National Fire Danger Rating System

John Saltenberger

Fire Weather Office

Northwest Interagency Coordination Center

5420 NE Marine Drive

Portland OR 97218

The National Weather Service uses the Lower Atmospheric Severity Index (LASI) to quantify the potential for forest fire growth. This index was developed a few years ago by a USDA Forest Service research meteorologist named Don Haines. Commonly called the "Haines Index," this index is a simple way to measure the atmosphere's contribution to the growth potential of a wildfire. The Haines Index combines two atmospheric factors that are known to have an effect on wildfires: Stability and Dryness.

1. Stability.

Stability is a term used to describe the tendency for vertical air motion in an airmass. A stable atmosphere tends to discourage any vertical air motion while an unstable atmosphere promotes it. A wildfire stands a much better chance of growing large in an unstable airmass because vertical motion encourages the growth of the smoke column. This would increase the chance of spotting and crowning and a very strong smoke column can eventually create its own wind. i.e. become a plume dominated fire.

2. Dryness

The drier the airmass, the drier the fuels are likely to be. Thus, more combustive energy is released and fire spread is enhanced.

Since the Haines Index is intended to be used all over the United States it is adaptable for three elevation regimes: Low Elevation, Middle Elevation and High Elevation.

Low Elevation is for fires occurring at or very near sea level. Not applicable for most fires in Oregon.

Middle Elevation is for fires burning in the 1000-3000 foot elevation range. This is applicable to some coast range and lower Cascade fires in Oregon.

High Elevation is intended for fires burning above 3000 feet elevation..i.e. most eastern Oregon fires.

Fire weather offices regularly calculate the Haines Index. Haines Index data becomes available shortly after weather balloons are released at selected locations across the nation at 5:00 am PDT and 5:00 pm PDT each day during the fire season.

There are some strengths and weakness associated with the Haines Index which I will elaborate on.

1. Advantages

* The Haines Index is a quick and easy way to combine two important atmospheric factors into one simple value. If upper atmospheric data is available, the Haines Index is quite easy to compute. The Haines Index can then be calculated for daily operational use or for historical weather research.

* The Haines Index tends not to "cry wolf" too often. A 1990 study at the Boise Fire Weather Office found that a Haines Index value of 6 occurred on only 6% of the days during the fire season. However, it was on these days that 75% of the season's total acreage was burned! On the other hand, days with a Haines Index of 2, 3, or 4 occurred much more often but only 7% of the total acreage was consumed. Informal studies in Oregon suggest similar results.

2. Drawbacks

* The Haines Index does not take wind into account at all. It is possible to have a low Haines Index value while the wind is creating a very dangerous fire situation.

* The Haines Index does not take into account fuel moisture. It only measures moisture at the 5000 and 10000 foot levels in the free atmosphere. Moisture at these upper altitudes may not be representative of the relative humidity conditions at the surface where the fuels are.

* The Haines Index does not account for terrain, fuel continuity, slope or ignition risk. The index quantifies the growth potential only of existing fires.

* The Haines Index can only be calculated at locations where a radiosonde weather balloon sounding is available. These soundings tend to be about 300 miles apart in the contiguous U.S. If a fire occurs in a region between these locations the meteorologist must estimate what the Haines Index is at the fire location. The estimates tend to be pretty good but terrain effects may modify the local conditions.

* The Haines Index is available only twice a day...5am and 5pm during the summer. Atmospheric conditions may change between these reporting times.

* The Haines Index is difficult to predict. Since the index is computed from temperature and humidity values at several levels of the upper atmosphere, precise forecasts of these factors are necessary at each level. New computer modeling technology is allowing meteorologists to attempt such fine scale forecasts but only time will tell whether they are accurate enough to be useful.

Don't let all the drawbacks listed above discourage you! The Haines Index is very useful for evaluating wildfire growth potential. The danger comes with the mentality that this one index "solves the whole problem". It does not! It is only one more tool that partially measures the atmosphere's contribution to fire growth potential. The fire manager must still evaluate the other legs of the fire triangle...fuels, terrain and all aspects of weather.

John F. Saltenberger is a meteorologist with the N.O.A.A. National Weather Service and a graduate student at Oregon State University. His specialty is forecasting the weather for the control of forest fires. He has served as incident meteorologist at several severe wildfires such as the 1991 Spokane Complex and the 1994 siege in Colorado. John lectures on general weather topics including small scale and mountain meteorology. John enjoys hiking, soaring and scuba diving...he likes to get outdoors often.

Direct any inquires to:

John F. Saltenberger

National Weather Service
Fire Weather Office
5420 N.E. Marine Drive
Portland OR 97218

phone: 503-326-2420

internet email: jsaltenb@oce.orst.edu

website: nimbo.wrh.noaa.gov/Portland/fwx.htm