PART VI — PRECIPITATION AND REGIONAL WEATHER PHENOMENA 



3. FOG 



Modification of Warm and Cold Fog 



The principal impetus for the de- 

 velopment of methods for modifying 

 fog has come from civil and military 

 aviation. Despite improvements in 

 instrument-landing techniques, dense 

 fog over an airport severely restricts 

 or prevents aircraft landings and 

 takeoffs. Such occasions, even if they 

 last only a few hours, impose sub- 

 stantial financial penalties on the air- 

 lines, cause inconvenience and loss 

 to the traveling public, and delay or 

 abort military missions. Dense fog 

 is also a serious hazard for marine 

 and surface transportation. (See Fig- 

 ure VI-8) On the other hand, fog is 

 beneficial in certain forested regions 

 in which the fog-drip from the trees 

 supplies significant moisture. 



The time- and space-scales of fog 

 and its frequency of occurrence are 

 all small enough that no large-scale 

 changes of climate appear likely even 



if all fogs were to be dissipated. 

 However, the climate of certain local 

 areas with a high incidence of fog 

 would certainly be changed if the 

 fog were eliminated. 



Cold Fog 



Modification of supercooled, or 

 "cold," fogs by seeding them with 

 ice nucleants has developed to the 

 point of operational use at a number 

 of airports where such fogs are rela- 

 tively frequent. The scientific basis 

 for modifying supercooled fogs is 

 well established; the remaining prob- 

 lems involve the engineering of reli- 

 able and economical operational 

 equipment and procedures. 



Nucleants — Cold fogs are seldom 

 supercooled by more than a few de- 

 grees centigrade and, therefore, the 



Figure VI-8 — A DRIVING HAZARD 



The photograph shows a section of an interstate highway running through the valleys 

 of central Pennsylvania. The valley in the foreground is clear, with excellent driving 

 conditions. Once the driver enters the gap between the valleys, however, visibility 

 begins to decrease until it reaches near zero. Although a local phenomenon, this 

 condition causes many accidents each year. 



ice nucleants must have the highest 

 possible threshold-activation temper- 

 ature. Dry-ice pellets and liquefied 

 propane, carbon dioxide, and freon 

 have typically been chosen to meet 

 this condition. Silver iodide is not 

 expected to be effective above — 5 

 centigrade. Consideration should be 

 given to the use of certain organic 

 nucleants such as urea and phloro- 

 glucinal, which have been reported 

 to have relatively high activation 

 temperatures. 



Dispensing Methods — To be effec- 

 tive, the nucleants must be distrib- 

 uted fairly uniformly through the 

 volume of fog to be modified. The 

 earliest, and still the most effective, 

 procedure is to distribute dry-ice pel- 

 ets from aircraft flying above the fog; 

 vertical distribution is assured by the 

 rapid fall of the pellets through the 

 fog. Nucleants in the form of fine 

 particles or liquefied gases must be 

 introduced directly into the fog, 

 which may involve hazardous flight 

 levels. The costs of aircraft seeding 

 and the limited storage life of dry 

 ice have led to the development of 

 ground-based dispensers. Liquefied 

 refrigerant gases are commonly used, 

 often with fans or blowers to dis- 

 tribute the resulting ice crystals 

 through the fog. 



Fog is almost always accompanied 

 by a wind drift, and the location 

 and timing of the seeding operation 

 must be selected so that the clearing 

 moves over the airport at the desired 

 time. This requires timely wind ob- 

 servations, precise navigation for 

 airborne seeding, or extensive arrays 

 of fixed seeding dispensers. A wind 

 shift during the operation may cause 

 the clearing to miss the airport. 



Cost Considerations — Operational 

 successes in the clearing of cold fog 



180 



