have been reported by the U.S. Air 

 Force in West Germany and Alaska, 

 by Orly Airport in Paris, and at 

 several commercial airports in north- 

 western United States. Cold fog at 

 most American airports is so infre- 

 quent, however, that the standby cost 

 of a cold-fog modification system 

 probably cannot be justified. (It 

 should be noted that the ice fogs 

 that form in cold regions such as 

 Alaska cannot be modified by seeding 

 with ice nucleants.) 



Warm Fog 



Warm fog is much more common 

 than cold fog. Many methods have 

 been proposed over the years for 

 modifying warm fog, but those that 

 have shown significant success all in- 

 volve the evaporation of the fog 

 drops. The evaporation may be 

 achieved by heating the air, by dis- 

 tributing hygroscopic particles in the 

 fog, or by forcibly mixing the fog 

 with the drier and/or warmer air 

 above the fog layer. 



Heating was employed at military 

 airfields in England during World 

 War II with considerable operational 

 success. This so-called FIDO (Fog In- 

 vestigation and Dispersal Operation) 

 method was further developed at Ar- 

 eata, California, after the war, and an 

 operational system was installed at 

 Los Angeles Airport. Moderate suc- 

 cess was claimed, but the method 

 was abandoned because of the large 

 amounts of fuel required and the 

 psychological and safety hazards of 

 operating aircraft between two lines 

 of flames. 



The fundamental unsolved prob- 

 lem of thermal-fog modification is 

 the uniform distribution of heating 

 throughout the fog. In a typical fog, 

 heating sufficient to raise the air 

 temperature by about 1° centigrade 

 will cause the fog to evaporate in a 

 short time. Arrays of point heat 

 sources, particularly linear arrays, can 

 be expected to lead to convection, 



non-uniform heating, escape of heated 

 air aloft, and horizontal convergence 

 of fog near the surface. The U.S. Air 

 Force has had some success using jet 

 aircraft on either side of a runway 

 as heat sources. Further engineering 

 developments aimed at providing 

 reasonably uniform heating by means 

 of blower-heaters specifically de- 

 signed for the task may be worth- 

 while in view of the basic attractive- 

 ness of thermal-fog modification. 



Hygroscopic particles introduced 

 into fog grow by condensation, 

 thereby reducing the relative hu- 

 midity and leading to the evaporation 

 of the fog drops. This transfer of 

 the liquid water to a small number 

 of larger solution droplets leads to 

 an improvement in visibility in the 

 fog. More complete clearing occurs 

 as the solution droplets fall out under 

 the action of gravity. Hygroscopic 

 particles act something like ice crys- 

 tals in a cold fog, with the important 

 difference that the equilibrium vapor- 

 pressure over the solution droplets 

 rises rapidly as the droplet is diluted, 

 approaching that of pure water. 



To minimize the total quantity of 

 hygroscopic material required to 

 modify a fog, the hygroscopic parti- 

 cles should be as small as possible, 

 consistent with the requirements that 

 they be large compared to the fog 

 drops and that they fall out of the 

 fog in a reasonable time. Since the 

 solution droplets become diluted as 

 they fall, the deeper the fog the 

 larger must be the initial size of the 

 hygroscopic particles. When the 

 depth of the fog is more than a few 

 hundred meters, accretion of the fog 

 drops by the solution becomes an im- 

 portant mechanism in the lower por- 

 tion of the fog. 



Mathematical models of the modi- 

 fication of warm fog by hygroscopic 

 particles have been devised and used 

 to guide field experiments. The the- 

 ory of the growth of hygroscopic 

 particles and the evaporation of fog 

 drops is well established. Reasonably 



adequate information is available on 

 the drop-size spectra and liquid 

 water content of natural fogs. Tur- 

 bulent diffusion is arbitrarily intro- 

 duced on the basis of a few estimates 

 of the eddy-diffusion coefficient in 

 fogs. However, these mathematical 

 models are static in that they do not 

 model the natural processes that form 

 and dissipate fog. Dynamical models 

 must be developed that incorporate 

 these processes. Among other ad- 

 vantages, such models should yield 

 the characteristic time of the fog- 

 formation process. It seems evident 

 that any artificial modification must 

 be accomplished in a time that is 

 short compared to this characteristic 

 time of fog formation. This is of 

 the utmost importance in the design 

 of fog-modification experiments. 



In field experiments, hygroscopic 

 particles have been released from 

 aircraft flying above the fog. The 

 usual assumption that the trailing 

 vortices uniformlv distribute the par- 

 ticles in the horizontal is highly 

 questionable. Failure to achieve uni- 

 form distribution of the seeding par- 

 ticles is probably one of the principal 

 causes of unsatisfactory modification 

 experiments. A non-uniform distrib- 

 ution can be countered only by in- 

 creasing the total amount released to 

 insure that there is an adequate con- 

 centration everywhere. A closely re- 

 lated problem is the marked tendency 

 of the carefully sized hygroscopic 

 particles to emerge in clumps. Imag- 

 inative engineering design is needed 

 to solve these problems, and nothing 

 is more important at the present time. 



Air Mixing — Mechanical mixing of 

 the warmer and/or drier air above 

 a relatively thin fog layer will usually 

 cause the fog to evaporate. The U.S. 

 Air Force has produced cleared lanes 

 by utilizing the strong downwash 

 from helicopters; this technique is 

 effective only in shallow fogs, how- 

 ever. The cost/effectiveness ratio is 

 probably large, but it may be justified 

 for certain military purposes when 

 the helicopters are available. 



181 



