Blodget.] 150 [Mayl, 



A DOWNWARD ATMOSPHERIC CIRCULATION, AS ONE 

 CAUSE OF EXTREMES OF COLD. 



By Lorin Blodget. 

 {Read hefoi^e the American Philosophical S-^ciety, May 1st, 1874). 



The system of atmospheric circulation which gives us a general easterly- 

 movement in temperate latitudes, scarcely needs further explanation, yet 

 the recent establishment of observatories on Mount Washington and on 

 Pike's Peak, are found to afford positive evidence in verification of such 

 movement that is full of interest. The easterly current on the top of 

 Mount Washingon, at least, is almost constant and with extreme velocity, 

 iu a direction the resultant of which is almost due east, and there are no 

 conditions apparent to throw doubt on the general assumption that this 

 is the returning current of a vast system of atmospheric circulation to 

 and from the tropics, primarily, through which the heat and humidity 

 of the tropics are widely diffused at both the northern and southern tem- 

 perate belts. 



But I propose only to refer to some deductions that have for some time 

 past impressed me with great force, as to the origin of certain almost in- 

 explicable facts of our climate, at points near the northern border of this 

 system of circulation ; and particularly in the colder parts of the United 

 States, east of the Rocky Mountains, in winter, and indeed, in all the 

 cold months. I had the honor to lay before this Society on a former oc- 

 casion, sf>me suggestions as to the origin of the extremes of cold observed 

 at vai-ious points, chiefly of the northwestern interior, and to express the 

 conviction that these extremes were not propagated, or transferred along 

 the surface, as a part of what is usually thoirght to be the surface circu- 

 lation from the west ; and also that they do not move down — that is, 

 along the surface — from the north, or from any other point of the com- 

 pass. On the contrary, they appear to be instituted or established at the 

 point of their most extreme existence, as if brought down from the upper 

 atmosphere, or as if the result of the action of causes extraneous to the 

 earth's atmosphere. 



The recent extension of observations to the territories of the plains, 

 and to posts on both sides of the Rocky Mountains, has given us a new 

 basis of facts for the discussion of the symmetrical climates of the eastern 

 United States, as I may call them, since their principal changes are 

 usually quite symmetrical ; — and it has disclosed the fact that no sym- 

 metry or cori-espondence of phenomena can be traced across the Rocky 

 Mountains, connecting any great storm, or any area of excessive heat, or 

 excessive cold, with any like condition at the east. I have been particu- 

 larly observant of such facts as I could obtain in regard to this point 

 along the northern belt, for the purpose, first, of tracing, if possible, the 

 origin of the remarkable extremes of cold occurring in Dakota and Min- 

 nesota ; and have spent much time in examining these cases, with the 

 result of coming to the conclusion that there is absolutely no connection 



