|A cone funnel above a whirl of dust and dirt on the ground signal the birth of a tornado very close to the town of Attica, Kansas on May 12, 2004. Notice the 'clear slot', the bright area cutting into the cloud base just behind the funnel. This marks the presence of the 'RFD', or rear flank downdraft, wrapping around the mesocyclone. The formation of a RFD and its associated 'clear slot' often precede the formation of a tornado. Also note the large hailstones on the road, which were falling when this image was taken. Image is from video.
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