This picture shows the cloud that very quickly grew and grew until it became a cumulonimbus. The `cap` indicated that the air was ascending very quickly at a speed of at least 20kmh. This `cap` is defined as a thin sheet of cloud above cumulus or cumulonimbus and is initially not part of the cloud itself but forms because the air just above a quickly rising column of air is pushed upwards hence cooling and condenses into a `cap`. Eventually this cap becomes part of the parent cloud as can be clearly seen in the thumbnail below. The weather situation showed a cold pool of air at 500mb above warm, moist air below leading to lots of instability. As there was wind shear waterspouts were also reported. `Cb pileus` and `Cb mammatus` are related with each other.