action of a temperature inversion layer. The layer need only be
a few feet above the surface (the vertical scale here is exaggerated).
Parallel rays from the setting sun follow two paths to the observer
above the layer and up to three solar images are seen (the
uppermost is erect and for clarity is not shown here). The mirage
can greatly magnify small angular differences in ray directions.
The separation between red and green images is enhanced and can
give a green flash.
As the sun sinks its rays start to pass through the temperature inversion.
The action of the inversion is most easily understood by concentrating
on the interface between the warmer layer and the cool more dense
air beneath. Rays passing downwards through it are refracted so that
slightly towards the earth.
The upper parallel sun ray glances through the inversion and is most
strongly curved. To the eye, the ray appears to be coming from a point
higher in the sky and it contributes to an upper (and inverted) image
of the sun. The lower parallel ray is less deflected and contributes
to a lower solar image.
In a direction tangential to the base of the inversion layer small
vertical differences in ray directions are highly magnified. The normally
insignificant green rim of the sun is vertically stretched to produce
a green flash.
When the inversion layer, as shown here, is at constant height the
mirage effects are all below the astronomical**
horizon. Sometimes inversion layers follow vertical atmospheric waves,
the mirage and green flash can then appear rather higher in the sky
as in the example here.
See Andrew Young's pages for further explanation
and a simulation.
Sometimes the inversion can be sufficiently strong that the ray curvature
equals or is stronger than that of the earth. The rays are then guided
along the inversion layer - a duct.
Ducting can make objects at a
considerable distance below the horizon visible.
The astronomical horizon
is defined by a horizontal plane passing through your eye. When you
are even a small distance above sea level the real horizon is always
below the astronomical one. From an aircraft the real horizon is obviously
lower. Pure I-Mir and M-Mir flashes always occur below the astronomical
horizon which is why a clear horizon, and preferably a sea horizon,