Blodget,] 152 [May 1, 



On several occasions during the present month of April, the weather in 

 tlie seaboard States has exhibited this phenomenon. All of the severely- 

 cold weather, for the season, has been initiated at the point where its 

 greatest severity was experienced ; not being transferred along the sur- 

 face from any point at the west, or at the north. For many days of the 

 present month (April) these cold and heavy winds have been felt in the 

 country east of the Alleghanies, when in no single instance that I can 

 trace, has there been any connection or conformity of movement from the 

 western or northern interior. Like severity has often existed there, but 

 the fact, and all its relations, was local in this sense, or was not connected 

 or continuous with other districts. 



When the enormous friction of atmospheric contact with the surface is 

 taken into account, it must be apparent that there can be few winds of 

 propulsion. I think it may be fairly assumed that the greater number of 

 winds in cold weather particularly, are winds that descend, and that to 

 this descent most of their continued force is due. On Saturday, April 11, 

 and Sunday, April 12, the thermometer fell at Washington under the in- 

 fluence of these obviously descending winds until in the night of the 12th 

 it reached a minimum of 19°, while for three days previous no place west 

 of the Alleghanies in the same latitude was below 50°, and the average 

 temperature at Fort Sully, on the Missouri, 1200 feet above the sea, and 

 in latitude 45^ north, was as warm as at Philadelphia, at sea level, in 

 latitude 42° N. This remarkable depression of temperature could not 

 have been due to radiation, since all the areas west and north were even 

 more exposed to radiation, being clear and calm ; nor was it diie to north 

 or northwest winds propagated along the surface, for there had been no 

 cold winds from these points at the west or north for several days. Nor 

 was there any general storm to effect a displacement or shrinkage, at 

 least no storm on the continent. There may have been some general 

 storm, or shrinkage at sea, however, facilitating or inducing a descent of 

 heavy masses of cold air from above to supply the partial vacvium. 



I venture to assume, therefore, a large measure of influence in causing 

 extremes of cold in these latitudes to the descending volume of air inci- 

 dent to the shrinking and wasting of heat and moisture from the atmos- 

 pheric current eastward in the course of traversing the continent. Its 

 northern border is perpetually invaded by fitful alternations of displace- 

 ment ; sometimes getting calm and intensely cold, to reduce the tempera- 

 ture in winter to 10°, 20° or 30° below zero ; and in spring, when the 

 general accession of heat gives a more free play of the forces, a frequent 

 recurrence of heavy northwest dry winds potired from above, and from 

 the north, displacing and condensing the local, or surface atmosphere ; 

 and this overflow is almost constantly repeated until the whole system of 

 circulation has been swept beyond our limits at the north, by the advance 

 of summer. During most of the summer months the rarifying and ex- 

 panding forces prevail so completely, as to remove all these phenomena 

 far to the north, or possibly to another hemisphere. 



