I too have seen this effect when flying above clouds and therein lies the clue. This is an example of the Opposition Effect seen when you are illuminated from behind and are looking at something that contains many reflective particles, in this case the spherical water droplets in a cloud.  The light is reflected straight back along the path whence it came except where it was blocked out by the observer; making it look as though you're seeing an image when in fact it's a sharply cast shadow contrasted by the reflected light.  Similar effects can be seen when approaching highly reflective roadsigns with the sun directly behind you and lunar astronauts have also noticed the effect from the lunar regolith (again with the sun behind the astronaut) which is composed largely of small, reflective, glassy spherules.

All emergent consequences of the reflective properties of transparent spheres.