Funnel Cloud
Before a tornado, a cloud that looks like a funnel may appear to drop from the dark clouds overhead. Often, the funnel drops from what is called a wall cloud, a low-hanging mass of rotating air. While the funnel remains aloft, not touching the ground, it is called a funnel cloud. If it extends to the ground, it becomes a tornado. A funnel cloud is not a sure sign that a tornado will occur, but signals that one is very likely to form within a short period of time—often within a matter of minutes.

A tornado may be in progress even if the funnel doesn't appear to reach the ground. If you spot debris or dust being thrown about at ground level underneath a funnel, the tornado is already on the ground. As the tornado develops, dust and debris drawn into it make it easier to see.