We associate mirages with adventure stories such as Beau Geste, with desert sands and Rudolf Valentino as The Sheik, and with fertile oases that disappear when they are needed most. But a mirage can occur quite close to home; they are common sights in Ireland in hot weather.
A ray of light, left to its own devices, will travel through the atmosphere in a more or less straight line. If, however, the temperature of the air changes dramatically with height - i.e. the temperature in one particular layer of the atmosphere is very different from that of the layers above it, or below - the light rays passing from one layer to another are bent, or refracted.
It is because of this phenomenon that we see our Irish mirages, usually in the form of an illusory patch of water on a concrete or tarmacadam surface on a hot day.
Any object illuminated by the sun - let us say a tree - transmits rays of light in all directions. It is the rays travelling directly towards us in a straight line, for example, that allow us to see the tree in the normal way. In hot weather, however, some of the rays of light proceeding downwards from the tree towards the ground in our general direction may be bent upwards again towards our eyes as the light is refracted on passing through the very hot layer of air in immediate contact with the heated ground.
As a result, we may see a second image of the tree - upside down, and underneath the normal one. More commonly, as we look at a distant tarmacadam surface in the heat, it will seem to us to be covered with pools of water; but what we take for water is nothing more than hot air, and what we see is an apparent reflection of the clear blue sky above.
A mirage like this, where a distant object looks as if it is reflected in a "mirror" on the ground, is called an inferior image. But a superior image - an image which is apparently suspended above the object we are looking at - can also occur if the thermal characteristics of the atmosphere are suitable - usually when a temperature inversion exists and the air temperature increases, rather than decreases, with height through a depth of several hundred metres. It is this process that leads to the phenomenon we normally think of when the term mirage is mentioned - the alluring image of a fertile not-too-distant oasis that has traditionally tantalised the thirsty desert traveller.