VERTICAL DISTRIBUTION OF THE REFRACTIVE INDEX 213 



and North Platte, Nebr. At this time the entire cross section is charac- 

 terized by weak to moderate gradients of refractivity. On figure 5.46 

 12 hours later, this front had moved some 300 km southward. The 

 contrast of the southward push of polar air and the northerly advection 

 of tropical maritime air from the Gulf of Mexico into the developing warm 

 sector of the polar front wave is evidenced by the relatively large gradients 

 in the neighborhood of Dodge City. The core of tropical maritime air has 

 evidently not progressed far enough northward to displace the warm but 

 dry air that had been over the Great Plains prior to the outbreak, with 

 the result that a region of low A values is found between the front and the 

 tropical maritime air. On figure 5.47 the effects of the Pacific front are 

 apparent in the buildup of a secondary region of high A some 400 km 

 ahead of the polar front, located at this time over Dodge City. Twelve 

 hours later (fig. 5.48) the core of tropical maritime air has become more 

 extensive and now reaches to a height of 3 km. The second (Pacific) 

 front is picked up now on the cross section and the area of low A values is 

 confined between the two fronts. By 0300 UT on the 20th, figure 5.49, 

 the polar front is approaching Lake Charles and the Pacific front is re- 

 ported on the daily weather map as a squall line. Finally, by the morning 

 of February 20 (fig. 5.50), both fronts have passed to the south of Lake 

 Charles, and the polar air just behind the front is characterized by rela- 

 tively low A values. 



The use of space cross sections does not always yield measurements 

 at the most desirable points along a frontal zone. Another method of 

 arriving at the probable refractive index structure about the frontal 

 interface is to plot radiosonde observations for a single station arranged 

 according to observation times as on figures 5.51 to 5.57. 



The time cross section for Glasgow, Mont. (fig. 5.51), which is in the 

 cold air behind the polar front for the entire period of observation of the 

 storm, displays gradients of A values that are generally weak. An excep- 

 tion occurs on the evening of the 19th (20/0300 UT) apparently as a 

 result of subsidence effects. Rapid City, S. Dak. (fig. 5.52), similar to 

 Glasgow, shows generally weak gradients throughout in the cold air be- 

 hind the front. North Platte, Nebr. (fig. 5.53), exhibits moderate 

 gradients with increasing A values in the post-frontal, higher-density polar 

 air. This station is too far north to record much in the way of moisture 

 effects at this time of year. Dodge City, Kans. (fig. 5.54), represents 

 dry low ahead of the polar front and increasing A values in the cold air 

 just behind the front. Oklahoma City, Okla. (fig. 5.55), represents a 

 classic synoptic situation in which advection of tropical maritmie air 

 from the Gulf of Mexico produces a strong high ahead of the front and a 

 low within the cool, dry, polar continental air. Again in the region 

 around Little Rock, Ark. (fig. 5.5G), there is warm, moist air of extremely 

 high refractivity ahead of the front being replaced by polar continental 



