markedly lower ; but having no second sun-thermometer, I could not as- 

 certain this by the comparison of simultaneous thermometric observa- 

 tions. When staying at Ventnor, in the winter of 1872-73, I noticed 

 that a not inconsiderable proportion of the total solar heat falling upon 

 a house on a cliff, near the shore, was reflected from the sea. M. Duf our 

 has since observed the same phenomenon between Lausanne and Yevay 

 on the Lake of Geneva*, and has actually measured the proportions of 

 direct and reflected heat incident at five different stations on the northern 

 shore of the lake. He found that the proportion of reflected heat was as 

 much as 68 per cent, of the heat directly incident from the sun, when 

 the sun's altitude was between 4 38' and 3 34'. At about 7 altitude 

 the proportion was between 40 and 50 of reflected to 100 of direct heat. 

 Even at about 16 altitude the proportion was between 20 and 30 of 

 reflected to 100 of direct heat ; but when the sun was higher than 30, 

 the reflected heat was hardly appreciable. It will be seen that this 

 action of extensive reflecting surfaces of snow or water must exert a 

 powerful influence upon the maximum temperature of places favour- 

 ably situated for receiving the reflected rays ; and, moreover, where 

 the proportion of heat reflected varies (as it has been proved to do in the 

 case of water, and as it doubtless also does in the case of snow) inversely 

 as the angle formed by the incident rays and the reflecting surface, this 

 action must materially contribute, especially in winter, to the main- 

 tenance of an approximately uniform sun-temperature throughout the 

 day. At Davos and similar elevated stations, however, the comparative 

 freedom of the air from suspended liquid and solid particles must 

 obviously contribute, to a still greater extent, to such a result ; for as pure 

 and dry air is transcalent and reflects light but slightly, the horizontal 

 sunbeams, passing through such air, would be nearly as powerful as ver- 

 tical rays. 



The peculiar winter climate of Davos appears, therefore, to depend 

 upon the following conditions : 



] . Elevation above tlie sea, which causes greater rarity of the air, and 



* Comptes Eondus hebdomadah-es des Seances de 1'Academie des Sciences, 

 June 30th. 1873. 



