Simulation of a Weak Mock-mirage Sunset


In this simulation, the thermal inversion is moderately weak: a change of only 0.8 K between 30 and 50 meters above sea level. That is, the bottom 30 m of the model atmosphere are just 0.8 degrees colder than the Standard Atmosphere, which applies above 50 m. Between 30 and 50 m, the temperature profile is nearly linear, but with a lapse rate considerably different from that in the rest of the lower atmosphere. The edges of the inversion have been smoothed with a 2-meter-wide Gaussian to satisfy the requirements of thermal physics. The observer here is at 60 m elevation, just 10 m above the top of the inversion layer.

animation of a mock-mirage sunset

Main features

Until the lower limb reaches the top of the inversion, this is a standard sunset. When the lower limb reaches the top of the inversion, it flattens out and pauses momentarily before popping back into view as a separate blob below the main disk, which it quickly joins. (Of course, this blob is a double image of the lower limb.) The blob first appears as a small red flash; however, let's skip this red flash until we have a better example.

Notice the narrow, pointy indentations in the lower limb, just where the blob joins the disk above it. These indentations become pointy “spikes” sticking out of the upper limb when it reaches the altitude where the line of sight is just tangent to the 25-m top of the inversion layer. The pair of spikes separates from the rest of the Sun when they reach the top of the disk, forming a “plume” that turns green as it disappears. This weak green flash is usually drowned out by the brighter disk below it. For a better look at it, see the next page.

© 1999 – 2002, 2005, 2006, 2013 Andrew T. Young

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