262 The National Geographic Magazine 



which, like the dark shading, do not ap- 

 pear on chart I, which was purposely 

 left clear of these symbols, so that the 

 movement of wind in accordance with 

 pressure gradients could be the better 

 shown. These red lines connect places 

 having the same temperature. Note 

 how, on both charts, they trend from 

 the Atlantic coast northwestward into 

 the southeast quarter of the cyclone, 

 and where they leave the storm center 

 how precipitately they drop away to- 

 ward the southwest. A cause can be 

 easily found for this by examining the 

 direction of the arrows. In the first 

 case the isothermals are being pushed 

 northward by southerly winds, and in 

 the other forced southward by winds 

 from the northwest. As the cyclone 

 proceeds eastward the regions now un- 

 der the influence of warm southerly 

 winds will be, in less than 24 hours, on 

 the west side of the storm, and cold 

 northwest winds will sweep over them. 



The line of arrows leading from west- 

 ern Wyoming to the center of the storm 

 on chart III shows the place where the 

 cyclonic circulation of wind began that 

 constitutes the storm and the course 

 pursued by the storm center. The 

 small circles surrounding crosses mark 

 the places where the storm was central 

 at each 12-hour interval. The figure 

 above the circle indicates the date, and 

 the letter below evening or morning. 



As previously explained, the large 

 figures give the average temperature for 

 each of the four quarters of the storm 

 within a radius of 500 miles from the 

 center. The same information may be 

 gathered from the isotherms, but cannot 

 be so strikingly presented. Now, remem- 

 bering that the air ascends as it spirally 

 moves around the center, one may see 

 how the cold air of the northwest quar- 

 ter is mingled with the warm air of the 

 southeast portion, which in each of the 

 three cases presented by the charts so 

 far brought into the discussion is more 

 than three times as warm. On chart 



III the two quarters are represented — 

 one by 13 degrees and the other by 47 

 degrees. The mixing of such cold and 

 such warm masses of air and the addi- 

 tion of cold due to expansion as the mix- 

 ture rises is a fruitful cause of precipi- 

 tation, but not the only one, for we see 

 that rain has fallen in the Gulf states, 

 as exhibited on chart III, probably only 

 as the result of cold northwest winds 

 flowing into and mingling with the 

 warm air of the south. Precipitation 

 may also occur as the result of the warm 

 humid air of southerly winds under- 

 running cold and heavier air, and by 

 other processes not yet understood. 



ANTI-CYCLONIC STORMS 



Attention is now directed to the anti- 

 cyclone or high-pressure area shown on 

 these three charts as resting over the 

 Rocky Mountain plateau. Here all the 

 functions of the cyclone are reversed ; 

 hence the name anti-cyclone. The air 

 has a downward component of motion 

 at and for a considerable area about the 

 center, instead of an upward compo- 

 nent ; the winds blow spirally outward 

 from the interior, instead of inward, and 

 are deflected to the left of their initial 

 direction, instead of to the right, and 

 the air is mostly clear, cool, and dry, 

 instead of cloudy, warm, and humid. 

 The center of this high moved but little 

 during the three 12-hour periods, but 

 its area expanded eastward as the low 

 advanced, and if the chart of Decem- 

 ber 17, 8 p. m., were shown the high 

 pressure would be seen to cover with 

 clear, cool weather the region now em- 

 braced within the limits of the low 

 pressure. 



These are winter conditions that are 

 being described. The storms are gen- 

 eral, not local, as in summer, when the 

 highs and the lows exhibit small differ- 

 ences of pressure, move slowly, and sel- 

 dom embrace large areas. The summer 

 type of local storms gradually merges 

 into general storms as the heat of sum- 



