What are mammatus clouds and how are they formed?

True to their ominous appearance, mammatus clouds are often harbingers of a coming storm. Composed primarily of ice, they can extend for hundreds of miles in each direction and individual formations can remain visibly static for 10-15 minutes at a time. While they may appear foreboding, they are merely the messengers. As upward draughts carry precipitation enriched air to the cloud top, the momentum begins to diminish, air sinks down making the clouds round and heavy pouches.