!76 



PHILOSOPHY OF STORMS. NO. VHI. 



BY PROF. W. L. ATLEE, M. D., PHILADELPHIA, PA. 



Wlifiiiever, therefore, the dew-point is very little below the tempera- 

 ture of the air, and the cloud very narrow and very lofty, and reaches 

 down so as to touch the earth, the storm will take the form of the loatcr 

 spout if at sea, and the tornado if on land. The lower part of the cloud, 

 or that which forms below the original base, in consequence of the levity 

 of the- cloud itself, will be in the form of an inverted cone. 



The length of this inverted cone will vary with the difTerence be- 

 tween the dew-point and the temperature of the air within the ascending 

 column under the base of the cloud. For example, if the dew-point be 5 

 degrees below the temperature of the air, the inverted cone will be 500 

 yards long ; if it be 6 degrees, it will be 600 yards long ; and thus for 

 every additional degree of diflerence between the dew-point and temper- 

 ature, the cone will be 100 yards longer. 



This forming of the cloud lower and lower in the up-moving col- 

 umn under the cloud is not only indicated by the thermometer, but de- 

 pends upon the same circumstance, which causes the sinking of the bar- 

 ometer, and corresponds also with the fluctuations of the latter instru- 

 ment. For every fifth of an inch that the barometer sinks, the cloud 

 will beo-in to form about 100 yards lower, so that, if the barometer 

 should fall, in one of these tornadoes, two inches, the air, on coming in 

 under thq cloud, will cool by diminished pressure about 10 degrees, and 

 the inverted cone might be 1000 yards long, and would then reach to 

 the earth, if the dew-point was only 10 degrees below the temperature 

 of the air, at the time the cloud began to form. 



The velocity of the air upwards in one of these spouts will be iu 

 proportion to the fcill of the barometer in the centre of the column, in- 

 creased a little by its rise in the annulus. This may be calculated by 

 an observer over whom the middle of the cloud passes, by the following 



formula : Note the height of the barometer at the moment of the calm 



which precedes the storm, and also at the moment of the calm in the 

 middle of tlie storm; take the difference in inches — 8 times the square 

 root of 900 times this difference will be the velocity in feet per second 

 of the upward motion of the air in the centre of the storm. For ex- 

 aniple : if the barometer should sink one inch in the centre of a storm, 

 the air would rush upwards with a velocity due to a head of pressure 

 equal to one inch of mercury. This is equal in weight to about 900 

 feet of air of mean density at the earth's surface. On the supposition 

 of its having this density, the pressure would of course be this much 

 less in the inbidc than the outside of the column. Now, if we subject 



