Mixing height (also called mixing depth) is the height above ground level through which relatively vigorous vertical mixing occurs. Low mixing heights mean that the air is generally stagnant with very little vertical motion; pollutants usually are trapped near the ground surface. High mixing heights allow vertical mixing within a deep layer of the atmosphere and good dispersion of pollutants. As such, mixing heights sometimes are used to estimate how far smoke will rise. The actual rise of a smoke plume, however, considers complex interactions between atmospheric stability, wind shear, heat release rate of the fire, initial plume size, density differences between the plume and ambient air,and radiant heat loss. Therefore, an estimate of mixing height provides only an initial estimate of plume height.