Under ordinary circumstances, the reflections from very distant targets rarely confuse the operator. The curvature of the earth tends to shield the radiation, and the distance factor alone reduces the intensity to a negligible value. But weather can cause peculiar returns. A layer of warm air above cooler air at the earth’s surface has much the same effect on radio waves that it has on light waves. A temperature inversion can produce radar “mirages”--commonly called “phantoms,” “ghosts,” or “angels.” Relatively small amounts of warm air, even mere warm bubbles in a layer of colder air, will suffice. When the scope records a series of blips, the operator ordinarily assumes that all are returns from a single object. If inversions of temperature or humidity exist in the atmosphere, however, the series of returns may represent several different ground objects rather than a single object in the sky. Since these inversion layers do not remain fixed but move, change, and shimmer, on one sweep the radar may reflect one ground object and on the next sweep some fifteen seconds later may reflect a totally different ground object five or six miles away from the first. An inexperienced operator might conclude, wrongly, that both echoes came from a single object that had traveled five miles in a fraction of a minute (see Figure 14). Similar mistakes in identity have caused many reports of radar flying saucers. [Illustration: _Figure 14._ Deflection of radar beams by temperature inversion. Top, radar picks up ground target. Bottom, on next sweep, radar picks up different ground target, which seems to indicate a fast-moving UFO.]