Simplify the following text for a Grade 10 native English-speaking person.
Original: Cumulonimbus clouds sometimes form mammatus clouds .
Simplified: The clouds can form alone , in clusters , or along a cold front in a squall line . Cumulonimbus clouds form from cumulus clouds ( namely from cumulus congestus cloud ) and can further develop to a supercell , a severe thunderstorm with special features .