cumulonimbus cloud

A cumulonimbus cloud produces rain.

I'm guessing that you meant to type "stratus". Stratus clouds are the lowest forming cloud in the atmosphere (even fog is a type of stratus). They can produce light precipitation. Also those little "pieces" of cloud that you see sometimes are a type of stratus known as stratus fractus. Cumulus form mid-troposphere. And the cumulus family consists of cumulus humilis, mediocris, and congestus. Cumulonimbus are in the cumulus family and consist of such formations as cumulonimbus calvus, cumulonimbus with pileus, cumulonimbus capillitus, and cumulonimbus incus. Cirrus clouds are those "thin and wispy" looking clouds way up in the troposphere. They are the highest cloud formation.

First. A wall cloud is not a cumulonimbus clouds, but is a smaller cloud that extends from the base of a cumulonimbus. Tornadoes often form from wall clouds.

What causes a cumulonimbus cloud is the cold and warm fronts that colided.