THE THUNDERSTORM— EVANS AND McEACHRON 



193 



the side. Thus, in the thunderstorm shown in plate 1, figure 1, the 

 downward and outward sweep of the rain Hne is undoubtedly due to 

 such air currents. In this particular photograph, which was taken 

 by one of the authors from a mountain top in northern Idaho, the rear 

 of the rain area is shown. Similar air currents blow from the fronts 

 of thunderstorms. By assuming that this photograph shows the 

 front of a storm moving toward the right, the shovelhke action of the 

 air currents can be readily visualized. 



The upward movement of warmer air, which is caused by the 

 shovel action of the cold air currents, is made evident by the upward 



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Figure C— Schematic cross-section of a thunderstorm showing adjacent downward and upward air currents 

 which help maintain the storm. A, ascending air; D, descending air; C, storm collar; S, roll scud; 

 D', wind gust; II, hail; T, thunderheads; R, primary rain; R', secondary rain. (After Humphreys.) 



movement of small cloud fragments called scud cloud which frequently 

 are present below the main cloud base. At times these can be seen 

 moving up toward the cloud base at very high velocities. The 

 presence of the adjacent upward and downward moving air columns 

 is also shown by the rotation of a mass of condensed vapor (called the 

 roll scud) which is sometimes plainly visible just below the cloud base 

 at the front of the rain area. Its direction of rotation and location 

 are shown at S in figure 6. A roll scud rotating as shown in the 

 diagram has been seen at the front of a storm by one of the authors 

 during thunderstorm studies ^ in Colorado. 



THUNDERSTORM PREDICTIONS 

 FREQUENCY 



An estimate of the number of thunderstorm days to be expected in 

 any part of the United States per month and per year can be made from 

 Alexander's^^ isokeraunic ^^ maps, shown in figures 7-11. These 



" Alexander, W. H., Distribution of thunderstorms in the United States, Mon. Weather Eev., vol. 52, 

 p. 337, 1924. 

 » Equal frequency of thunderstorm days. 



31508—38 14 



