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A mirage is understood to mean an optical phenomenon in the atmosphere that consists of an image of a distant object. For example on the horizon, the reverse image of the ship. This image may be single or multiple, steady or wavering, upright or inverted, vertically enlarged or reduced. So something is observed that is not observed in ordinary, normal circumstances. The reason is that the light coming from the object is refracted by the atmosphere, and so undergoes a change in direction. This happens when the refraction index of air shows great differences from one place to another. The value of the refraction index depends very much on the density of the air, which in turn is related to the temperature.
A mirage occurs if big temperature differences or density differences are present in the atmosphere. This happens mainly in the lower layers of the atmosphere, close to the surface of the earth or sea. A distinction is made between two types of mirages, upper mirages and lower mirages. The upper mirage occurs if for example the surface of the sea is much colder than the air above it. The temperature of the air will as a result increase rapidly in the lowest layers with the height, while the density of the air quickly falls with the height. The light rays coming from object located low on the horizon are then curved so strongly by the presence of the temperature inversion layer above the observer and the object that it looks as though the object is reflected in the layer above it. This mirage will mainly occur at higher latitudes when the surface temperature of the seawater is very low and when there is a light wind, so the air in the lower layers is not mixed too much. Another effect may occur under these circumstances for objects that are not normally visible, as they are beyond the horizon. Because of the curvature of the light rays they then become visible. This phenomenon is called 'looming'. This is not then a reflection. The horizon also looks higher and further away. The lower mirage occurs when the surface of the sea is much warmer than the air above it. This may for example be the case if cold air flows over a relatively warm sea surface or over the coast when the beach is heated strongly by sunshine. The density of the air then increases in the lower air layers with the height. In this case for example a ship is observed or a coastline becomes visible in the air suspended above an unclear, glistening horizon, or an inverted image of the object below the object itself. Sometimes above the inverted image, adjoining it, a further upright image is observed. The refracted rays of light have a faint curvature. The higher the observer is above the reflecting layer, the further away from him the reflection takes place. Objects appearing at a great height therefore lie far beyond the point of reflection. If the object lies in the far distance, the curvature of the earth's surface also plays a part. Below a so-called vanishing line the bottom part of the object is invisible, the top part is reflected. Often the inverted image is very flattened. Here the opposite phenomenon to looming may also appear: objects located on or even above the normal horizon become invisible. The horizon seems to be closer.
On land the phenomenon is mainly observed above an asphalt road on a sunny day; in the distance there seem to be puddles of water. The water is however nothing other than the reflection of the sky; the glistening is caused by irregular convection currents in the hot layer.