Growing thunderstorm front
The approach of a thunderstorm begins with a change in the cloud, a cell rapidly growing in all directions, shooting after a thunderstorm, by nightfall there was again a thunderstorm.
Height of base: 1,100 - 6,500 ft
Latin: cumulus - heap; nimbus - rainy cloud
Cumulonimbus are heavy and dense low-level clouds, extending high into the sky in towers, plumes or mountain shaped peaks. Commonly known as thunderclouds, the base is often flat and very dark, and may only be a few hundred feet above the Earth's surface. Cumulonimbus clouds are associated with extreme weather such as heavy torrential downpours, hail storms, lightning and tornados.
If there is thunder, lightning or hail, it's a cumulonimbus cloud rather than nimbostratus.