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Saucers reported by competent observers could not be explained by searchlight spots, but the beam-of-light analogy gave Menzel something to work with. He looked around for other tricks of nonmaterial light which might convince an observer that he had seen a material object zipping through the air at unearthly speed.
Radar Ghosts. He did not have far to look. During World War II, Menzel had left astronomy to become a radar expert. One job (as chairman of the Wave Propagation Committee of the Joint Chiefs of Staff) was to study the effect of atmospheric irregularities on radar waves. Sometimes a layer of warm air makes the waves wander oddly, producing deceptive ghosts on the radarscope. Warships have shelled empty ocean, thinking an enemy was there. Since light waves and radar waves behave in much the same way, Menzel reasoned that the same irregularities might produce optical ghosts resembling flying saucers.
Some optical ghosts are common: the ordinary mirages which nearly everyone has seen. Commonest of all is "water-in-the-road," which is caused by a thin layer of warm air above sun-heated pavement. The two layers (cold and dense above, hot and less dense below) "refract"* upward the light that reaches them from the distant sky. A motorist sees shining water (really sky) lying in the road. In hot deserts this sort of mirage is extremely deceptive.
Moon Disks. Menzel is convinced that rarer types of mirages explain most flying saucers. Part of his conviction comes from something he saw while driving across New Mexico from Holloman Air Force Base to Alamogordo. It was a clear, cool night and a full moon had risen. Menzel noticed near the moon two bright objects which he took at first for the stars Castor and Pollux. His astronomer's knowledge told him that Castor and Pollux would not be visible at that season, so he lowered the car window to get a better look. The stars turned into fuzzy disks with about one-quarter of the moon's diameter, and they kept up with the moon in its apparent motion past objects in the foreground. After five miles, Menzel told the driver to stop the car. At once the disks vanished.
Menzel wrote a report on this "sighting" and sent it to the Air Force. He never thought his disks were flying saucers; they were close to the moon and obviously associated with it. But they puzzled him for a long time. Now he believes they were caused by the motion of the car distorting a layer of warm air just above its roof and forming two displaced images of the rising moon. A more ignorant man might well have reported them as flying objects. At any rate, they led Menzel to his present theory about the saucers.
Normally, he explains, the atmosphere grows cooler as altitude increases, but under some conditions it may contain layers of warm air with cold air below them. These are called "inversions." They occur in all climates but are commonest in deserts, where both the ground and the air get very hot in daytime. As soon as the sun sets, the ground cools off, radiating its warmth into the sky. The air for a few feet up grows cool by contact with the cool earth, but the air a little higher stays warm.