National Weather Service United States Department of Commerce

Pollution Data and Contents


Air pollution information is also used as a guide for prescribed burning and for issuing burning permits. Each day the data released will be as follows:

1.   Afternoon mixing height and transport wind

2.   Ventilation Index - which is the product of the afternoon Transport Wind and Mixing Height.


        LVORI stands for Low Visibility Occurrence Risk Index

         LVORI links dispersion and relative humidity concepts to calculate the magnitude of impact that  occurs when smoke particles interact with fog to further reduce visibility.

* The critical meteorological problem is with forecasting fog probabilities, as some parameters that support less dispersion also support fog formation (light winds and clear skies).  Thus, significant emphasis must be placed on incorporating fog potential in such an index.

* We can use the Short Range Ensemble Forecast (SREF) computer model system that looks at 21 different model members and calculates the proportion of those members that predict a visibility less than one mile at any given location using several meteorological variables.

LVORI Values Category Interpretation:

1 - Lowest proportion of accidents with smoke and/or fog.

2 - Physical or statistical reasons for not including in category, but proportion of accidents not significantly higher.

3 - Higher proportion of accidents than Category 1 by about 30 to 50%.   

4 -  Significantly higher than category 1 by a factor of 2.

5 - Significantly higher than category 1 by a factor of 3 to 10.

6 - Significantly higher than category 1 by a factor of 10 to 20.

7 - Significantly higher than category 1 by a factor of 20 to 40.

8 - Significantly higher than category 1 by a factor of 40 to 75.

9 - Significantly higher than category 1 by a factor of 75 to 125.

10 - Significantly higher than category 1 by a factor of 150.



 >100     Very good (but may indirectly indicate hazardous conditions)

61-100   Good (typical-case burning weather values are in this range)

41-80     Generally good (climatological afternoon values in most  inland forested areas of the US fall in this                        range).

21-40    Fair  (stagnation may be indicated if accompanied by persistent  low windspeeds)

13-20    Generally poor, stagnation if persistent (although better

             than average for a night value).

7-12      Poor , Stagnant by day (but, near, above average at  night).

1-6        Very poor. (frequent at night, represents a majority of most locations at night)


     Category Day: Guideline for Prescribed Burning in Louisiana

1.   A Category Day of ONE - which is No Burning. It has a ventilation rate of less then 2000.

2.   A Category Day of TWO - which is No Burning after 11:00 am and not until surface inversion has lifted. It has a ventilation rate of 2000 to 4000.

3.   A Category Day of THREE - Daytime Burning only, but not before surface inversion has lifted. It has a ventilation rate of 4000 to 8000.

4.   A Category Day of FOUR - Burning anytime. For Night Burns use backing fire with surface wind speeds greater than 4 mph. It has a ventilation rate of 8000 to 16,000.

5.   A Category Day of FIVE - Unstable and Windy. Excellent smoke dispersal. Burn with Caution. It has a ventilation rate of greater than 16,000.

Estimating the Stability Class of the atmosphere out of Arkansas.

1.   A is very Unstable(Excellent dispersion and Windy)

2.   B is Unstable

3.   C is Slightly Unstable

4.   D is Neutral

5.   E is Slightly Stable

6.   F is Stable(Strong Inversion in place...Capped)

The Stability Class used in Fire Weather forecasts is based on wind, incoming solar radiation, and cloud cover. The wind speed is the average wind expected for the period of concern, either daytime or nighttime.

Calculations are being made from the NAM and GFS NWS Weather Model and will be compared to AWIPS model soundings as well as area soundings for comparison purposes. Temperature and transport wind information is calculated for each forecast zone in the County Warning Area. During the comparison, quality control adjustments will be made to the mixing heights and transport wind, to ensure as much accuracy as possible.

Mixing heights are in feet/meters and transport winds in meters/second and mph. These are calculated values from a standard plot of the vertical temperature, moisture, and wind profiles. The mixing height defined as that height above ground level to which a parcel of air will rise using dry adiabatic thermal bouyancy. The transport wind is the average windspeed through the mixing layer and average direction...using eight points of a compass.

The Mississippi Forestry Commission only issues burning permits when the mixing height is at least 500 m and the transport wind is at least 3.5 m/s. Can not do prescribed burning when humidity is less than 25 percent.


The Louisiana and Arkansas Forestry Agencies are similar and will only issue permits if the mixing height is at least 500 meters or 1700 feet with a transport wind of 4 m/s or 7 mph.

The remarks section will explain the nature of the wind shift and its significant affects on Forestry Operations.

Fire Weather Danger Surface Indices numbers 0 through 5. 0 is no fire danger; 1 is little fire danger; 2 is low fire danger; 3 is moderate fire danger; 4 is high fire danger; and 5 is extreme fire danger