Rare wave-like cloud formations were spotted in the Midwest earlier this week. Known as asperitas, the cloud structures form on the bottom of a cloud, creating a wave-like ceiling when viewed from below, according to the UK Met Office. The process that forms asperitas clouds is still unknown. Several theories abound, including one that involves mammatus clouds and changing wind direction and height, the UK Met Office said. 📷 KasperAg35/Twitter #clouds #cloudsofinstagram #cloudformations #illinois