Immediately after the Washington crisis, the Technical Development and Evaluation Center of the Civil Aeronautics Authority was assigned the problem of finding out exactly what had produced the radar returns. Investigation showed that the phantoms were not a new or unusual phenomenon. They had appeared on the Washington radars on many nights before the first “invasion,” appeared twice during the week between the two, and many times after the second. Abnormal returns are commonplace during the hot summer months when temperature inversions and inequalities in the moisture of the air are most frequent. On the nights of July 19 and 26 the Weather Bureau at Washington recorded small temperature inversions and an abnormal distribution of moisture in the atmosphere, conditions that regularly produce radar angels. The experts also carried out a series of experiments in the Washington area on several nights in August when conditions of temperature and humidity closely resembled those on the “invasion” nights. During these experiments unidentified targets appeared in profusion on the radar screens. The first observation period began on the evening of August 13, 1952. At about 9 P.M. E.D.S.T., suddenly “a group of seven strong stationary targets became visible in an area about fifteen miles north-northeast of the radar antenna. During the next two or three antenna revolutions, the area on the scope between Washington and Baltimore became heavily sprinkled with stationary targets in a belt about six miles wide. A group of additional targets became visible in an area approximately ten to fifteen miles south of the radar antenna. This was evidence of the beginning of a temperature inversion.”[VIII-6] Two temperature inversions were involved, one just above the earth’s surface, and one at about 8000 feet. The investigators concluded that the unidentified targets observed on Washington MEW (Microwave Early Warning) and other radar in the summer of 1952 were to be attributed to secondary reflections of the radar beam, caused primarily by temperature inversions[VIII-5]. Saucer enthusiasts protested (and still insist) that the inversions were not large enough to produce radar anomalies, revealing how superficial was their acquaintance with radar. Although pronounced temperature inversions are responsible for the superior and inferior mirages resulting from the bending of light rays, large inversions are not required to produce the mirages resulting from the refractive bending of radio waves. At radar frequencies, refraction is influenced by both temperature differences and the distribution of water vapor in the atmosphere. A pronounced drop in moisture content at higher altitudes can easily cause radar rays to bend earthward and pick up ground targets, even though temperature conditions in the lower atmosphere are entirely normal.