Mixing height refers to the height above ground level at which vertical mixing of the air occurs. Low mixing height indicates the air is stagnant, and pollutants are held closer to the ground. The lowest mixing heights often occur at night and early morning, with the highest occurring at mid to late afternoon.
Source:Ferguson, Sue A. 2001. Smoke Management Meteorology. A chapter in Smoke Management Guide. by Achtemeier, Gary, Brenner, James D., Core, John E., Ferguson, Sue A., Hardy, Colin C., Hermann, Sharon M., Jackson, Bill, Lahm, Peter, Leenhouts, Bill, Leuschen, Tom, Mutch, Robert E., Ottmar, Roger D., Peterson, Janice L., Reinhardt, Timothy R., Seamon, Paula, and Wade, Dale. A Publication of the National Wildfire Coordinating Group.