The desk or table top fan has an impeller with radial blades on a hub which is mounted directly on the shaft. There is a slot in each blade. The effect of the slot is to break up the air and thus increase the volume delivered by the fan to increase the cooling effect at periods of very high temperature. This avoids the expense of a special motor to operate the fan at high speed. The slot is preferably parallel to the axis of the blade but may also be perpendicular or inclined to the axis of the blade.