Smoke Dispersion

All of the weather elements that affect the behavior of fire will affect the production and dispersion of smoke. In this section, we will discuss those that are most important to managing smoke.

Atmospheric Stability

There is vertical as well as horizontal motion in the atmosphere. Stability is an indication of how rapidly vertical mixing is taking place. The more unstable the atmosphere, the more quickly smoke is lifted and dispersed. The behavior of the fire will also be more intense and unpredictable. Stability is usually expressed as very unstable, unstable, slightly unstable, neutral, slightly stable, stable, or very stable. It is a component of the forestry forecast in many states, and is expressed in some type of index. During slightly unstable or stable periods, higher concentrations of smoke may come back to the ground some distance downwind even though it was lifted initially by the heat of the fire.

Unstable  Conditions
 Stable Conditions

The atmosphere is most unstable during the afternoon. This is because of the heating of the earth's surface during the day. The air just above the surface becomes heated and expands. In turn, it begins to rise, since it is lighter, resulting in vertical movement in the atmosphere. The more unstable, the higher smoke will be lifted.

After dark, atmospheric conditions change rapidly becoming stable. Under stable conditions, smoke will not rise except from the heat of the fire and then, only for a short distance. The fire itself will not burn as intense. Any smoke still in the air will drop back down to the ground. A temperature inversion is an extreme example of stable conditions. It will act as a "lid" on the fire.

The circulation around a low-pressure area causes horizontal converging of air at low levels and lifting of air near the center. For this reason, low-pressure areas usually are areas of cloudiness and precipitation. Frontal lifting is frequently combined with convergence.

Indicators of Unstable Conditions

Indicators of Stable Conditions

Mixing Height

Mixing height is the maximum height that rapid vertical mixing takes place in the atmosphere.  The more unstable the atmosphere, the higher the mixing height is as a rule.  It acts as a lid on the height smoke can reach.  The higher the mixing height, the higher the smoke will rise although it will not reach the mixing height except possibly when fuel loading is high and the fire is very intense.  This in turn allows it more room to disperse.  The bottom of cumulus clouds is usually a good indicator of the mixing height. In Alabama, the mixing height is included in the forestry forecast. The same is true in many States.

Mixing height of 1700 feet or less are indicators of stagnant conditions. Prescribed burning should not be conducted under such conditions.

Transport Winds

Transport wind is an average of the horizontal wind speed and direction from the surface to the mixing height.  This is the wind that moves smoke out of an area and helps to disperse it in the atmosphere. However, it will generally not reach the mixing height. High intensity wildfires might. The smoke will spread out both horizontally and vertically as it moves down wind. Wind speed usually increases with height. It is usually greatest in the afternoon. When transport winds are less than 9 mph, caution is indicated if smoke sensitive areas could be impacted.

Local eddies form in the lee of each tree stem and affect the behavior of surface fires.

Surface Winds

Surface wind is measured at 20 feet in open areas, and is an indication of the wind that will affect the fire.  Measurements are made at 20 feet to get away from the local changes at ground level, but still close enough to indicate the wind at the surface over a broad area.  The forestry forecast usually includes both morning and afternoon surface winds.
Dispersion Index

The Dispersion Index is a numerical indicator of how well and how rapidly smoke will be dispersed.  It utilizes stability, mixing height, and transport wind as the major factors.  Other factors are: the amount and angle of sunlight and temperature. Most states include some type of dispersion (or stagnation) index in their weather forecast.    Fire Weather Forecast National Weather Service Birmingham, AL

SMOKE DISPERSION INDEX

Numerical
Rating
Interpretation
100 or over Excellent Dispersion
  • May indicate hazardous fire control conditions.
61-100 Good Dispersion
  • Typically, good prescribed burning days.
  • Most good prescribed burning days will fall in this category.
41-60 Generally Good Dispersion
  • Use care with marginal burns.
  • Afternoon values in most inland forested areas of the United States usually fall in this range.
21-40 Fair Dispersion
  • Marginal day for burning.
  • Stagnation may be indicated if accompanied by persistent low windspeeds.
13-20 Generally Poor Dispersion
  • Stagnation will develop if poor conditions persist. (Better than average for most nights.)
7-12 Poor Dispersion
  • Stagnant conditions during the day. (Near or above average at night.)
1-6 Very Poor Dispersion
  • Do not burn (Represents the majority of nights in most locations).

Note:  The decision to burn should not be based solely on the dispersion index.

Remember ... the mixing height often drops rapidly around sunset.  During the night, dispersion will usually be poor or very poor.

Sky Conditions

Sky conditions are also part of a forestry forecast.  These include:

The weather service determines visibility by whether or not known landmarks can be seen.  Visibility of five miles or less is an indication of existing pollutants in the air.  This fact should be carefully considered before adding to the concentration, especially where smoke-sensitive areas could be impacted by smoke from the burn.

Upper Air Measurements

Stability, mixing height, and transport winds are determined from measurements taken with a weather balloon.  Although these measurements are taken regularly (twice each day), only one or two measurements are made in each State.  Neither are they taken during the middle of the day. They are taken at 6:00am and 6:00pm in the southeast. (All upper air measurements are taken at the same time world wide.) Consequently, prescribed burners cannot blindly depend on the dispersion index alone.  Look at the forecast mixing height, transport winds, and sky conditions.  Observe the atmospheric conditions at the burn site as well. 

Forestry Weather Forecasts

A forestry forecast is made by the National Weather Service and disseminated to the state forestry agencies and national forests.  In turn, the states broadcast this forecast over the radio and it can be monitored on the designated frequency.  Some States put it on the internet. Its made for today, tonight and tomorrow.  Forecasts can also be obtained by calling the nearest district office of the state or federal forestry agency.

NOAA Radio

The National Weather Service has added a continuous weather forecast which is broadcast throughout the state on a designated frequency.  Radios which are on the proper frequency for a given area are inexpensive to purchase.  Many of these NOAA radios also have an alert system, allowing the weather service to activate transmission, and broadcast information when severe weather (thunderstorms, tornados, etc) threaten a specific area.  It is possible to add this frequency to two-way radios used by most forest industries.

In some areas, the National Weather Service has added the forestry forecast on NOAA radio.  If the forestry forecast is not offered, a special agricultural forecast can be used.  It will contain all the needed information except the upper-air soundings for stability, mixing height, and transport winds.  Special or spot forecasts are still available by calling the National Weather forecast office.  

Observations on Site

Weather observations should be made at the actual site of the burn:

Observations at the site will provide a ground check on the forecast. During the burn, watch for changes in the wind and behavior of the fire. After the burn, check to see if there are any changes that will determine where your smoke will go.

Smoke Dispersion is Affected By:

Surface Winds
Relative Humidity
Temperature
Atmospheric Stability
Mixing Height

Transport Winds
Long-Range Transport
Down Drainage
Plume Rise
Dispersion Index

Weather Conditions Change Continuously -- Stay Updated!