Fog is a cloud on the ground. It is no friend of the sailor the driver, or the aviator. Nearly three-quarters of a century after the Wright Brothers first flew at Kitty Hawk, fog was still making mischief for the flier.

When the flier is the pilot of a twenty-million-dollar jet airliner with hundreds of people aboard, circling the field waiting for the fog to clear, the mischief can be a grave matter. Fog costs the airlines around one hundred million dollars a year, in airport tie-ups, cancellations of flights, di versions to other airports, and general delays. A single tie-up can cost one hundred thousand dollars.

The first attempts to do something about fog at flying fields came in World War II. The British burned oil in drums placed around the edge of the airport. The heat from the fire cleared out the fog by evaporation up to one hundred feet overhead, but any breeze soon brought the fog back, often undoing the work of the oil fires before the circling planes could come in.

When electronic instruments were developed to help the pilot land, the oil fires were given up. At last, flying men thought, they were done with the problem of fog. Not quite, as it turned out. Old habit was too strong. In the final moments of the approach to the runway, the pilots still wanted to see the ground with their own eyes. The fog battle was on again.

The wartime idea of the oil drums was brought back with refinements, but it proved to be too expensive. It cost one million to two million dollars to install the system, and up to five hundred dollars a minute to operate it.

When the fog is supercooled, it can be got rid of by seeding it with silver iodide or dry ice. But how does one do the seed ing when the cloud lies at the ground? The United States Air Weather Service is testing several ways to do it from the surface, as well as from aircraft and balloons. A number of airlines also are experimenting with fog-seeding techniques.

Clearly, there are hazards to seeding the fog over an air- port with airplanes, since they are flying in the very conditions which caused the field to be shut down in the first place. And balloons tethered overhead could be dangerous in their own right, recalling the use of balloons to snag invading German aircraft in the Battle of Britain in World War II.

So it looked as if the answer finally worked out would be something that operated from the ground.

Alas, however, it wouldn't make much difference how the seeding was done. Most of the fog problem would still be there. Only about five percent of fog is supercooled, responsive to seeding. The rest is "warm."

And warm fog is even more stubborn than cold fog. The United States Air Force, doing research on the puzzle at its Cambridge Research Laboratories in Bedford, Massachusetts, finally got the idea to try helicopters. If the warm, dry air above a bank of fog were driven down into it, they reasoned, this ought to cause the water droplets to evaporate.

After developing a technique and practicing for three weeks, the experimenters were ready to give the idea a try. Using big, turbine-powered choppers, they hovered five hundred feet above the fog cover as the whirling rotors beat the air down- ward.

Soon a ring a mile across appeared on the top side of the fog. The experimenters dropped down to a hundred feet from the center of the ring, and at thirty miles an hour flew in gradually widening circles until they reached the outside of the ring.

In one minute, the fog at the center of the circle began to fade. Ten minutes later, the fading had spread all the way across the circle and there lay the airport, in a mile-wide clearing.

But others who have tried helicopters to disperse airport fog have done less well. They succeeded in clearing out the fog directly underneath the hovering chopper, but ridding the fog from a six-thousand-foot runway proved to be some thing else. In eighteen attempts, only two succeeded all the way-not good enough.

It appeared that the best tool to dispel warm fog is something you can buy at the grocery store. This is ordinary salt. Like silver iodide, salt is a nucleating material, but it works in warmer conditions.

Salt acts as a hygroscopic nucleus, which means it absorbs or attracts moisture from the air. The salt gathers the droplets of fog and builds them into raindrops, just as silver iodide builds drops from supercooled moisture in a rain cloud, start ing with ice crystals.

It was known that salt would work, but nobody knew how well until seven hundred pounds were loaded aboard an air plane and dumped into a fog bank over Chemung County Airport, Elmira, New York, one autumn day in 1968. The salt opened a hole in the fog a quarter mile wide and a half mile long in fifteen minutes.

Very important was the size of the grains, it was found. Table salt is too big-by about one hundred times, which makes sense when one remembers the size of silver iodide particles. In a heap of them no larger than a grain of salt would be ten thousand times too hundred million of them!

There is one problem with salt, however. If the fog layer is deeper than three hundred or four hundred feet, the drops that collect around the salt particles as they sift downward through the fog reach capacity too soon and stop growing before they are all the way through to the bottom.

Another drawback to salt as a fog dispersant at airports is its corrosiveness. No automobile not under cover would be safe.

Despite the problems, reports the National Academy of Sciences, "There is optimism among various investigators that it will be possible to devise a practical scheme of clearing warm fogs by seeding them with hygroscopic aerosols."

Perhaps there is something better than salt.