A more detailed survey of the weather conditions, however, quickly showed that this picture was greatly oversimplified. At 8:12, the time of the sighting, the night had already become quite dark. Yet the sun had set only forty-five minutes earlier and, according to the almanac, twilight should not have ended until 9:01 local time. Thus there must have been a dense cloud bank low in the west. Also, according to Captain Nash, there was probably some unstable air, which in itself indicates inequalities of temperature and/or humidity. A thorough study of the situation showed that inversions of both temperature and humidity must have been present. In the summer of 1952 all the eastern states were suffering from an intense heat wave and drought, and the ground cooled rapidly after sunset, because of the lack of cloud cover during the day. In a period of heat and drought, the nightly cooling produces marked inversions favorable to extreme refraction or reflection. Small in extent, existing only briefly in one place, constantly changing location, such inversions may not be detected by radiosonde observations[XII-9]. During July and August, temperature inversions occurred almost every night in the coastal regions and accounted for the radar angels so frequently observed in the Washington area during those weeks (see _Chapter_ VIII). The fact that the sighting occurred over Chesapeake Bay is significant. A body of water cools more slowly than the land, and the air over water is warmer than that over land. The cooler air from the land is carried over the water by convection currents, flows in and under the warm air, is heated by the water and rises, to be replaced in turn by the further flow of cold air from the land. The air over a lake, river, or other body of water also has a higher moisture content than over the land and can form an invisible haze. All these facts lead inescapably to the conclusion that sharp localized discontinuities of both temperature and humidity must have existed over Chesapeake Bay on the night the UFOs appeared. A light on the Virginia coast, shining northeast toward the plane, could easily have been spread out into a series of images like those observed. A change in the orientation of the light or a shift in the location of the inversion would account for the abrupt change of course made by the disks. Since the plane was flying at a ground speed of about 195 knots (225 to 250 miles an hour), it would have traveled about a mile during the twelve or fifteen seconds the disks were in view. This distance would have changed the relation between moving plane and stationary ground light, so that the images would no longer have been visible from the plane. By flying on, the witnesses left the phenomenon behind them.