Here are 8 seconds of the inferior-mirage red flash at sunset, seen from a height of 4 meters above sea level, and displayed in real time:
(The animation is about 143 kB; allow time for it to load.)
Dispersion makes the lower limb and its inverted image approach one another first at the longest wavelengths (just as it makes everything happen last at the shortest ones, at sunset). So there is a sort of red flash when the two first meet to form an “hourglass” shape.
This is exactly the red counterpart of the inferior mirage's final green flash: it occurs just at the horizontal line where the erect image folds over to join the inverted one. This fold line is where there is very large vertical magnification. Just as the final green flash of the inferior mirage is a greatly stretched (in the vertical direction) image of the green upper rim, this red flash is a stretched image of the red lower rim. Of course, the shape is different, because it's the opposite limb of the Sun.
© 1999 – 2002, 2005, 2006, 2008 Andrew T. Young