Screening System

Introduction

Use of one or more of the “Guidelines for Managing Smoke” may be enough to meet your needs for managing smoke. (Especially for maintenance burns.) However, this screening system will give you an idea how far smoke could be a problem.

This screening system is simple and easy to use.  It is designed to be used on the initial planning phase as part of the written prescribed burning plan, and again just before the burn to suggest alternatives when weather conditions aren’t as prescribed.  However, it should not be used without a working knowledge of fire behavior and smoke management.  A thorough understanding of all the factors that affect smoke is needed to interpret the results safely.

This system can only suggest that:

  1. A smoke problem probably will occur,

  2. The probability of a smoke problem is marginal, or

  3. A smoke problem probably will not occur.

There are many variables which affect the amount and type of smoke that is produced and how it will be dispersed.  Also, limited research has been done to determine how these variables affect smoke.  This system utilizes only the major parameters and is based on an ‘worse-average’ weather and fuel conditions and ‘worse-case’ events.  All the data used is based on very limited research and verification.  Some elements of the needed research was never started by the time the project was closed down.

Some elements not included in this system are:  Size of burn, (except on a very large scale); fuel loading, moisture, size, shape and compaction.  Temperature, relative humidity and surface wind are not included except as part of the Dispersion Index.

This system is designed as a starting place - and to get a "feel" for managing smokes. It should be used with judgement by the burning prescriptionist to estimate what may happen with the smoke and how to manage it to avoid any smoke problems.

With experience, those responsible for prescribed burning can modify this system to more closely fit the size of the burn, type and condition of the fuel, weather conditions, and type of burning techniques used.

The Screening System (Daytime Use)

This system consists of five steps:

  1. Determine distance and direction of possible smoke impact.

  2. Identify each smoke-sensitive area (SSA).

  3. SSA within first 1/4 impact distance.

  4. SSA within last 3/4 impact distance.

  5. Interpreting screening system results.


STEP I:  Determine Distance and Direction of Probable Smoke Impact


  1. On an appropriate map or overlay, plot the area that is within five chains of the perimeter of the planned burn.  (Consider using up to ten chains if a large burn or heavy fuel loading.)
    This indicates the area that may be impacted regardless of the wind direction.

  2. To use the table below, estimate the Category Day on which you plan to burn. For example, a numerical rating of 21 to 40 would be “Fair Dispersion” or a category 3 day. You might plan to use a category “3” or “4” day  because they occur more frequently, or because category “6” might produce too hot a fire.  Whichever category day you choose, consider it tentative at this point.  From the table below, under the Category Day you have chosen, determine the maximum distance that smoke could be a problem corresponding to the fuel type you have and the firing technique you plan to use.  Visible smoke may be present for this distance.

 

Distance of Possible Smoke Impact

Fuel Type or Firing Technique Distance in Miles by Category Day
(Dispersion Index)+
1*
(7-12)
very poor
2
(13-20)
poor
3
(21-40)
fair
4
(41-60)
good
5
(61-100)
very good
6
(>100)
excellent
1. Grass, crop residue, light understory, or --using backing fire with any fuel type. 8 4 2 1 1/2 1/4
2a. Heading fire, spot fires, or <300 ac. 16 12 8 4 2 1/4
@2b. Heading or spots >300 ac. do not burn 20 12 6 3 1/2
#3a. Scattered logging debris <200 ac. do not burn 16 8 4 2 1
#3b. Scattered logging debris >200 ac. do not burn do not burn 12 6 4 2
#4. Small dry piles. do not burn do not burn 12 6 4 2
#5. Large, wet piles or windrows. do not burn do not burn do not burn 15 10 5
* When Dispersion Index Falls below 7 (seven), Do Not Burn!
@ When the fuel type is Palmetto-Gallberry more than 5 ft. high, --use 1.5 x above distances.
# Firing should be completed by 3:00 p.m. because dispersion will deteriorate at night--usually to "poor".
+ Based on Interstates which are most sensitive to smoke.
  1. On the same map or overlay, plot the planned transport wind direction for the burn the distance determined from the table. (Step 1B)  Draw lines from each flank of the fire at an angle of 30 degrees outward from the wind direction and the length of the impact distance.  Connect the ends with an arc as shown in the sketches below.  If actual or forecasted winds are light or variable, when rechecking on the day of the burn or the day before, replot impact area using a 45 degree angle. This step indicates the area that may be impacted by smoke during the day.  (The heaviest concentration will be along the centerline).

  2. Next, follow down-drainage the same distance determined from the table in “Step 1B.”  Draw in a narrow area covering only the “bottom” or width of the drainage area.  This is the area that may be impacted after dark by the residual smoke that is produced.  (If grass-type fuel, or only one-year old, skip this step.  You will have very little residual smoke.)  


STEP II:  Identify and List Smoke Sensitive Areas


  1. Identify any smoke-sensitive area adjacent to or within five (or ten) chains of the perimeter of the planned burn, regardless of direction from fire as determined in Step 1A above.  List these in your written prescription.

  2. Identify and list any smoke-sensitive areas (SSAs) located within the probable downwind impact area determined in Step 1C above.

  3. Identify and list the SSAs located within the down-drainage impact area determined in Step 1D.  

  4. If no SSAs are located as described in “A,” “B,” or “C” above, You should have no smoke problem.  Go Directly To Step V.  If any SSAs are found, you need to take steps to reduce the production or dispersion of smoke so that no SSA will be adversely impacted.  Continue screening system by going to step III.


STEP III:  SSAs in First 1/4 of the Impact Distance.


  1. Fuel Type is Windrows.*

Do not burn windrows within the first 1/4 of the impact distance.  Change the prescribed wind direction to redirect smoke, or increase the Category Day to shorten smoke impact distance, so that no smoke-sensitive areas lie within the first 1/4 distance.  If this is not possible, don’t burn the windrows.  When they are dry, repile in small, round piles and continue screening system.

  1. Fuel Types Other Than Windrows

    1. SSAs adjacent to or within five (or ten) chains of fire perimeter, regardless of direction from fire, may be impacted.  Create a Safety Zone.

      Plow out a safety zone at least three-to-five chains (200-300 feet) wide on the edge of the burn area facing the SSA.  Burn this zone first during the middle of a day when steady winds (at least 4 m.p.h.) are blowing away from the SSA and preferably on a high (5 or 6) category day (see table in Step I).  If SSA is down-drainage, burn the plowed-out zone in small blocks of five acres or less.  If the SSA is a highway, have flagmen available and be ready to plow out the burn.

* Note:  Windrows are the most polluting of all southern fuel types due to compacting and slow drying, large size fuel, dirt and lack of oxygen for the combustion process.  Dirt in piles or windrows will drastically increase the amount of smoke produced, especially the residual smoke.

If all SSAs adjacent to or within five (or ten) chains of the burn are not down-wind or down-drainage, it should be safe to burn once the safety zone is established. If there are no other SSAs, Go To Step V. If there are other SSAs, continue with the screening process.

  1. If any SSAs are within the first one-quarter (1/4) of the smoke impact distance determined in Step 1, either downwind or down-drainage, a smoke problem would be likely [this includes any SSAs identified in B(1) above, if down-wind or down-drainage].  If possible, adjust your prescription by either (a) or (b) below:

  1. If any SSAs are downwind, use a different wind direction that will miss them.

  2. Burn only during a better category day that will result in the SSAs not being within the first 1/4 of the probable impact area.  If SSAs are still within the last 3/4 impact distance, go to Step IV.

If following (a) or (b) above removes all SSAs from the total impact area, Go To Step V.

If following (2a) or (2b) above is not feasible, continue . . .

  1. If an SSA within the first 1/4 impact distance is unavoidable:

  1. Burn only on a Category 5 or 6 Day.

  2. If the fuel is scattered logging debris or small piles, or there will be residual smoke at night, complete active firing by 3:00 p.m.  Burn only in blocks of ten to forty acres and monitor. The closer the SSA to your burn, the smaller the blocks should be.

  3. If fuel is an understory or field, take as many other steps as needed to reduce the possibility of any adverse impact of smoke (see Minimizing Risks).

(Go To Step V if there are no other SSAs)


STEP IV:  SSAs Within Last 3/4 of the Impact Distance


  1. Fuel Type is Windrows or Large Piles:

    1. If the SSAs are downwind, change the prescription for wind direction to miss the SSAs.  Then Go To Step V

    2. If down-drainage or you cannot miss the SSAs, then use techniques proved by experience to reduce any smoke impact, such as:

      1. No interstate or major highway within three miles down-drainage – especially in fog prone areas.
      2. Shake to remove excess dirt and repile in small piles.
      3. Remove stumps and larger material and do not burn them.
      4. Burn small blocks of 40 acres or less.
      5. Complete active firing by 3:00 p.m.
      6. Monitor after dark and before daylight.
      7. Mop-up if needed.

(Go To Step V if there are no other SSAs.)

  1. If (1) or (2) above cannot be accomplished, Do Not Burn.

  1. Fuel is Either Scattered Logging Debris or Small, Round, Dry Piles
    Do One or More of the Following:

    1. Area to be burned at any one time should be 160 acres or less.

    2. No interstate or major highways within two miles down-drainage --especially if fog prone area.

    3. Complete firing by 3:00 p.m.

    4. Monitor smoke after dark and before daylight.

    5. Mop-up if needed.

  1. Fuel Type is Understory or Fields:

    1. If fuel includes brush (such as palmetto, gallberry and titi) that is head high or over:

      1. Each burning block should not be over 160 acres.

      2. Complete firing lines by one hour before dark.

      3. If there will be residual smoke, monitor after dark and before daylight.  Go To Step V.  

    2. If fuels are light and less than head high:

      1. If backing fire, or less than 300 acres, Go To Step V

      2. If heading fire and large burn (more than 300 acres):
        Burn when mixing height is 2,000 feet or more and transport winds are 10 miles per hour or more, and---complete firing by one hour before dark.  


STEP V:  Interpreting Screening System Results


If all the requirements in the smoke screening system have been met to this point, the burning can be done safely as prescribed smoke should not be a problem.

If all the smoke screening system requirements have not been met:

Caution:  The smoke screening system does not consider other sources of smoke which may be reducing visibility in the area.  Minimum visibility from the site of the planned burn should be five miles or greater.

If some conditions are marginal, smoke could still be a problem.  On the other hand, it may be possible to burn without causing a smoke problem even though the screening system indicates otherwise.  You may not have a problem if:

There is not enough information, nor could the burning prescriptionist measure all the variables necessary to predict how much visibility will be reduced at a certain distance or the effect it could have on human health or welfare.  Nor is the interaction of all these variables well understood.  Potential fog at night compounds this problem.

The final decision must be made by the person in charge.  That person must be experienced and knowledgeable about prescribed burning and smoke management.  He must use his experienced judgment along with this screening system or guidelines to determine if his particular burn will cause a smoke problem with the weather conditions and burning techniques desired.  Any time an extra measure of caution is desired or dictated for any reason, follow the applicable recommendations in “Minimizing Risk”.

 

SMOKE SCREENING SYSTEM AT NIGHT

By complying with the requirements below, the smoke screening system can be used at night.

Logging debris burning should not be done at night, even when scattered.  Neither should any other type except understory burns in young stands where low temperatures are required.  Fuel loading should be light and small areas burned (80 acres or less). The dispersion index forecast should be at least 13 for the night (20 would be better). Atmospheric stability will almost always be stable to very stable with little or no winds.

If SSAs might be impacted, night burning should only be done when there is a forecast of a strong cold front with steady winds lasting all night.  The weather should be continuously monitored during the night as well as the fire itself and the smoke.  Have a plow unit at site so the burn can be plowed out if it becomes necessary.  Have lighted “smoke” signs available.  Check with the sheriff and state troopers to alert them to the burn and to find out how often they patrol the roads in the area.

 

GAINING EXPERIENCE IN SMOKE MANAGEMENT

To get a better understanding of how much smoke is produced and what happens to it in various types of fuel, weather, and topography conditions in your area, do the following:

  1. Check the production and dispersion of smoke on all your prescribed burns and document--even when no SSAs are identified.  Check downwind and down-drainage to observe the smoke during the  day, after it is completely dark and next morning at daylight.  Document the distance that smoke could become a visibility problem on each prescribed burn.  This is good information that you can use later when burning the same type fuel under the same general conditions and you have SSAs to consider.

  2. Check the smoke after dark.  If there is much residual smoke, monitor all night. If not, check again before daylight the next morning.

  3. Become aware of where and at what time fog generally occurs in your area. Check all the roads in your area during times when there is fog. Locate and mark the fog-prone areas on your administrative map. When making the prescriptions, check to see if your smoke might impact a fog-prone area. If so, take extra cautions to take pictures and document your burn and smoke.

After getting sufficient data, you may determine that you can safely reduce the distances in the table for some burns or modify some other items of the screening system. Due to limited data, this system tends to be conservative.  Your prescribed burns may be smaller in size, or for some other reason produce less smoke than this system suggests.

Remember there are large gaps in the information needed for such a system. Consequently, the data in this system is very conservative.


Smoke Management is Good Management!