A friend of Philip Behnke (Member 55,797) watched this dark Cumulonimbus loom over the rooftops of Walthamstow, London, England. It spread out at its summit in a wide incus, or anvil, upon reaching the top of the troposphere, where it could rise no further. The fibrous appearance near its top, where ice crystals have formed, identifies this Cumulonimbus as the ‘hairy’ capillatus species. The pouch-shaped protuberances hanging from its shadowed underside are known as mamma, meaning ‘udders’ in Latin.

It was likely part of a quasi-linear convective system, better known as a squall line. This cloud often brings strong, gusty winds, thunder, and sudden downpours of heavy rain or hail. We advise the pigeons to take cover among the chimneys as this summer storm rumbles across the city.