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   Air Temperatures, Mirages & Green Flashes

  Sunset mirages and green flashes result from unusual refraction by air layers at different temperatures and hence density and refractive index. The refractive index differences are minute but their effects accumulate as sunset rays travel large distances through the atmosphere.

    Air temperature falls smoothly with height. This usual state of affairs arises primarily because (1) the air is heated by heat transfer from by the ground or sea and (2) because air pressure, produced by the weight of overlying air, decreases with height. Imagine a pocket of dry warm air rising by buoyancy. As it rises it encounters lower pressure and so it expands. However, to expand it must do work - expend energy. The work is at the expense of its heat content and so its temperature falls. Air pockets ascend until they reach air at the same temperature. When equilibrium is reached the air column will be found to be cooler at its top. The temperature drop - lapse rate - is about 6.5°C/km (3.6°F per 1000ft) but depends much on local conditions and especially the air moisture content.
  A temperature inversion is the presence of a kink in the normal temperature profile whereby layers exist that are warmer than usual, warm air overlays colder air. Inversions can form when warm air from inland blows over colder marine air. This happens off the Californian coast when warm Santa Ana winds from inland overlay air cooled by the cold Californian ocean current. When you are above an inversion layer, so that sunlight from space enters and leaves it before entering your eye, the conditions are right for a mock mirage sunset and M-Mir green flash. Mock mirage ray paths here.
  Ground or the ocean surface heated by solar radiation can produce an abnormally hot air layer above it. These are the conditions for the inferior mirages seen over a hot road and they similarly produce "Etruscan vase" sunset mirages and I-Mir green flashes. Mirage ray paths.