Mixing height, or mixing depth, is the height above the ground where pollutants can disperse.

 Mixing height, or mixing depth, is the height above the ground where pollutants can disperse. It's an important parameter for forecasting air quality because it determines the volume of the atmosphere available to dilute pollutants. 

Here are some things to know about mixing height:

How it's calculated

There's no single definition or accepted method for calculating mixing height, but one way is to draw a line from the ground surface's temperature and elevation, with a slope equal to the dry adiabatic lapse rate. The mixing height is the height where the line intersects the 2 PM temperature profile. 

How it varies

Mixing height varies by season and time of day. For example, in the summer and spring, the highest mixing heights are usually between 11 and 17 UTC. In the winter, local heating from pollution can significantly impact air quality. 

How it affects air quality

A high mixing height brings in drier air from above, which lowers surface dew points and increases surface temperature. 

How it affects smoke dispersion

During surface temperature inversions, which usually occur at night with clear skies, the mixing height is zero, and smoke dispersion is minimal. 

Mixing height is the height at which the lower atmosphere mixes turbulently, creating a nearly uniform air mass. It's also the height at which pollutants, like smoke, can spread. 

Here are some things to know about mixing height:

Mixing layer

The layer of air between the surface and the mixing height is called the mixing layer. 

Mixing height and wildfires

The mixing height determines how high wildfire smoke can reach. Days with high mixing heights allow smoke to be transported higher. 

Mixing height and prescribed burning

The Clean Air Act requires a minimum mixing height of 500 meters (1700 feet) for prescribed burning. 

Mixing height and surface temperature inversions

During surface temperature inversions, which usually occur at night with clear skies, the mixing height is zero and smoke dispersion is minimal. 

Mixing height and solar heating

Mixing heights are usually highest during the day when there's strong solar heating. 

Mixing height and high wind speeds

High wind speeds can also create deep well-mixed layers.