Inferior mirage

Inferior mirage distorting islands and ships

This is the mirage that creates an illusion of water on asphalt or of an oasis in desert. On this page you have samples of inferior mirage at sea. The phenomenon is well known by boatists: islands seem to hang in the air and look a bit like reflected on a calm sea surface.

Some samples of the setting sun and the inferior mirage are shown on the bottom of the page. The last row of thumbnails consists of composites showing how changing observing height affects on the mirage.

About inferior mirages. Word 'inferior' refers to the place of the mirage with respect to the 'true' object: mirage is below the 'true' image of the object and is inverted. To see these mirages you need a warm surface, which heats up a thin layer of air just above the surface. Then, if cool air is blown over this warm layer, a steep thermal gradient is created (i.e. temperature changes very rapidly with respect to height). On my observing site (at sea) this kind of situation is found especially in Aug-Nov on crispy autumn afternoons. Cool air sets over warm sea surface. Inferior mirages are very common and saddly - always look the 'same'. By changing your vantage point you can see how objects disappear when observing near the warm surface, while at higher vantage point hardly any traces of the inferior mirage are seen. Curvature of large water surfaces creates an impression of concave surface, horizon line is closer than normally and distant islands may completely disappear. During night this mirage often disappears due to mixing of warm and cold air.

For a more comprehensive approach to the subject see Andew T. Young's great pages.