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
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