Home
  OpticsPOD
  What's New
  Rays & Shadows
    Crepuscular rays
    Anti-crepuscular
    Cloud shadows
    Mountain Shadow
    Earth's Shadow     Blue Sky
    Sunsets
    Sunset mirages
      Air Temperatures
      Etruscan vase
      M-Mir Sunset
      More Images
    Green Flash
    Moonrise/set
    Opposition effect
  Water Droplets
  Rainbows
  Ice Halos
  High Atmosphere
  Links & Resources
  Search - Index

OPOD





                            

123456789012345678
   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.