Mirage - Superior Mirage

Superior Mirage

A superior mirage occurs when the air below the line of sight is colder than that above. This is called a temperature inversion, since it does not represent the normal temperature gradient of the atmosphere. In this case the light rays are bent down and so the image appears above the true object, hence the name superior. Superior mirages are in general less common than inferior mirages, but when they do occur, they tend to be more stable, as cold air has no tendency to move up or warm air to move down.

Superior mirages are most common in polar regions, especially over large sheets of ice with a uniform low temperature. They also occur at more moderate latitudes, although in those cases they are weaker and less smooth and stable. For example, a distant shoreline may appear to tower and look higher (and thus perhaps closer) than it really is. Because of the turbulence, there seem to be dancing spikes and towers. This type of mirage is also called the Fata Morgana or hafgerdingar in the Icelandic language.

A superior mirage can be right-side up or upside down, depending on the distance of the true object and the temperature gradient. Often the image appears as a distorted mixture of up and down parts.

Superior mirages can have a striking effect due to the Earth's curvature. Were the Earth flat, light rays that bend down would soon hit the ground and only nearby objects would be affected. Since Earth is round, if their downward bending curve is about the same as the curvature of the Earth, light rays can travel large distances, perhaps from beyond the horizon. This was observed and documented for the first time in 1596, when a ship under the command of Willem Barents in search of the Northeast passage became stuck in the ice at Novaya Zemlya. The crew was forced to endure the polar winter there. They saw their midwinter night come to an end with the rise of a distorted Sun about two weeks earlier than expected. It was not until the 20th century that science could explain the reason: The real Sun had still been below the horizon, but its light rays followed the curvature of the Earth. This effect is often called a Novaya Zemlya mirage. For every 111.12 kilometres (69.05 mi) the light rays can travel parallel to the Earth's surface, the Sun will appear 1° higher on the horizon. The inversion layer must have just the right temperature gradient over the whole distance to make this possible.

In the same way, ships that are in reality so far away that they should not be visible above the geometric horizon may appear on the horizon or even above the horizon as superior mirages. This may explain some stories about flying ships or coastal cities in the sky, as described by some polar explorers. These are examples of so-called Arctic mirages, or hillingar in Icelandic.

If the vertical temperature gradient is +12.9°C per 100 meters (where the positive sign means temperature gets hotter as one goes higher), then horizontal light rays will just follow the curvature of the Earth, and the horizon will appear flat. If the gradient is less (as it almost always is) the rays are not bent enough and get lost in space, which is the normal situation of a spherical, convex "horizon".

In some situations, distant objects can get elevated or lowered, stretched or shortened with no mirage involved.