You’ve seen this before and you’ve probably taken a picture of it as well, a large scary looking cloud that can occur with a thunderstorm. A shelf cloud is one of the most photographed weather phenomenon but how exactly does it form.
A shelf cloud occurs within the leading edge of a thunderstorm, especially if you have a line of storms together. As the thunderstorm grows, rain cooled air will begin to sink down to the surface. Out ahead of the storm, the warm air will rise and will allow warm moist air to feed into the thunderstorm. Thunderstorms love warm, moist air as this provides energy to the storm and keeps it alive and moving. The air is then pushed forward and the warm air cools and condenses into a shelf cloud. Think of this as a bulldozer pushing through dirt and forcing it up.
A shelf cloud is often confused for a wall cloud, which produces tornadoes. The difference between a wall cloud and a shelf cloud is that shelf clouds tend to cover the entire horizon and will appear at the leading edge of a storm. Shelf clouds are also larger and less compact than wall clouds.
If you see a shelf cloud you can expect the wind to come first, which may be strong, followed by very heavy rain.