Explanation:
Normal cloud bottoms are flat because moist warm air
that rises and cools will
condense into water droplets at a very specific temperature,
which usually corresponds to a very specific height.
After water
droplets form that air becomes an opaque cloud.
Under some conditions, however,
cloud pockets can develop that contain large droplets
of water or ice that fall into clear air as they evaporate.
Such pockets may occur in
turbulent air near a
thunderstorm, being seen near the top of an
anvil cloud, for example.
Resulting mammatus clouds can appear especially dramatic if sunlit from the side.
These
mammatus clouds
were photographed over
Monclova,
Mexico.
APOD presents: Astronomy Pictures of the Year for 2007
