Atmospheric Optics Home     Mock-Mirage Sunset Previous Feature Next Feature Subscribe to Features on RSS Feed

Mock-Mirage Sunset, Oregon Coast imaged recently by Alan Clark of the University of Calgary. Images ©Alan Clark, shown with permission.

Air pressure decreases with height. What about its temperature? Imagine a rising pocket of air, it must expand as it climbs and the pressure falls. But the expansion requires energy and to supply it in the absence of any other source the air pocket's temperature drops - Temperature is purely a measure of the violence of the air molecules' random motions. The stable equilibrium state of the lower atmosphere is therefore one where the air temperature falls with height.    Frequently there are conditions, temperature inversions, where there are layers with above equilibrium temperatures. When the temperature and air density differences are strong enough the rays of the setting sun slanting through the inversion are refracted and curved sufficiently to form a mock-mirage. The sun appears cut into at least three slices, some descending and others descending. These are ideal conditions for mock-mirage green flashes and they were probably occurring here but were masked by sea mist.

OPOD  -  Optics Picture of the Day   -   OpticsPOD         |          Home   -   Archive   -   Submit Image   -   Atmospheric Optics