Certain types of airplanes - the McDonnell Douglas DC-10 and MD-11 are notorious examples - create funnel clouds along their wings when the flaps are deployed, during the landing. The flaps induce a lateral (sideways) component of the airflow under the wing. When this air recombines with the air flowing over the wing backwards, the low pressure and rotational shearing causes a funnel cloud which extends several tens of meters behind the airplane. They can be quite spectacular when seen from the ground near a runway, in humid foggy weather. The funnel clouds cause extreme turbulence.
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