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Mixing Heights

The States of Washington and Oregon have responsibility through federal legislation for maintaining air quality according to the provisions outlined in the Clear Air Act. They do this through the administration of their individual Smoke Management Plans. NWS fire weather forecasters assist state officials by issuing "Transport and Stability" forecasts which include guidance on air mass stability, mixing heights of surface-based air, and transport or free air winds within the mixed layer. The goal of the smoke management program is to allow the burning of forest residue when meteorological conditions are favorable for good smoke dispersion and prohibit slash burning when meteorological conditions would result in the situation pictured below.

Poor smoke dispersion

Good smoke dispersion

Mixing Height Calculations

Mixing heights represent the top of the layer through which relatively vigorous mixing will take place. It is the height at which smoke will lose its buoyancy and stop rising. A well mixed layer is a layer in the atmosphere in which the lapse rate is roughly dry adiabatic (5.5 degree Fahrenheit per 1,000 ft). Mixing heights commonly go through large diurnal variations and seasonal variations.

The morning mixing height is calculated as the height above the ground at which the dry adiabatic extension of the morning minimum surface temperature plus 5 degrees centigrade intercepts the vertical temperature profile measured at 1200 UTC. The afternoon mixing height is defined as the height above the ground at which the dry adiabatic extension of the afternoon maximum temperature intercepts the 0000 UTC sounding. An estimate of this afternoon's mixing depth can be obtained by finding the height above the surface of the dry adiabatic intersection of today's forecast maximum temperature with today's 1200 UTC upper air sounding.

The height of the afternoon mixing (or boundary) layer based on accelerometer and temperature measurements made with light aircraft have been found to be in good agreement with mixing heights calculated using the methods described above. However, daily forecasts of afternoon mixing heights using 1200 UTC soundings introduces errors into the mixing height calculations since 1200 UTC soundings are not always conservative into the afternoon hours. Modifications to the 1200 UTC sounding need to take into account processes that tend to modify atmospheric temperature and moisture profiles such as 1) low-level heating or cooling, 2) synoptic-scale lift or subsidence, and 3) mid-level temperature and moisture advection.

A more accurate approach to predicting late afternoon mixing heights is to use forecast soundings valid for 0000 UTC. Forecast soundings valid for 0000 UTC will (should) take into account the various processes mentioned above that modify morning upper air soundings.

Links to Forecast Soundings

Storm Chase Model Soundings

U of W MM5 Model Soundings

Mixing Height Climatology

	Isopleths of mean spring afternoon mixing heights in feet agl.


	Isopleths of mean summer afternoon mixing heights in feet agl.


	Isopleths of mean autumn afternoon mixing heights in feet agl.


	Monthly Average Mixing Heights (1990 data)


	Average Daily Mixing Heights at Quillayute from 1984 to 1991


	Average Daily Mixing Heights at Spokane from 1984 to 1991


	Comparison of Average Daily Mixing Heights

Please refer any questions or comments about this web site to: john.werth@noaa.gov