While on a search for wild Onaqui horses with photographer friends south-west of Salt Lake City, Utah, US, Sara Goodnick (Member 50,827) saw this herd of Cumulonimbus capillatus clouds over the plains. These are mature storm clouds that have grown tall enough to reach the tropopause, the invisible temperature lid that marks the top of the atmospheric region where weather happens, which we call the troposphere. Clouds cannot easily grow through this lid on the lower layer of Earth’s atmosphere. When storm clouds reach the tropopause, usually at around 10-12 km (33,000-39,000 feet) over Utah, they may want to keep growing, but they can’t. The towering clouds are only able to expand outwards. That’s how the Cumulonimbus storm cloud develops its distinctive flat top, which we refer to as an incus, or anvil.

Having thundered across the afternoon sky, Sara’s wild atmospheric horses were calming in the evening sun. ‘I was on my way back to the hotel,’ she said. ‘The other photographers had already departed, so I was the only one left to enjoy this moment.’ The cloudy stallions here appear to be dissipating – doubtless, looking for a place to rest for the night.