260 The National Geographic Magazine 



circles. The word "high" is written 

 at the center of the region of greatest 

 air pressure and the word ' ' low ' ' at 

 the center of the area of least pressure. 

 Under the influence of gravity the air 

 presses downward and outward in all 

 directions, thus causing it to flow from 

 a region of great pressure toward one 

 of less. The velocity with which the 

 wind moves from the high toward the 

 low will depend largely on the differ- 

 ence in air pressure. To better illus- 

 trate : If the barometer read 29.5 at Chi- 

 cago, 111., and 30.5 at Bismarck, North 

 Dakota, the difference of one inch in 

 pressure would cause the air to move 

 from Bismarck toward Chicago so rap- 

 idly that after allowing for the resistance 

 of the ground there would remain a 

 wind at the surface of the earth of about 

 50 miles per hour, and Lake Michigan 

 would experience a severe ' * north- 

 wester." 



CYCLONIC STORMS 



- Chart No. p shows a winter storm (cy- 

 clone) central in Iowa at 8 a.m., Decem- 

 ber 15, 1893. The word "low" marks 

 the storm center. It is the one place 

 in all the United States where the ba- 

 rometer reading is the lowest. The 

 heavy, black lines, oval and nearly con- 

 centric about the low, show the gradation 

 of air pressure as it increases quite uni- 

 formly in all directions from the storm 

 center outward. 



The arrows fly with the wind, and, 

 as will be seen, are almost without ex- 

 ception moving indirectly toward the 

 low or storm center, clearly demonstrat- 

 ing the effect of gravity in causing the 

 air to flow from the several regions 

 marked high, where the air is abnor- 

 mally heavy, toward the low, where the 

 air is lighter. As the velocity of water 

 flowing down an inclined plane depends 

 both on the slope of the plane and on 

 the roughness of its surface, so the ve- 

 locity of the wind as it blows along the 

 surface of the earth toward the storm 



center depends on the amount of the de- 

 pression of the barometer at the center 

 and the resistance offered by surfaces of 

 varying degrees of roughness. The 

 small figures placed at the ends of the 

 arrows indicate wind velocities of six 

 miles per hour and more. At Chicago, 

 where the wind is blowing at the rate 

 of 40 miles per hour, the anemometer is 

 270 feet high, while at Minneapolis, 

 where the instrument is so low as to be in 

 the stratum whose velocity is restricted 

 by the resistance encountered in flowing 

 over forests to the northward, the rate 

 is not great enough to be marked by a 

 figure. At Chicago and Davenport the 

 wind is blowing against the pressure 

 gradient, away from the low. This is 

 due to the fact that it has flowed swiftly 

 from the south and gained such momen- 

 tum that it rushes by the storm center 

 before the gradient on the north of the 

 center can overcome its movement and 

 turn it. 



Now picture in your mind the fact 

 that all the air inside the isobar (heavy 

 black line) marked 30.2 as it moves 

 spirally inward is rotating about the low 

 in a direction contrary to the movement 

 of the hands of a watch, and you have a 

 very fair conception of an immense at- 

 mospheric eddy, or cyclone. 



Have you ever watched the placid 

 water of a deep running brook and ob- 

 served that where it encountered a pro- 

 jecting crag little eddies formed and 

 went spinning down the stream ? Well, 

 storms are simply great eddies in the 

 air that are carried along by the general 

 easterly movement of the atmosphere in 

 the middle latitudes of both hemispheres 

 and by the westerly movement of the 

 general circulation in the tropics. But 

 they are not deep eddies, as was once 

 supposed. The low marks the center 

 of an atmospheric eddy of vast horizon- 

 tal extent as compared with its thickness 

 or extension in a vertical direction ; thus 

 a storm condition extends from Wash- 

 ington to Denver in a horizontal direc- 



