Remember how mountains were often depicted in paintings by early visitors to New Zealand? That’s how Mt Taranaki appeared to some of us at Labour Weekend, from the ridge above Waiopehu Hut, on a very still clear morning. It seemed to tower much higher above the horizon than usual. Paul Bruce, weather forecaster, was on the trip and his explanation was as follows:
A mirage is an optical illusion that is the result of refraction of light by air with unusual density patterns. The desert mirage makes you think that you have found water when you are actually seeing blue sky (Fig. 1). It occurs on the sunniest of days when the ground becomes so hot that it superheats the air lying just above it. Several meters above the ground the air is colder and heavier. Therefore, light waves travel faster near the ground than several meters up, and so they bend away from the ground.
Our view from the Tararua Ranges was through a relatively warm, dry layer above a shallow cool layer left over following a period of southerlies. When the surface of the earth or bottom layer is very cold and the air touching the surface is colder than the air above, light rays will bend in the opposite way. Small objects may then appear as “castles” in the air. This is sometimes called the fata morgana after the magician-sister of King Arthur. (Fig 2).
''Depending on the precise temperature structure of the air, the images of objects in the distance may appear either right side up or upside down, and may appear either magnified or compressed. Several images of the same object may be visible. Some distortion is to be expected in these unusual situations, and this tends to reinforce feelings that something supernatural is happening...
(Edited extracts from The Science and Wonders of the Atmosphere by Stanley David Gedzelman.)