Mirages

 Refraction due to the temperature gradients in the atmosphere can cause a number of mirage effects.

• Inferior mirages occur over hot ground surfaces such as roads. They are caused when rays of light bend upwards leading to an inverted image, producing the illusion of water, because the lower part of the inverted image is often of the sky.
• Superior mirages result from a temperature inversion when a layer of colder air lies on a layer of warmer air, causing rays to bend downwards so an image appears above the true object.
• Looming is a refraction phenomenon in which the visible horizon is lifted or lowered and distant objects appear displaced or distorted; the effect can occur when warm air lies on cold water. Looming is also associated with warm winds, such as the Alpine föhn, which can cause vertical temperature differences of 15° over 100 m. The temperature differences lead to changes in the refractive index of air causing the looming effect.

A fata morgana arises from more complicated temperature variations than the layered effect found in looming and leads to mirages in which an image may appear a number of times. The name arises from the crystal castle in the sea inhabited by Morgan Le Fay, King Arthur’s half-sister. The effect can lead to greatly elongated images and are common in the Strait of Messina, between Sicily and mainland Italy.

Visibility is normally limited by the curvature of the Earth but refraction in the atmosphere can extend the visible area.

•  In the hillingar effect, a vertical temperature gradient of over 0.11 Km-1 can make distant land visible.
•  In the Novaya Zemlya effect, refraction can give the impression that the Sun is rising earlier than expected.
•  The Min Min light phenomenon, whereby lights appear to follow travellers and which has been reported in western Queensland, has also been explained by over-thehorizon refraction.