

occur are well understood, stemming 

 from the research of Schaefer, Lang- 

 muir, and Vonnegut in 1946. Al- 

 though the dry-ice technique is 

 theoretically effective in converting 

 supercooled water to ice crystals only 

 at temperatures colder than —4° cen- 

 tigrade, operational experience has 

 demonstrated unequivocally that this 

 technique is effective up to centi- 

 grade through proper sizing of the 

 dry-ice pellets and proper control of 

 the seeding rates for the conditions 

 prevailing. 



This method of dispersing cold fog 

 is about 80 percent effective. The 

 failures that do occur are primarily 

 related to operational problems such 

 as miscalculating wind drift, which 

 results in the cleared area moving off 

 target. Occasionally, too, the tech- 

 nique is stretched beyond the capa- 

 bility of the physical reactions to take 

 place, typically in supercooled fog 

 decks whose upper layers are several 

 degrees warmer than centigrade. 



Ground Dispensing Methods — Be- 

 cause of such operational problems 

 and the complex logistics that are 

 required in dispersing an airport fog 

 by means of aircraft, a ground dis- 

 pensing system, which employs essen- 

 tially the same physical principles, is 

 more desirable. Liquid propane has 

 been used effectively as the seeding 

 agent; it has reached a degree of 

 sophistication in France, where con- 

 trol of supercooled fogs at Orly Air- 

 port is completely automated through 

 the use of seventy fixed dispenser 

 heads deployed around the target 

 area. Liquid propane has been used 

 operationally to combat cold fogs in 

 the United States, but, primarily for 

 economic reasons, the technology has 

 never been developed beyond the use 

 of a few portable dispensing units. 



Researchers have suggested that 

 liquid propane and other cryogenics, 

 in addition to providing the cooling 

 mechanism, also alter the fog drop- 

 lets through a clathration process. 

 Since this latter process may increase 

 the effectiveness of liquid propane 



in fog temperatures several degrees 

 warmer than 0° centigrade, further 

 investigation is warranted. Many air- 

 ports are subjected to dense winter 

 fogs with characteristic temperatures 

 slightly warmer than freezing. De- 

 velopment of this clathration process 

 would pay off in benefits at many 

 airports that cannot support the 

 more expensive warm-fog dispersal 

 programs. 



Warm Fog 



Since all but about 5 percent of 

 the dense fog that closes airports and 

 cripples other forms of transporta- 

 tion in the populated latitudes is of 

 the warm type, it would be expected 

 that there has been some preoccupa- 

 tion with measures to alleviate the 

 warm-fog problem. Formal research 

 into fog physics and development of 

 laboratory techniques for dispersing 

 fog have, however, been under way 

 less than forty years. Out of desper- 

 ation, some brute-force methods for 

 evaporating fog have been under- 

 taken where economics was not a 

 factor. 



Houghton's work at the Massachu- 

 setts Institute of Technology in the 

 1930's was the first formal research 

 aimed at fog dispersal. A number 

 of other studies on warm fog were 

 subsequently undertaken by federal 

 military and civilian agencies, but 

 until the 1960's none of the fog- 

 modification concepts was applied to 

 routine commercial or military activi- 

 ties. Economics, problems of logistics, 

 or deleterious effects on the environ- 

 ment were the deterrents. 



Modern Techniques — At least one 

 installation of a refined thermal sys- 

 tem for evaporating fog at a busy 

 airport is planned for 1972. Other 

 thermal methods that utilize energy 

 more efficiently are under develop- 

 ment. All of these systems are expen- 

 sive and will probably be limited to 

 application at major airports or other 

 sites where the economic pressure of 

 fog paralysis is high. 



For two years, warm fog has been 

 regularly dispersed at a few U.S. air- 

 ports through chemical seeding tech- 

 niques that had been partially con- 

 firmed by fog physics research and 

 laboratory testing. This approach is 

 feasible, and is producing economic 

 benefits exceeding costs of the pro- 

 grams by a factor of about 3 to 1; but 

 it is considered in the developmental 

 stage because aircraft dispensing is 

 required. For full reliability and opti- 

 mum benefit/cost ratios, a ground 

 dispensing system must be developed 

 that will use the most effective mate- 

 rials. A number of promising con- 

 cepts have been conceived and some 

 have been laboratory tested. Further 

 development work is required, but 

 success will depend on better basic 

 knowledge of fog makeup than we 

 have today. 



Basic Warm-Fog Physics — Suffi- 

 cient knowledge of fog physics exists 

 to disperse warm fog with heat. The 

 more attractive and economically 

 feasible approaches to warm-fog dis- 

 persal, which do not employ heat, 

 require more basic physical knowl- 

 edge in order to develop the most 

 efficient system. 



Recent research involving the use 

 of hygroscopic materials as seeding 

 agents has provided some much- 

 needed knowledge about fog, but 

 there are still some baffling blind 

 spots. This new knowledge came fif- 

 teen years after successful feasibility 

 tests were conducted, using the same 

 principle, but which were not con- 

 tinued because of logistic problems. 

 It is hoped that another long delay 

 will not develop before we can ex- 

 plain, for example, why polymers, 

 surfactants, and other substances, 

 when diffused properly, produce posi- 

 tive results, apparently through a 

 strong ionization process. Supersatu- 

 rated solutions of nontoxic materials 

 with endothermic properties, and 

 the electrogas-dynamic principle, are 

 promising dispersal materials and 

 techniques which require further de- 

 velopment, as does research on the 

 physics of fog. 



183 



