Meteorology: Wash Day on the Runway

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Nantucket Memorial Airport was socked in, and all flights to and from the island resort off the southern hook of Cape Cod were canceled. The only thing moving through the mist was an awkward contraption that looked like an oversized giraffe with a bad case of neck strain. As it rumbled along, the monster seemed to be generating additional fog, spewing a fine white spray out of its tall tip. But the machine's passage produced remarkable results. In less than half an hour, some of the thick ocean-born fog overhanging the field began to disappear.

That impressive test was part of a program sponsored by the Air Transport Association to clear the fog from the nation's airports. Known as a Fog-Sweep, the big machine is actually a mobile blower with a 100-ft. flexible plastic tube that pops up, jack-in-the-box style, once its fan starts whirling. Out of the tube comes a spray of chemicals that are close kin to ordinary household detergents. And 70% of the time, they can "wash" away enough fog to let planes fly in and out of closed-down airports.

Next to overcrowding in the skies and on the ground, fogbound airports are the airlines' most vexing and expensive operational problem. Fog costs them some $75 million a year in flight delays, diversions and cancellations. Meteorologists have been battling it in various ways ever since the R.A.F.'s primitive World War II efforts to burn away British pea-soupers by placing barrels of flaming fuel along airport runways. Yet, to the airlines' annoyance, the most promising ventures in the laboratory have often proved impractical at the airfield.

A major hitch has been temperature. Aerial "seeding" with crystals of dry ice can be used easily enough to turn fog into snow when the water droplets are at temperatures below freezing. That technique is regularly used at 21 major U.S. airports. But such "cold" fog accounts for only 5% of airport shutdowns in the continental U.S. The rest are caused by fog at temperatures above freezing, which until now could only be dispelled by chemicals that corroded metals, destroyed plant life or simply cost too much.

Fog-Sweep's chemicals are of a nontoxic, noncorrosive variety commonly used in the disposal of sewage and industrial wastes. One type, known as polyelectrolytes, imparts tiny electrical charges to the billions of airborne water droplets. Once charged, the droplets attract one another, combine, and often plunge to the ground as rain. Even if no precipitation occurs, the reduction of the number of droplets in the air alone improves visibility. Other chemicals, called surfactants, push the fog-clearing process along by relaxing the droplets' surface tension, the contracting tendency that helps give them their particular size and shape.