Viewer question: How do shelf clouds form?

CHARLOTTE, NC (WBTV) - If you've seen pictures of yesterday's storms that rolled through portions of the Carolinas, you may have noticed some interesting clouds.

This particular picture submitted to (at right) gave us a great view of what is known as a shelf cloud.

We know that thunderstorms need warm, moist air to form, and these storms grow by pulling that warm air upward into the storm, where it eventually cools and condenses.  Once rain starts falling from the storm, it cools the air around it, which sinks due to its higher density.

The air descends until it hits the ground, then spreads out in a circular motion away from the storm.  Think of the edge of that cold air as a miniature cold front.  It's more specifically known as the gust front.

As the gust front moves away from the storm, it encounters warm, moist air which rides up over the cooler air behind the front.  As the warm gets cooler, eventually condenses and forms this interesting phenomenon known as the shelf cloud.

Since it requires that cold air caused by rainfall, you'll only see shelf clouds in mature storms.  You won't see them as they're still organizing.

If you have pictures of yesterday's storms, we would love to see them.  Submit your pictures here.