These passed overhead, moving east, with a few rumbles of thunder, at about 10-ish this morning. The first time I've seen mamma clouds in Suffolk.
My Cloud Book says,
They are caused by powerful downdraughts, when pockets of cold, moist air sink rapidly from the upper to the lower parts of the cloud, reversing the usual cloud-forming convection of warm, humid air. Their shapes and forms can vary considerably, from near-spherical pouches to tubular, rippled or merely undulating globules, arranged in cellular formation.