A deflection of air downward relative to an object that causes the deflection.

The deflection of a gas flow downward, brought on by eddies that form in the lee of a structure (the area protected from wind or airflow by the structure itself) when the wind is blowing.

The downward movement of pollutants or smoke plumes immediately to the lee of flow obstacles, such as buildings, bluffs, or smokestacks. This downwash is caused by wake turbulence or lee cavity circulations generated by the obstacle and brings higher-concentration pollutants down toward the ground. Stack downwash occurs when the average vertical effluent velocity 0 out of the top of the smokestack is less than 1.5 times the mean wind speed at stack top. The average height decrease of the pollutant plume centerline is of order 2[() − 1.5], where is the diameter of the top of the smokestack.

The term downwash has two meanings within the field of aerodynamics.