How "Shelf Clouds" are formed

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Quad Cities (KWQC) - Numerous shelf clouds were seen early Wednesday across the QCA as strong thunderstorms moved into the area. Marking the arrival of cooler air, wind and rain, they look intimidating but their bark is usually worse than their bite. There will be occasional shelf clouds that are accompanied by 60 to 70 mph, or higher, gusts. THOSE are the troublesome ones but, luckily, they aren't as frequent! Here's the simple process in which they form. Rain-cooled air, and cold air from deep within the parent thunderstorm drops to the ground. As this air spreads out ahead of the storm it runs into air that's usually warmer and somewhat humid. The heavier, cold air acts as a wedge and gets under the lighter warm air, pushing it upward as the storm moves ahead. This warm air cools as it gets higher in the sky and eventually cools enough that the moisture within it condenses. You can see just where that happens as a cloud feature called a "shelf" develops, marking the lowest level of condensation, and the path of that air as it rides up and over the wedge of cold air that's moving along the ground. If you can't move out of the storm's path, shortly after the passing of the "shelf" you'll notice the temperature drop, the wind pick up, and you'll start to get rain. Shelf clouds aren't dangerous but they ARE harbingers of an approaching strong storm. Sometimes, if they outline a complex of grouped thunderstorms, they can be miles long and stretch from one horizon to another!