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a) (upper left) 7:21:10 p.m. Early stage in the
sunset, showing the normal extinction gradient. The
indentations about 1/3 the way up from the bottom of the
disk indicate temperature inversions.
True sun altitude: + 40'
b) (upper right) 7:24:15 p.m. This stage shows a
reversed extinction gradient: the widest part of the
image is brighter and yellower in color than the upper
part. The sudden decrease in width below the wide
yellow strip shows that we are looking down through an
inversion layer, and out the other side. The flattened
bottom of the image is not the horizon, but is due to strong
refractive compression of the lower limb.
True sun altitude: + 2'
c) (lower left) 7:26:09 p.m. Three separate images are
visible. The upper one is the brightest, and yellow in
color. The bottom image, which appeared in the dark
space below the image in part (b), is dimmer and redder,
indication that it is seen through a longer aerosol
path. The middle image, just appearing between the
others at this stage, is still redder and darker; it has
passed through the most aerosol. The equal widths of
the top and bottom images indicate that they are nearly
complete, but highly compressed, images of the sun.
True sun altitude: - 20'
d) (lower right) 7:26:50 p.m. The middle image has
expanded to join the top and bottom images of (c). The
disappearance of the dark strips in (c) is characteristic of
looking down through an inversion. Note the serrated
edges of the image, which indicate a complex series of
thermal inversions in addition to the main ones. All
three images now show the full width of the sun.
True sun altitude: - 29'
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