The first sign of a tornado may be a strong whirlwind of dust from the ground surface, often at the same time as a short funnel grows from the storm cloud above it. The funnel then becomes more organized and descends further from the cloud, sometimes touching the ground. Meteorologists are not sure how tornadoes form. But they do know that they are the result of great instability in the atmosphere and are often associated with sever thunderstorms or in advance of cold fronts. Weather forecasts include tornado alerts when these conditions arise.
Tornadoes can occur, however, ahead of warm fronts or even behind cold fronts.
The existence of a strong updraft, such as that generated by a severe thunderstorm, and the conservation of rotational momentum, are clues to how they are formed. Tornadoes occur most often in spring during the late afternoon or early evening. In the United States, they are most common on the Great Plains. In fact, tornadoes are so common that this part of the United States is often called Tornado Alley. The diameter of an average tornado is only about .4 kilometers.