Normal cloud bottoms are flat.
This is because moist warm air
that rises and cools will condense into water droplets at a specific temperature,
which usually corresponds to a very specific height.
droplets grow, an opaque cloud forms.
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
Resulting mammatus clouds can appear especially
if sunlit from the side.
mammatus clouds were photographed over
during the past summer.
Craig Lindsay, Wikipedia