Glossary

Air Quality Index (AQI)

The Air Quality Index (AQI) is a standardized index used by the U.S. EPA for looking at overall ambient pollution. The AQI is typically normalized so that a value of 100 represents a 24-hour National Ambient Air Quality Standard (NAAQS) exceedence (note:  for PM2.5 the AQI is normalized to 40ug/m^3 = 100).   When fire smoke is the major issue, the AQI will be driven by fire smoke PM2.5 concentrations and the AQI values can be used as a proxy for PM2.5 levels. 

See also:  AirNow website AQI discussion

Haines Index

A simple measure of the probability that an existing fire will become a dangerous, erratic fire in the absence of strong surface winds.  It reflects atmospheric stability and moisture in a layer of the atmosphere roughly 1 to 5 km above the surface.  High values indicate high risk of dangerous behavior.  The Haines Index is calculated across three pressure layers, Low (950-850 mb), Middle (850-750 mb), and High (750-500 mb) based on the surface elevation at the location of interest.  In general terms, the Low variant is appropriate for locations below 200 m elevation, the Middle variant is for locations between 200 m and 1000 m, and the High variant for locations above 1000 m elevation.  A Haines Index value of 2 or 3 is considered "Very Low;" 4 is "Low;" 5 is "Moderate;" 6 is "High."

Mixing Height

The depth of the atmospheric layer where turbulent mixing causes momentum, moisture, and other properties to be more or less uniform. High values indicate more rapid smoke dispersion and greater risk of gusty, turbulent winds.  They can also indicate a greater chance of wind and moisture conditions aloft coming to the ground, which can influence fire behavior.

PM2.5

PM2.5 stands for particulate matter <= 2.5 microns.  In practical terms, this is the fine-scale component of aerosol particulates, and ambient levels are regulated under the U.S. EPA National Ambient Air Quality Standards (NAAQS) (the current standard is a 24-hour average of < 35 ug/m^3). Wildland fire produces PM2.5 (along with larger sizes), and can contribute to NAAQS exceedences when smoke becomes heavy in an area. 

Forecasts of PM2.5 must be taken in context as modeled PM2.5 levels are dependent on a chain of models (fire consumption, emissions, plume injection, and dispersion).  Additionally, wildland fire smoke models often omit other (non-fire) sources than can raise the observed level in a region.  Often it is best to view PM2.5 levels in context of previous forecast values. 

Smoke Trajectories

Smoke trajectories are a simple model for where the smoke plume is expected to go.  The trajectories follow a single parcel of air as it is advected downwind.  Note that smoke trajectories do not disperse, and therefore cannot represent fully the smoke plume.  Trajectories are used because the computational requirements is significantly less as compared with dispersion or full chemistry modeling.

Surface Winds

This refers to the wind speed and direction at the ground, at a given location and time.  Typically this is measured at 2.5 m above the ground, although it can vary (see below).  It influences the rate of spread of a fire, its intensity, and the transport and dispersion of smoke from the fire. 

Within the WFDSS-AQ collected tools, surface winds refer to somewhat different things.  For the National FCAMMS products, surface winds are shown for the 10m wind.  For the RAWS Wind Rose products the surface wind is measured at 20ft.  For VCIS, surface winds are modeled at 10m. 

Transport Winds

Transport wind is defined as the wind that determines the direction and speed with which smoke is transported away from its source.  In these maps, this is the average speed and direction of the winds from the ground to the mixing height.  Note that some state or local agencies calculate transport winds differently, but the convention used here is the most common.

Ventilation Rate (or Ventilation Index)

The mathematical product of the Mixing Height and the Transport Wind.  This provides an estimate of how high and how far smoke (or other pollutants) will disperse: a high ventilation rate suggests that smoke will spread out quickly and through a deep layer of the atmosphere, so that surface concentrations downwind will be lower than they would be with a lower ventilation rate.