MISCONCEPTIONS CONCERNING THE WEATHER 135 



aceompaii}'mg or immediately following great battles is not unlike that 

 which might have been expected in the course of natural events. Bear- 

 ing in mind the fact, already stated, that throughout large areas rain 

 occurs on an average once in three or four days, and also the subjective 

 fact that rain associated with July 4 celebrations or with battles would 

 doubtless not have been remembered had it not been for such associa- 

 tions, the hypothesis appears to have no foundation. In 1892 the U. S. 

 Government disproved the idea by experiments in which violent explo- 

 sions of dynamite were produced within clouds by means of kites and 

 balloons, with no rain following as a direct or even as an indirect result. 

 The practise, still followed in various European countries, of attempt- 

 ing to prevent hail by bombarding approaching clouds or of projecting 

 vortex rings of smoke upward, also is without scientific basis. The 

 relatively feeble convectional currents resulting from these artificial 

 attempts to influence the weather are too meager to have any appre- 

 ciable effect upon the massive convection accompanying storms and are 

 wholly inadequate to influence precipitation. 



It is often maintained that cold waves are produced by a descent 

 of cold air from aloft. While it is true that the air aloft is colder than 

 that at the ground, and that up to a height of about six miles there is a 

 more or less uniform decrease of temperature with increase of height, 

 cold wave* owe their origin to a number of factors. Nearly all cold 

 waves of the United States occur in the area forming the rear of a pass- 

 ing cyclone and the front of an approaching anticyclone. During the 

 winter half-year this region is characterized by relatively strong north- 

 erly or northwesterly winds, clearing skies, decreasing humidity, and the 

 conspicuous fall in temperature. There is a distinct gyratory move- 

 ment in large disks of air, clockwise, outward from the center, and to a 

 slight extent descending, in the anticyclone, while it is counter-clockwise, 

 inward toward the center, and to some degree ascending in the cyclone. 

 The sharp fall in temperature forming the cold wave is caused primarily 

 by the horizontal transportation of huge masses of cold air from the 

 cold continental interior, and is heightened by the increased radiation 

 from the ground through clear, dry air thus brought in. Vertical cur- 

 rents are probably only of secondar}^ importance in this connection. 



In comparing the climates of different places too much stress is 

 generally laid upon mean, and not enough upon extreme conditions 

 of the weather. For example, the average annual temperature, often the 

 only climatological fact quoted in the description of a place, may be 

 very deceptive. Based upon the records of 33 years, the mean annual 

 temperatures of "Washington, D. C, and San Francisco, Cal., are prac- 

 tically the same, being 54.7° F. and 54.9° F., respectively. The 

 climates of the two cities are greatly unlike, however. Washington has 

 a semi-continental climate, with daily maximum temperatures in sum- 



