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dirty air Sailor's jargon for air flow downwind of a sailboat sailing to weather that is disturbed by the boat's sails. It is weaker wind, flowing in various directions. When attempting to pass a sailboat ahead, it is crucial to avoid its dirty air. This is not really a huge effect but it can prevent two identical boats from passing each other. It is typically spread over a relatively narrow band. Likewise we can think of air downwind of any obstacle (island, hill, cliff) as being dirty air, and even on the near upwind side the air will be lifted and disturbed. To see examples of this do a Google search on "sailboat's dirty air"

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