PART VI — PRECIPITATION AND REGIONAL WEATHER PHENOMENA 



Summary 



In summary, the modification of 

 cold fogs with ice nucleants is an 

 operational success, and further en- 

 gineering improvements are to be 

 expected; but there are only a few 

 regions where the frequency of cold 

 fogs is sufficiently high to justify the 



expense of a permanent installation. 

 Warm-fog modification by heat or 

 by seeding with hygroscopic particles 

 is achievable in the relatively near 

 future. The requirements for success 

 are more adequate numerical models 

 of fog and, most importantly, imagi- 

 native engineering design so that the 

 assumptions made in the experi- 



mental design can be realized in 

 practice. However, it remains true 

 today, as thirty years ago, that the 

 total cost of warm-fog modification 

 will be high enough to discourage its 

 extensive application. Some recent 

 benefit/cost figures are shown in Fig- 

 ure VI-9. 



Figure VI-9 — RESULTS OF FOG-SEEDING PROGRAMS 



The table lists the operational benefits versus costs experienced by United Airlines 

 during the winter of 1969-70. Benefits have been calculated as monies that would 

 have been spent were fog dispersal not available or unsuccessful. Cost of delays 

 were computed from crew salaries, aircraft maintenance, and fuel and oil costs. 

 Diversion costs included alternate ground transportation, meal and hotel costs, 

 and overtime charges for ground personnel. Not included were intangibles or 

 incomputables such as maintenance dislocation, ferrying equipment, need for 

 reserve aircraft, and mispositioning of flight crews when flights were diverted. Also 

 not included is the cost of customer inconvenience when fog disrupts operations. 

 It is of interest to note that benefits were twice the costs of the program at Los 

 Angeles even though fog was successfully dispersed in only 32% of the cases. 



Fog Dispersal Techniques 



To assess the present state of fog- 

 dispersal techniques and define the 

 work to be done, it is necessary to 

 consider three types of fog. 



Ice Fog 



This type of fog is an increasing 

 problem for aviation and other forms 

 of transportation in a few high-alti- 

 tude localities. Comparatively little 

 research has been done to develop 

 economical methods of combating ice 

 fog. The only technique available at 



present is the brute-force method of 

 applying heat to evaporate it. Fur- 

 ther research is required to assist in 

 the development of more efficient 

 means of thinning or dispersing this 

 type of fog. 



Supercooled (Cold) Fog 



In the contiguous United States, 

 approximately 5 percent of the dense 

 fogs that close airports to opera- 

 tions are of the cold type. In more 

 northerly latitudes, the percentage is 



higher during the winter. Other 

 forms of transportation are equally 

 affected when visibility drops below 

 one-half mile, but the economic im- 

 pact is probably not as great as it is 

 on aviation. 



Dry-Ice Dispersing Techniques — 

 Dispersal of cold fog by seeding 

 crushed dry ice from light aircraft is 

 an operational reality at approxi- 

 mately a dozen airports in the United 

 States. Some of these programs have 

 been established each winter since 

 1962. The physical changes that 



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