A clear slot is a visible area of clearing that appears near the rear side of a wall cloud or mesocyclone. It marks the arrival of the rear flank downdraft, or RFD, at cloud-base level as dry, sinking air pushes into the storm's low-level circulation.
How it forms
The RFD is a current of air brought down from higher in the storm along the back of the mesocyclone. As it sinks, it warms and dries by compression, and it may bring drier midlevel environmental air down with it. When this air reaches the cloud base, it erodes cloud material and opens a visible slot or notch behind the wall cloud.
The slot usually appears as a bright wedge of sky cutting into the darker, ragged cloud base on the storm's rear flank. In the classic conceptual model, it wraps counterclockwise around the wall cloud in the Northern Hemisphere and can appear to squeeze the wall cloud from behind.
How spotters interpret it
A wall cloud with a fresh, sharpening clear slot behind it is treated as a heightened risk situation. The RFD helps concentrate low-level rotation, and its arrival often coincides with rapid changes in the wall cloud's behavior, such as tightening, rising motion, or a visible funnel.
Spotters usually pair the clear slot with other cues: is the wall cloud rotating faster, is there rising motion right at the leading edge of the slot, is the inflow tail cloud strong. A clear slot with none of those other signs may be a passing feature that never produces a tornado.
Where it misleads
Not every clear slot is tornadic. Many supercells cycle through several clear-slot episodes without producing a tornado, and some tornadoes occur without a visually obvious slot, especially in high-precipitation storms where rain hides the rear flank.
The clear slot is also easy to confuse with ordinary breaks in a ragged cloud base or with the visual gap between two separate convective towers. A true RFD clear slot is anchored to the rear of the wall cloud and evolves with the mesocyclone, not somewhere else in the cloud field.