Heat and its Importance in Meteorology. 377 



(5.) The passage of a steady atmospheric current over a 

 mountain range depositing rain and snow on the windward 

 side but descending as dry air on the leeward side gives rise to 

 a standing wave such as may be seen above any obstacle in a 

 rapid river. Thus kinetic energy is converted into static pres- 

 sure and therefore on the leeward side under the summit of 

 the wave down to the earth's surface there is a somewhat higher 

 pressure than there would be in case no such current existed. 

 The clear descending air by its dryness has also a slightly 

 greater density than before at the same temperature and pres- 

 sure which adds somewhat to the barometric excess. The cool- 

 ing by radiation annuls approximately the warming by com- 

 pression and again gives increased density. These three factors 

 therefore conspire to increase the pressure at the earth's surface 

 and this is again further increased largely by the influences 

 of the earth's rotation and the southward flow as explained by 

 Ferrel and Helmholtz. This is the best explanation that T 

 have yet been able to frame of the formation of the high areas 

 and cold waves that move southeastward over Canada and the 

 United States. I believe that I first stated my conclusion as 

 to this mechanism when, in 1876, I had occasion to urge the 

 importance of maintaining meteorological stations in Alaska. 

 The reports from these stations as subsequently established, as 

 well as the international maps of the Signal Service, served to 

 confirm that view. The original current on the Pacific side of 

 the Rocky Mountains may be due either to a special cyclone 

 or to' the general circulation of the atmosphere resulting in a 

 standing wave whose summit is over the Mackenzie River ; 

 when the current temporarily ceases, the summit and the high 

 area die away ; and when it is strongest, the high pressure is 

 the greatest ; generally the original current may be considered 

 as a temporary overflow from northern Siberia along the arctic 

 circle to Alaska. 



Washington, March, 1892. 



[P. S. As I finish this paper I perceive from the last number 

 of the Meteorologische Zeitschrift that Dr. William Trabert of 

 Yienna has repeated and confirmed Maurer's calculation of the 

 coefficient of radiation. But the values (respectively 0*036 

 and 0'032 calories per hour per kilogram of air) thus deduced 

 from meteorological observations cannot be preferable, as I 

 have above stated, to the direct experimental determination. 

 Trabert gives the values of the annual average temperature of 

 the enclosure toward which radiation is supposed to have taken 

 place during the night, which values are between 1 and 8 

 degrees Centigrade below the temperature of the radiating 

 mass.] 



