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    If your airline trip was delayed, was it because the traffic controller waited for fog to lift off the runway?

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    Script is copyright Reuters Limited. All rights reserved

    Background: If your airline trip was delayed, was it because the traffic controller waited for fog to lift off the runway?
    Fog has long been a matter of concern to traffic safety engineers and very little progress has been made in solving the problem. In World War II airfields in England were "dried out" by heating the air with oil burners placed along the runways to burn off the fog. But this very expensive method was only justified by military necessity.

    Other methods for "drying out" fog over an airport have been attempted including the use of an electrical charge but this method has been all but abandoned as impractical.

    One other method which has been tested with good results in the laboratory is now being pursued by the National Aeronautics and Space Administration. It involves a "seeding" operation to combine the tiny water droplets of fog into large water droplets. The heavy droplets then fall like rain, giving increased visibility. The laboratory research on this technique is being conducted for NASA by the Cornell Aeronautical Laboratory, Buffalo, N.Y.

    The Laboratory has doubted the project "Fog Drops" because fog disposal -- and improved visibility -- is based on the size of the moisture drops, or "droplets" in the fog.

    In a fog, visibility is restricted because light is scattered by the water droplets in the optical path. Any procedure that can reduce the amount of light scattered can therefore be expected to improve visibility; increasing the droplet size does just this.

    Here is how the fog disposal system would work in airport operations. As the fog builds up and the pilot's vision is impaired, a light would be sent aloft carrying containers of sodium chloride.

    The chemical would be released over the runway approaches to drift down through the fog. Since the salt is extremely hygroscopic (absorbs water), the minute water droplets would cling to the salt in ever increasing numbers until the atmosphere would consist mostly of large drops of water moisture. Thus, with the dissipation of the tiny droplets which had been scattering the light, visibility would be greatly improved.

    But it is not all that simple. The grain size of the salt must be of a very precise dimension. If the particles are too small they would not result in drops of water large enough to cut down light diffraction; if too large, they would fall out of the fog before condensing enough water to be effective.

    The NASA-Cornell research program is determining the exact size required to do the job.

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