Superior mirages

When you see an inferior mirage of an object, at least part of the object is directly visible to you. This, of course, is only possible if it lies somewhere

Figure 3.5 Superior mirage. If there is a temperature inversion a few metres or more above the ground an inverted image of a distant object may be seen. This is due to rays that are refracted towards the ground as they pass through the temperature inversion. Under such circumstances some rays reach the observer from the sky and in reverse order (diagram a) so that what is seen is a squashed, inverted image of the topmost part of the distant object. At the same time the object itself appears raised above the horizon and drawn out vertically (diagram b).

Figure 3.5 Superior mirage. If there is a temperature inversion a few metres or more above the ground an inverted image of a distant object may be seen. This is due to rays that are refracted towards the ground as they pass through the temperature inversion. Under such circumstances some rays reach the observer from the sky and in reverse order (diagram a) so that what is seen is a squashed, inverted image of the topmost part of the distant object. At the same time the object itself appears raised above the horizon and drawn out vertically (diagram b).

between you and the true horizon. If, however, there is a layer of air a few metres above ground in which temperature increases with height (instead of the normal situation in which it decreases with height), in other words a temperature inversion, then you may see images of objects that are beyond the horizon. This is the cause of a superior mirage.

A temperature inversion brings objects into view above the true horizon because rays of light are refracted within the inversion so that their path is concave to the warmest layers of air.

An observer thus sees light that would otherwise pass overhead. This light will appear to come from a point that is above its actual source, which is why, when there is a widespread temperature inversion, objects are seen above their true position. Since temperature gradients within the atmosphere are never uniform, the amount of refraction is not the same for all rays and this can lead to towering and stooping.

Figure 3.6 Superior mirage of land. A superior mirage of a distant archipelago appears to hover, inverted, above the archipelago. (Photo Pekka Parviainen)

There are several ways in which a temperature inversion can be brought about. For example, the lowest layer of a mass of air resting on a much cooler surface is cooled by the surface. At mid latitudes, such inversions frequently occur over lakes and bays on still, warm afternoons in spring and early summer. In Arctic and Antarctic regions, the source of warm air is usually a cell of high-pressure air sandwiched between weather fronts, while the cold surface is either a vast ice field, or a huge stretch of icy-cold water. In fact the optical effects of the superior mirage are sometimes referred to as high-pressure refraction. High-pressure air generally brings with it clear, stable conditions followed by short-lived but violent cold-front storms, and a feature associated with the Arctic mirage is excellent visibility followed by storms. Sometimes the temperature inversion occurs at some distance above the Earth's surface. These so-called 'lifted' inversions are responsible for unusually great visual range and the inverted images that characterise a superior mirage.

Temperature gradients within a lifted inversion can vary greatly with height: for example, the gradient may be more pronounced (i.e. there is greater change in temperature with change in height) a few metres above the ground than at ground level. In such a situation, the rays of light that reach

Superior Mirage Ships
Figure 3.7 Superior mirage of a ship. The superior mirage of the ship appears as a mirror-image above the ship. (Photo Pekka Parviainen)

the observer through the layer of air closest to the ground will not be refracted as strongly as those that pass through the air at the level of the inversion. Under these conditions, the appearance of a distant object may undergo dramatic transformations because its image may tower or stoop strongly, and more than one image of it may be seen. In its most characteristic form, a superior mirage gives rise to three distinct images of an object: one on the horizon, with a reflected pair of images suspended some distance above it. The lower half of this pair is inverted while the upper half is compressed (i.e. stooped). Several examples of this type of superior mirage were seen by S.Vince when he looked out across the Straits of Dover from Ramsgate in 1798 'on August the first, from about half an hour after four o'clock in the afternoon till between seven and eight. The day had been extremely hot, and the evening was very sultry; the sky was clear, with a few flying clouds.'

What he saw were mirages of sailing ships, and of the coast of France. Although he used a telescope with a magnification of 30 to 40 times, he noted that the multiple images of the ships were visible to the naked eye. William Scoresby, a pioneer of Arctic exploration, saw similar mirages though a telescope when he was sailing in the icy sea east of Greenland,

'consisting chiefly of images of ships in the air.' during an evening of a beautifully clear summer day in 1820 during which there 'was scarcely a breath of wind. The sea was as smooth as a mirror.'

Don't imagine that superior mirages are to be seen only rarely, or only over the sea. Suitable conditions are frequently met at mid latitudes over lakes, bays and straits during the afternoon in late spring or early summer, giving us the chance to see superior mirages without having to journey to polar regions. Superior mirages can also seen over land if there is a suitable temperature inversion at heights ranging from tens to hundreds of metres from the ground.

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