Charles Gentry Jr., age 14, of Albuquerque, N.M., for his question:

WHAT CAUSES A MIRAGE?

A mirage is a realistic image of an object that is either totally imaginary or that appears to be in a location other than the true one. The imaginary vision is a psychological aberration sometimes experienced by persons suffering from such conditions as extreme thirst, or mental or physical strain.

The phenomenon that causes objects to appear out of place, usually in a desert or at sea, is the result of atmospheric conditions.

When heat radiates from a hot earth surface, as in a desert, it causes a diminution of the density of the layer of air just above the surface and forces a denser layer of air to remain above the hot, rarefied air instead of, as is usually the case, below it. The boundary between the two layers produces a lenslike effect and refracts or bends rays of light from a distant object. It also gives the appearance of a layer of water.

The image produced by the rays bent by abnormal vertical distribution of air density appears inverted and below the real object, just as an image reflected in water appears when observed from a distance.

The (ital) fata morgana (unital), which is a double mirage of the looming effect, produces exaggerated images of routine objects. It is sometimes seen over the Great Lakes.