Gust front. Shelf cloud (white, lower centre to lower left) at a gust front. A gust front is formed by downdrafts of cold air from a thunderstorm. Cold air moves down and out below the shelf cloud, often causing rain, pushing hot air up and over the shelf cloud. This can often lead to the formation of a new thunderstorm. This was previously a supercell (severe) thunderstorm. Photographed in Nebraska, USA.