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Where and When to Measure Forest-Fire Danger
G. LloydHayes
•
This article presentsthe results of a study to determine the place, time, and number of measurements that shouldbe made to obtain dependableratings of "average-bad"fire conditionswithout
an excessive number of stations or observations.
The author concludes that under the conditions
prevailing in the Priest River ExperimentalForest in northern Idaho a single measurementtaken
daily at noon at either a valley-bottomor a south-slope
stationis adequatefor the rating of fire
danger.
OREST-fire
danger
a is nowmeasured
in in manningfor fire control'in the longleafpine
everyforestregionof theUnitedStates.type.
Severalsystemsof measuringand evaluatIt was early recognizedthat the sameprining dangerhavebeehdeveloped
(3), all of which ciple that governs fire-suppression
practices
are of value in the current supervisionof fire shouldalsogovernthe placeandtime of making
suppression
and in fire-controlplanning.
danger measurements.Fire danger is known
Studies.at the Northern Rocky Mountain For- to vary greatly from place to place and from
est a•d RangeExperiment.Stationhave aimed time to time at the sameplace. For example,
consistently
to determine(1) what factorsaffect in the samefuel type the dangeris obviously
fire danger,(2) howtheymaybe measured,
(3) greateron souththan on'north slopes,at lower
wherethey shouldbe measured,(4) whenthey than at higherelevations,
and duringthe midshouldbe measured,and (5) how to integrate afternoonthan at night. Therefore,if an areais
thesemeasurements
into a reliable,practical,.and "manned"to copewith the averagefire danger
readily usablescale. Thesedistinctcomponents of all the aspects,
elevations,
and hours,the orof theproblemconstitute
the"what,how,where, ganizationshouldbe able to handlewith ease
and when" of fire danger. The "why" of fire- all fireson the less-dangerous-than-average
north
dangermeasurement
is primarilyto indicatethe slope,especiallythoseoccurringat night; but it
size of the controlorganizationneededto cope would be entirely inadequateto control fires
with existingfire danger.
starting on the more-dangerous-than-average
Presentpoliciesrequirethat preparations
be southslope,
particularly
duringthecriticalaftermade for controllingall fires that start under noon period. These important considerations
"averagebad" or easierconditions.This means were not adequatelytaken into accountin the
a fire-controlorganizationcapableof suppress- statisticalanalysesof fire-weatherstation dising .all fires,exceptthosethat might start in the tribution madeby Morris (11).
worst ]uela under the very worst burning conIn the early stageof measuringfire dangerin
ditions,and recognizes
the importance
of the law the NorthernRockyMountainRegion,the availof diminishingreturnsin fire-controlplanning. ability of men at an established
stationand conHornby (7) accepted
the factthat beyonda cer- veniencein makingthe ,measurements
were the
tain pointit is moreeconomical
to takethe loss controllingcriteria as to where and when the
than it is to bearthecostof preventing
that loss. average-bad
shouldbe measured.Theseare still
RecentlyKnorr (8) recognized
the application dominantfactorsin actualpractic&.By 1934,
of the average-bad
principlein selecting
the loca- however,progressin otherphasesof evaluating
tionsof fire-dangerstationsto be usedasa guide fire dangerhad showntheneedfor a betterbasis.
•Formerly attached to the Division of Forest Protec- Therefore,very appropriatelyat that time, a
tion, Northern Rocky Mountain Forest and Range Ex- study was startedat the Priest River Experiperiment Station, Missoula, Montana; now in charge of
howthe
torest-fi•e research at the Appalachian Forest Experi- mentalForestto find out moreprecisely
various fire-dangerfactois differed in mounment Station, Ashevill½, iN/. C.
a"Forest-fire danger" is used in its broad senseas defined in the Glossaryo! Terms Used in Forest Fire Control, U.S. Department of Agriculture, Forest Service,
1•39, namely,"A general term expressingthe sum total
tainous terrain in order to acquire more com-
plete and basicknowledgeof where and when
to measurefire danger.
ef both the constant and variable factors which deterThe resultsof that altitudeand aspectstudy
mine whether fires will start, spread, and do damage,
have been given in two previouslypublished
and which determine their difficulty of control."
744
WHERE AND WHEN TO MEASURE FOREST-FIREDANGER
745
reports(5, 6). Thispaperwill show,in sofar .as
Figure 1 may be likenedin somerespectsto
wind and the .atmospheric
factorsaffectingfuel- a contourmap savethat its "contours"are lines
moisture conditions are concerned, how the of equalburningindexratherthan linesof equal
average-bad
fire conditionin ruggedtopography elevation.At 6 a.m., for example,there are
may be evaluatedand whereand whenit can be "valleys"of low burning indexesat both low
measured.
and high elevations
with "ridges"of high burning indexbetween. Theseculminatein the highest"peak"around2 p.m.
The 'data are readily usableto approximate
the
burningindexat any elevationon northand
Fire-weatherdatawereobtainedat sevenfully
south
slopesbetween2,200 and 5,500 feet and
exposedstations. One of these,whichmight be
called the valley-bottomcontrol becauseof its at anyhourof thedayor night. For example,
similarityto a dangerstationat any rangersta- the figures'on the verticalline for 6 a.m. in
tion, waslocatedin a flat valleybottomat 2,300 Part A showthat the burningindex increases
feetelevation.
The remainingsixwereestablishedfrom valleybottomup to 3,500feetandthendetowardthe mountaintop, as follows:
in pairs locatedon true north and southslopes creases
BURNING INDEXESs AS A MEASURE OF AVERAGEBAD FIRE DANGEa
at elevationsof 2,700, 3,800, and 5,500 feet on
Elevation,
a ridge rising from east to west. All pairs
werewithin 100 feet elevationof the ridge crest.
At each stationwind velocityat 7• feet above
thegroundandmoisture
contentof duffandhalf-
feet
inch fuel moisture indicator
sticks were mea-
suredand recordedcontinuously
by an anemo-
Burningindex,
class
2,4OO
3,000
3,500
4,000
2.6
3.2+
3.5
3.4•
4,500
3.3+
5,000
5,5OO
3.2
3.1
hygrograph.
Similarly, the horizomalline for 3,000 feel
The fuel moistureand wind velocitydata of
elevation
on thissameslopeshowsthattheburneach station were integratedby the Northern
Rocky Mountain fire dangermeter (model 5) ingdndexratingsfor varioushoursof the day
into burningindexeson a scaleof sevenclasses. were as follow.s:
Theseembr.ace
theentirerangeof burningcondiHour•f
Burning
index,
day
class
tions, from no spreadof fire under Class1 to
10 p.m.
3.8
spreadsof 1,500 to 2,000 acresper hour when
2 a.m.
3.4
the explosive
conditi6ns
underClass7 prevail.
Figure 1 showsthe altitudinaldistributionof
the burning indexeson both north and south
6 a.m.
10 a.m.
2 p.m.
6 p.m.
3.2+
4.4
5.2-4.6
slopesfor 24 hoursof .themedianday4 in JulyIf average-bad
fire dangeris concretelyde.
August,1938. The conditions
shownare typical
or normalfor the clear,dry fire weatherthat is fined in termscapableof numericalexpression
so characteristic of northern Idaho summers.
(10), theinformation
in FigureI obviously
can
Median valuesare used,sincemean valuesor be used to determine when and where to measure
arithmeticaverages
aretoomuchaffected
by the ".average-bad"
with respectto aspect,elevation,
occasional,
unusualstormyweather(1, 4, 7) to and time. The chartclearlyshowsthat at idenhigherburninginbe typicalof the summer
climateof thisregion tical hoursand elevations,
withitshightemperatures,
low relativehumidity, dexes
existonthesoutherly
aspect!A) thanon
the northerlyaspect(B). Likewise•at all elevascantyrainfall,andlongperiods,
of drought.
tionsonthesoutherly
aspect
thedangeris great.
aThe Committeeon ForestryTerminologyof ;he Soer
between
10
a.m.
and
6
p.m.,
the so-called
ciety of AmericanForesters
has definedburningindex
as "a relative number denotingthe combinedevaluation burningperiod,than at any other hour of the
of the inflammability of forest fuels and the rate of entireday. Thesefactsleadto onelogicalconspreadof fire in suchfuels for specificcombinations
of
themeasure
of the average-bad
mustbe
fuel-moisturecontent,herbaceousstage,and wind ve- clusion:
upontheaverage
of fire-danger
conditions
ß 1ocity." In someformerpublications
(5, 6), burning based
indexes have been termed "fire-behavior classes."
asrepresented
by theburningindexeswhichpre•Median day is the day on which conditionswere
such that half of the July-Augustdays showedworse
fire danger and the other half less fire danger.
vail on the southerly
aspectandvalleybottom
between
thehoursof 10 a.m. 'and6 p.m.
746
JOURNAL OF FORESTRY
NIGHT
PERIOD
MORNING
DAY PERIOD
EVENING
5400
5200
5000
48,00
48,00
4400
4200
4000
38,00
3800
3400
3200
3000
2800
2800
2400
5400
,5200
,5000
4800
4600
4400
4200
4'000
3coo
3800
3400
3200
3000
2800
2600
2400
I0
MDNT.
2
4
6
A.M.
8
I0
NOON
2
4
8
8
I0
P.M.
Fig. L--Burning-index ratings for different altitudes, different hours of the day and night, and on
south (Part A) and north (Part B) alopesfor the median day of July-August, 1938, on the Priest
River Experimenta]Forest. The isograms,or "contour lines," are lines of equal burning index.
WHERE AND WHEN TO MEASURE FOREST-FIRE DANGER
747
measurements
at anindividual
staOn the medianday of July-August,
1938,the agingseveral
severalvaluesfrom different
burningindexof the average-bad
condition
was tion or combining
class 4.8.
This was obtained as follows: (1)
For the two monthsthe averageburningindex
of thecriticalperiod(10•a.m. to 6 p.m.) was
stations ?
AccuRAcY OF INDIVIDUALSTATIONRATINGS
with whichthe
calculated
separately
for eachof thethreesouth- Table1 showsthe accuracy
burningindexon anyJuly-August
slopeand the onevalley-bottom
stations.(2) average-bad
Next,thepercentage
of all fiats.andsouthslopes daywasapproximated
by singledailyratingsat
includedin the studyarea (6,000 acres)which eitherthevalley-bottom
or the mountain-top
stafell within the altitudinalzonebestrepresentedtion.Anyofthethreesingle
measurements
rated
by oneof eachof thefourstations
wasestimated average-bad
withan accuracy
sufficient
for pracfroma topographic
map.(3) The.average
burn- ticalpurposes,
but the oneat 2 p.m. on the
ing indexof eachstationobtainedin step1 w.as southslopenearthe lookoutstationwasbest.
then multipliedby the appropriatepercentage Whilethepractice
of measuring
onlyonceeach
fromstep2 andthe four products
wereadded
together.(4) Their sum,class4.8, was the
weighted-average
burning index of all south
slopesandvalley-bottom
areasembraced
by the
study.
Figure 1 (A) showsthat on the normalday,
between10 a.m. and 6 p.m., a burningindex
of class4.8 occurredtwice in the valley bottom
and at all elevationson the southslopeexcept
at thehighestpoint,whereit prevailedonlyonce
at 2 p.m. Consequently,
the average-bad
condition, as defined,could be measuredat any
placein thevalleybottomor onthe southslope.
In the valleybottomthesemeasurements
could
be made either at noon or 5 p.m. Measurements.at any other hours would give indexes
eitherdefinitelybelowor above"averagebad"
for the area. On the southslope,similar measurementswould have to be made at 3,000 and
4,000 feet around 11 a.m. or 5 p.m., and at
5,500 feet around2 p.m.
SOME PRACTICAL CONSIDERATIONS
dayandthenonlyat a lookout
station
wouldbe
readilyfeasibleon mostForestServiceranger
districtsin July and August,it wouldnot serve
duringearlierandlatermonths
whenthelookout stationsare oftenunoccupied.
RELATIVE ACCURACYOF THE THREE SETS
OF DATA
As shownin Table 1, the 2 p.m. burning
indexesfrom the south-slope
station at 5,500foot elevation were most accurate. There on 50
percentof all daystested,the ratingsdiffered
from the average-had
burningindex by only
___0.1
classor less;on 66 per cent of the days
the difference was +--0.24 class or less. How-
ever, at the valley-bottom
stationthe readings
were almost as reliable.
There the noon data
varied from the average-badrating by ___0.15
classor less on half the days,and by ___0.34
classor lesson two-thirdsof the days. Furthermorethan 5 p.m. data at this samestationdeviatedfrom .average-bad
by not over ___.20
and
+-0.33 classeson half and two-thirdsof the days,
respectively.
The largestoverestimate
exceeded
the averagebad ratingby 1.3 classes
and the largestunder-
As broughtoutabove,the average-bad
canbe
measured
at morethan oneplace,and at different hoursof the day in someplaces. Usually
menfor this purposeare readilyavailableonly estimate was 1.0 class too low. These errors
but
at valley-bottomranger stationsor mountain-top mightappearto be of practicalimportance,
lookoutpoints. The valley-bottom
and 5,500- they all occurredon dayswhenthe average-bad
foot south-slope
stationsusedin this studywere burningindex was low. On suchdays,when
typicalof suchlocalities.Measurements
of aver- fire will not give serioustrouble,errors of this
aswhendanage-badmade at thesestationswere testedto amountarenotof suchsignificance
in efficientfire control
answer several important practical questions. ger is high and success
vitally on accurateestimationof condi(1) How accuratelydo singleratingsat an indi- depends
dividualstationrepresentthe average-bad
from tions.
day to day; (2) for the stationsin question, The maximum overestimate of 1.3 classes ocwhichof the threeratingsapproximatedaverage- curred,for example,on July 6 whenaverage-bad
badmostaccurately;
and (3) howmuchcanthe for the 6,000-acrearea as a whol.ewas rated at a
accuracyof daily ratingsbe increased
by aver- burningclassof 2.4. At noonon that day,mea-
748
JOURNAL
OF FORESTRY
surementsat the valley-bottomstation indicated of about one-tenthof a class. Even its largest
the burning index at class 3.8. However,be- singleerror, an overestimateof 0.4 class,would
causeof rain which startedat 2 p.m. the same not be of seriousimportance
to a protectiveor-
day, the 5 p.m. index at this stationfell to class
1.6 or 0.9 classbelowthe average-bad.Similar•
ly, the diurnal trend was abnormalon the day
immediatelyfollowingowingto rapid drying of
unusuallywet fuels. Only on days which were
affectedby recentrain did errorsexceedingñ0.5
class occur. No error exceedingthat amount
occurredon any day unaffectedby rain which
ganization. Two measurements
in the valley
bottomand one on the mountaintop eachday
thereforecan be used, if justifiable on other
bases,to give maximumaccuracy.
DISCUSSION
The aboveanalyses
indicate
thattheoretically
the average-bad
burningindexcan be bestapratedclass3.6 (moderately
bad) or more. In proximatedfrom combinationsof three daily
fact, on the moderatelydangerousdays the rat- measurements
at two stations. From the pracings were generallyaccuratewithin ñ0.3 class ticalpointof view,however,
cananyfire-control
and less.
organizationreally make use of ratings as refined as this? To statethe questionfrom anINCREASING THE ACCURACY OF RATINGS
otherangle:Will the additionalcostof making
While suchaccuracyis reasonably
good,anal- morethan one daily measurement
be justified
ysisshowsthat averagesof two or moremeasure- by the financial benefitsderived from the most
mentsfrom a single station or from separate accurate
butalsothemostcostlyestimate
based
stationsgive greateraccuracythan any individ- on three measurements?
ual measurement.
Three different combinations
Suchquestions
mustbe answered
negatively
of two daily estimatesand one additionalcom- for the NorthernRockyMountainRegionwheu
bination of all daily measurements
were tested. consideration
is givento our knowledge
of fire
:Asis evidentfroin Table 2, averagesof three behavior,
the uniformityof dangerfrom dayto
daily measurements,
two in the valley bottom day, andthe degreeof refinement
employed
in
and one on the mountaintop, approximated present-day
fire-controlwork. Analysisindiaverage-bad
with remarkabledependability.
The catesthat on the two-thirds
of the daysthat a
median and standard errors of such an estimate
fire-controlorganizationwouldhave been most
werereducedto the inconsequentially
low points vitally concerned
with the fluctuatingdanger,
TABLE I.--AccuRACY WITH WHICH THE AVERaGE-BaBBURNINGINBZX WAS APPRGXlMaTED
BY
SINGLE,DAILY MEASUREMENTS
AT DIFFERENTSTATIONS,JULY-AuGuST,
1938
Station
and hour
Valley bottom, 2,300 feet
12:00 m...............................................................
5:00 p.m.........................................................
South slope, 5,500 feet
2:00 p.m..........................................................
Median
error of
estimate •
Standard
error of
estimate
Greatest
over.
estimate
Greatest
underestimate
Class
Class
Class
Class
4.0.15
4-0.20
4-0.34
4-0.33
+1.3
+0.9
--0.9
--1.0
4.0.10
4-0.24
+0.6
--0.6
XThemediandifferencebetweenthe measuredand the estimatedaverage.badburning index.
TABLE2.•AccuRAcY
WITH.WHICHTHEAVERScs-BiB
BURNINg
INBEXWASAPPROXIMATED
BY
COMBINING
THE DAILY MEASUREMENTS
OF TWO ANDTHREESTATIONS,
JULY-AuGusT,
1938
Station and hours
Valley bottomat 12 m. and 5 p.m........................
Valley bottom at 12 m. and south slope,5,500
feet, at 2 p.m.....................................................
Valley battom at 5 p.m. and south slope, 5,500
feet, at 2 p.m.................................................
Valley bottom at 12 m. and 5 p.m., and south
slope,5,500 feet, at 2 p.m...............................
Median
error of
Standard
error of
Greatest
over-
Greatest
under-
estimate
estimate
estimate
estimate
Class
Class
Class
Class
4-0.10
4.0.19
+0.8
--0.4
4-0.08
4.0.18
+1.0
--0.4
4-0.09
4.0.16
+0.3
---0.7
4-0.06
•0.11
+0.4
--0.2
WHERE AND WHEN TO MEASURE FOREST-FIREDANGER
749
was rated as 4.8 at noon. As the valley bottom
singlemeasurements
estimated
the average-bad
wasactually
5.2atnoononthedayofthis
fromonly-q-0.05to ñ0.18 classlessaccuratelyrating
or 0.3 class,mustbe added
than did two observations,
and only ___0.13to fire, the difference,
+__0.23
classlesspreciselythan the medianof to the 5.1 shown for the fire location. The estithree measurements.No fire control force in this matedburningindexat th6 siteof the firstfire
5.1 plus0.3 or class5.4.
regionis at presentso flexiblethat it can be wastherefore
variedfor'a changeof only0.23 classin fire
Similarly
thedangerat thesiteof thesecond
danger.
fire as estimated
from Figure1 and corrected
Moreover,thevariationsin fire behdvior
with- by the noonmeasurement
wasclass4.0 (class
ha the differentburning-index
classeshave not 5.2 of valley-bottom
stationat noonminusclass
yet beendetermined
to a degreethat economi- 1.2, thenormaldifference
between
ratingsof the
cally justifiesincreasingor decreasinga fire- valley-bottom
stationat noonand of a spoton
controlforce to meet a changeof -q-0.34class, thenorthslope
at 2,600feetelevation
at8 p.m.).
the largeststandarderror for estimatesbased Thelocalrangertherebyis ableto judgeandto
uponsingledaily measurements.
Until fire con- reportprobable
fire behavior
at thesetwofires
trol actioncanbe refinedto thepointof actually far moredependably
andspecifically
thanwould
makingchangesin the controlforceswith varia- otherwise
be possible.The actionhe takesfor
tionsof one-thirdclassof fire danger,onedaily a 5.1 conditionobviouslywill be both stronger
measurement is therefore recommended as adeandfasterthanfor theclass4.0, eventhoughthe
quateon anyunit similarto the onestudied.
latterrepresents
a spotat,lowerelevation,
hence
to burning.
Thesetestsare strictlyapplicable,
of course, withmoreareaaboveit andexposed
onlyto the samplingof .average-bad
as herede- While these are conditionsand actionsreadily
fined and computed. There is no reason,how- apparent
to the experienced
forestofficer,
they
ever, if someother index were wanted,why it are not so obviousand are often overlookedby
could not be approximated
by similar methods the newmanto whomall firesmaylook equally
and with similar accuracy. Average-bad
danger sinister.
The severalpossibilities
of making.dailymeaon forest areaslarger than 6,000 acrescould
wereanalyzedalsoto determine
which
not be determinedso accuratelyperhaps;but surements
estimates
an accuracywithin practicallimitationsshould wouldbe the bestbasisfor dispatchers'
be obtainableduring settledweatheron an area of this kind. Each combination of station meamanytimeslargerthan 6,000 .acresprovidingit surements was used in turn with values shown
in Figure1 to sstimate
burningindexesat three
is climaticallyuniform.
Burning-indexmeasurements,
in addition to differentsitesand widelydifferenthours. Furtheir value for rating dangeron an area as a thermore, additional charts were constructed
a
whole, also can be usedby fire dispatchers
to similarto Figure1 but with eachrepresenting
estimateburningconditionsat any spoton their certainlimited rangeof dangerat noonat the
unit where a new fire occurs. The manner in
valley-bottomstation. In using Figure 1 for
Which this may be done can be illustratedby this test, the estimateswere confinedto those
usingthevaluesin Figure1 to showtherelation- daysthatrated4.3to 5.3 at thevalley-bottom
shipbetweenthe dangeractuallymeasuredand station. Other charts should be used for lower
the dangerestimated
to be existing"at the fire." and higher dangers. This limited the number
in eachgroupto 30. Table3 shows
Let it be assumed
that two fires are reported of estimates
the
dependability
found for any one.of three
on a day when the noon burning index of the
different
measurements
when used as a basis for
valley-bottomstation was measuredand found
to be class 5.2, or 0.3 class above the index estimatingconditionson any July-Augustday
shownby Figure 1 for sucha stationand hour. at threeotherspotswithin the area.
In seven of the nine cases the estimated burn.
Further, assumethe first fire as occurringat 3
p.m. on a southslopeat 3,800feetandthe sec- ing indexesdiffered from the actual by only
classor less,on half of all the daystested.
ond at 8 p.m. on a north slopeat 2,600 feet. ___0.2
Accordingto Figure 1 (A) the aspect-elevationIn the other cases the differences were even
combination
for the first fire givesa rating of smaller,from ___0.1to ___0.15classor less. This
aboutclass5.1. But this chartshowsonlywhat uniformity of the median errors of estimate
can be expectedif the valley-bottomcondition showsthat on at least50 per cent of the July-
750
JOURNAL
OF FORESTRY
TABLE 3.--AccURACY WITH WHICH THE BURNING INDEX AT THREE DIFFERENT PLACES ANn HouRs
WAS ESTIMATED FROM MEASUREMENTSTAKEN AT THE VALLEY-BOTTOM STATION AT NOON
AND AT 5 P.M., AND AT THE SOUTH-SLOPESTATION 5,500 FEET ELEVATIONAT 2 F.M.
Situation and hour
of
measurement
Valley bottom,
noon
Median
error of
of
estimate
estimate
Valley bottom,
5 p.m.
South, 3,800 feet,
North, 2,700 feet,
South, 5,500 feet,
South, 3,800 feet,
North, 2,700 feet,
2 p.m.
8 p.m.
10 a.m.
2 p.m.
8 p.m.
South slope,
5,500 feet, 2 p.m.
South, 5,500 feet, 10 a.m.
South,3,800 feet, 2 p.m.
North, 2,700 feet, 8 p.m.
South, 5,500 feet, 10 a.m.
Standard
error of
estimate
Greatest
.overestimate
Greatest
underestimate
+0.15
ñ0.2
___0.2
ñ0.2
ñ0.1
+0.27
ñ0.25
+0.40
ñ0.41
ñ0.22
70.7
-{-0.7
71.3
70.7
70.6
---0.4
---0.3
•0.7
---0.7
-4).5
ñ0.2
ñ0.2
ñ0.2
ñ0.2
ñ0.39
ñ0.22
ñ0.40
ñ0.35
70.8
70.4
70.6
71.0
--0.9
---0.6
--1.3
•0.9
thenthe
Augustdaysconcerned,
the distributionof burn- velocityand direction,or by cloudiness,
of burningindex shouldand mustbe
ing indexeswassosimilarto that shownby Fig- estimates
ure 1 that it mattered little where or when the
modified by common-sense
allowancesbased
measurements
weremade. For example,on half upon experienced
judgment. Good dispatching
the daysthe burningindexesat 2 p.m. on the will alwaysdemandthe exerciseof soundjudgsouthslopeat 3,800feetelevationwereestimated ment. Thiscanbe guided,but neverreplaced,by
just as accuratelyfrom measurements
made 21 mechanical
methodsof estimating.
hoursearlierand 1,500feetbelow(valleystation
For usein dispatching,
burningindexes
should
at 5 p.m.) as from observations
taken at the thereforebe measuredas near as practicalto
sametime and 1,700 feet higher (south-slopebad fire zonesand preferablyjust prior to the
station5,500 feet at 2 p.m.) In eachcasethe time of day whenfires usuallyoccur. In this
median error was the same, ___0.2class.
connection,noon measurementsshould be more
The standarderrors in Table 3 show, however, valuablethan thosemadeat 2 p.m.; the latter
that on dayswhenthe distributionof fire danger in turn shouldhave greatervaluethan 5 p.. m.
wasnotsotypical,greateraccuracy
wasobtained data. Both for useby the fire dispatcherand in
when the measuredand estimatedburning in- over-allfire-danger
rating,measurements
should
dexeswere made near each other with respect not be made during those before-noonhours
to bothplaceandtime. Thus,the standarderror whenthe burningindexis changingrapidly,i.e.,
for estimates
of 2 p.m. burningindexesat 3,800 in valleybottomsfrom 7 through11 a.m., andat
feet on the south'slopewas only ___0.22class mid-elevationson south slopesfrom 8 through
when based upon measurementsmade at the 10 a.m. Measurements made at such times are
5,500-footsouth-slope
stationat 2 p.m. It was not so.reliablyindicativeof whatcondkionswill
___0.41
classwhen using5 p.m. measurementsbe elsewhere on the area as are measurements
made after the burning index has cometo its
of the valley-bottom
station.
Large overestimates
and underestimates
in ex- daily comparativeequilibrium.
cessof ___0.5classoccurredoccasionallyat all
SUMMARY
of the stationstested(Table 3). Errors of that
magnitudeare probablyless importantin the
Fire-dangerratings are necessaryto sound
presentinstancethan they would be in rating fire-controlplanning. Among other uses,they
generalfire danger. In actualdispatching
they furnisha very practicalindexof the pres.uppreswouldbe largelyavoided. No dispatcher,
for sionmanpowerneededon an area. Dangermeaexample,would acceptor use withoutmodifica- surements
madeat onespotand usedto rate the
tion an estimateof the burningindexfollowinga fire .dangerof the immediatelysurroundingarea
rain which was basedupon measurements
that must be representative
of the more dangerous
were taken before that rain started. Yet four of
portions of the area and of the more crkical
the largeit overestimates
in Table 3 werecaused hoursof the day, but not the most dangerous
by exactly such a combinationof conditions. exposuresat the very peak of the day. This
Similarly,when the typical topographic
dis- average-badconditioncan be sampledand extributionof fire dangeris disturbedby uneven pressednumericallythrough burning indexes
distribution
of rain,by unusualchanges
in wind obtainedby integratingmeasurements
of fuel
WHERE AND WHEN TO MEASURE FOREST.FIRE
moistureand wind velocity. For the area ineludedwithin the 6,000-acrePriestRiver Experi.
mental Forest, it was found that these measurementscouldbe madeeither in the valleybottom
at noonor 5 p.m., and on southslopes,depending upon elevation,at 11 a.m., 2 p.m., or 5
751
areas.andnearthe beginningof the daily critical
period. Fromthe dispatcher's
pointof view,the
noon measurement is the best of the three time
periodstestedand the 2 p.m. measurement
is
Single daily measurements
made at a valleybottomstationat noonor 5 p.m., andat a 5,500foot south-slope
stationat 2 p.m., represented
the average-bad
burningindexesfor the area as
a wholewith an accuracywell within the limits
of the practicalapplicationof fire-dangerratings. A combinationof two daily measurements
gave more representative
burning-indexvalues
than did singleobservations,
while three daily
measurements
improvedthe .accuracy
of averagebadratingsstill further. Few,if any,fire-control
organizationsare, however,sufficientlyflexible
to use effectivelyburning indexesmore precise
than thoseobtainedfrom singledaily measurements. Nor is present-dayknowledgeof fire
behaviorso completethat minutedifferences
in
estimatedfire dangerhavepracticalimplications.
Hence, there is no economicjustificationfor
financingadditionalmeasurements
in order to
obtainrelativelymeagerfurther refinement.
For dispatchingpurposesit matterslittle on
normalor averageJuly-August
daysin the Northern RockyMountainRegionwhereor whenthe
burningindex is measured. Each of three sets
of data taken at different sites and hours of the
DANGER
next best.
LITERATURE CITED
1. Gishorne,H.T.
not a normal.
171-175.
2.
1935. When a normal is
Amer.
Met.
$oc. Bul. 16:
. 1939. Hornby'sprinciplesof
fire-controlplanning.-Jour. Forestry 37:
292-296.
3.
. 1939. Fordstpyrology. $ci.
Monthly49(7): 21-30, illus.
4. Hayes,G. Lloyd. 1935. Don't let a "normal" deceiveyou. U. $. Forest•$erv. Bul.
19:4.
5.
1941.
Influence
of
altitude
and aspecton daily variationsin factorsof
forest-fire
danger. U.S. Dept.Agric.Cir.
591. 38 pp., illus.
6.
. 1942.
Differences in fire dan-
ger with altitude,aspect,and time of day.
Jour. Forestry40:318-323, illus.
7. Hornby,L. G. 1936. Fire controlplanning
in the Northern Rocky Mountain region.
North. RockyMountainForestand Range
Expt.Sta. (Multi.).
8. Knorr, Phillip. 1942. Variationsin firedangerfactorson a rangerdistrictin the
longleafpine region. Jour. Forestry40:
689-692.
day provided almostequally preciseestimates
of the burningindexat otherplacesandhours. 9. Mindling, GeorgeW. 1940. Do climaOn lesstypicaldayssuchas whenrain occurred,
tologicalnormalsserve adequatelyas nor-
however, better estimateswere obtained when
mMs?
Amer. Met. Soc. Bul. 21:3-6.
measuredand estimatedburning indexeswere 10. Morris, William G. 1939. What is "avermadeneareachotherwith respectto bothplace
age bad?" Fire ControlNotes3(3):25-26.
and time. Under these conditions the need of 11.
1940. Statisticalanalysisof
forest-fire weather station distribution. Jour.
the fire dispatcherwould be best servedwhen
Forestry38:318-321.
measurements
are made closeto high-danger