Every forest executive responsible for the protection of forest lands is confronted with the task of planning fire control facilities and action adequate for his area. These facilities and actions will be greatly different in Vermont-the state without a drought-from those required in some other parts of the country where nearly every summer brings a two-month or longer drought. In the various sections o£ the United States the fire control facilities and action justifiable will vary according to what might be called the normal fire danger of that section. As the cost of these facilities and action may vary from a few mills to several cents per acre per year, it behooves the forest executive to plan carefully. I£ he underestimates fire danger, he is likely to get burned out. I£ he overestimates, and overspends, his boss will very likely  burn him up.\u27\u2

Gisborne, H. T.

English

Every forest executive responsible for the protection of forest lands is confronted with the task of planning fire control facilities and action adequate for his area. These facilities and actions will be greatly different in Vermont-the state without a drought-from those required in some other parts of the country where nearly every summer brings a two-month or longer drought. In the various sections o£ the United States the fire control facilities and action justifiable will vary according to what might be called the normal fire danger of that section. As the cost of these facilities and action may vary from a few mills to several cents per acre per year, it behooves the forest executive to plan carefully. I£ he underestimates fire danger, he is likely to get burned out. I£ he overestimates, and overspends, his boss will very likely "burn him up.''</p

Gisborne, H.

Digital Repository @ Iowa State University (ISU)

Determination of NormalFire DangerBy H. I. GISBORNESenior Silviculturist, NQTthern Rocky Mountain Forest and RangeExperiment StationEVERY  forest  executive  responsible  for  the  protection  o£forest  lands  is  confronted with  the  task  of  planning firecontrol facilities and action adequate for his area.  These facili-ties and actions will be greatly different in Vermont-the statewithout a drought-from those required in some other parts ofthe country where nearly every summer brings a two-month orlonger  drought.   In the  various  sections  o£  the  United  Statesthe fire control facilities and action justifiable will vary accord-ing  to  what  might  be  called  the  normal  fire  danger  of  thatsection.  As the cost of these facilities and action may vary froma few mills to several cents per acre per year, it behooves theforest  executive  to  plan  carefully.   I£  he  underestimates  firedanger, he is likely to get burned out.  I£ he overestimates, andoverspends, his boss will very likely "burn him up.''When forest fire research was first  commenced  in  1916  and1917, the pioneer researchers,  S.  B.  Show,  J.  A.  Larsen,  C.  C.Delavan, J. V. Hofmann, and W. B. Osborne, all drove towardthe  same  goal:    indices  of  fire  danger.    Some  selected  wind,some  advocated  humidity  alone,  some  tried  all  the  weatherelements, but both Show and Larsen commenced investigationsof duff moisture and inflammability.  In those days no one spokeof the "fuels" of forest fires, but when Show and Larsen meas-ured the moisture content o£ forest duff, and tested its ignitibilitywith a match, they were studying specific fuels and they wereoriginating  a  phase  of  research  which  is  now  being  pursuedin several sections of the country.AT T.HE  Northern Rocky  Mountain  Forest  and  Range  Ex-perlment Station later studies of duff moisture resulted inAmes Forester 27the invention of the duff hygrometer for measuring the moist-ure content o£ this fuel.  These studies also pointed to the im-portance of other fuels such as dead branchwood, and the woodcylinder  or  "stick''  method  of  measuring  wood  moisture  wastherefore originated.   With  duff hygrometers  and wood  cylin-ders available,  it was found  easier to  measure  this  inflamma-bility factor of fire danger directly rather than measure all theimportant weather factors and then attempt to determine fuelmoisture by integration.Northem Rocky Mountain Station studies o£ fire behavioralso  vindicated  the  research  pioneers  who  had  stressed  theimportance of wind.  These studies showed clearly that rate ofspread of fire varied not only with fuel moisture but also withwind.  Consequently, fuel moisture alone could not be used asa  dependable  index  of  fire  danger.   Wind  velocity  also  mustbe  colnsidered.Analyses of the fire records also proved that,  in the North-ern  Rocky  Mountain  region,  lightning  must  be  included  inthe danger rating scheme.   And,  as more lookouts are neededwhen  atmospheric  transparency  is  reduced  by  smoke,  dust,mist,  and fog,  some  allowance  must be  made  for  visibility,  ifthe danger index is to be used as a guide to the action that afire control executive must take in protecting his property.Obviously,  no  instrument  was  needed  for  measuring  theoccurrence of lightning, but new instruments were needed formeasuring  wind  velocity   and  visibility   distance.    Standardanemometers at $80  or more per instrument were  out of for-esters' financial reach so the Northern Rocky Mountain Stationoriginated  a  cheap  device  for  measuring  wind.   Recently  amanufacturer has been found  able  to  produce  an  even  moreaccurate instrument at less than $5 each.  In the meantime thePacific  Northwest  Forest  and  Range  Experiment  Station  in-vented  a  visibility  meter  and  the  Northern  Rocky  MountainStation followed with a similar device so that visibility distancecan be measured instead of estimated.ITRtg¥hetF:srers:seexa:cchutlfvheeraes:laesc[ceoinlsi:tq:fe:ileye¥saednetiaalVfaai:::::of fire  danger  and  instrumental  methods  for  measuring  eachof them.  This still left to executive judgment, however, the in-tegration of all these factors into a terminology or scale whichwould  indicate  the  fire  control  action  needed.   To  solve  this28 Nineteen Thirty-ehghtbasic problem the Northern Rocky Mountain  Station  adopteda mechanical device used by photographers, called an exposuremeter,  and produced the  first fire  danger meter.   This  deviceprovided a consistent method of integrating the  essential fac-tors of fire danger into a numerical rating on a scale of 1 to 7.By this scale o£ fire danger class 1 includes all combinationsof  season,  lightning,  visibility,  fuel  moisture,  and  wind  suchthat fires will not spread and no men need be employed for firecontrol.  At the other extreme, class 7 includes such conditionsas  mid-July  and  August  vegetative  development,  lightningwithin past two days, visibility distance restricted to less than4 miles, fuel moistures under 5 percent,  and winds of 8 milesan hour or more.  If fuel moistures are between 5 percent and7 percent and the wind 25 m. p. h. or more, with other factorsas above, the day is also rated as class 7.  Under all such condi-tions fires molt immediately controlled can be expected to "blowup" and spread at rates of 1,500 to 2,000 acres per 7LOttr in mostof the timber types of this region.This numerical scale of fire danger therefore offers the for-est  executive  a  methodical  and  measured  determination  offorest fire danger upon which he may more  dependably basehis opinion of the  "normal  danger"  to which his forest  is  ex-posed.  Without such integrated measurements he must  guessand take his chances of gett,ing burned out or burned up.Ames Forester 29T:sEt¥:rotnhleyrnonReochkayv1¥gO=nutcahinm::sgiOrne:fe:?seI:rOraeS:esp:ordyico:®years.  Other Regions have barely begun or have yet to com-mence accumulating such data.  For Region One there are  infact good data for only four years, 1934 to 1937, inclusive, andthis is  clearly  inadequate for establishing  a  true  "norma1''  ormedian  in  the  sense  of  the  terms  as  used  by  meteorologists.The available data reveal a consistency, however, which is ob-viously indicative.  Furthermore,  in this region there is a col-loquial axiom to  the  effect that t{There  is no  such  thing  as  anormal fire season."  Every season differs from every other inone or more significant respects.The consistency of the Region One data is apparent in table 1,which shows the number of days  of each class  o£ fire  dangerduring July and August, on a typical north Idaho or westernMontana National Forest.TABLE 1.   Number of days of each class of fire danger, July and AugustClass  of  danger3                 4                 5Number  of  days1934193519361937AverageStandard  error C>JCO.I.I.C2CuOc5c5c5c5c5c5OL®coeC£eCeCDo5coL®OtlOICqOapCO.a2lJ,.cotl®®a)tip-hCuTII|lCqCuCqCurocoHTIII1.I+t`Cutlh-lc5tl'co-IIIICuCuC®CueOL®lL®C®TIIIIIOOd'o>'co'cjajoI+eco+.eOOIJ2L®.coetl'T+'coCuOE5EEI=er+Oeeec5c5c5c5c5c5As shown both by the yearly data and the averages in table 1,the  executive  cannot  depend  upon  a  single  day  of  absolutesafety from fire during July and August in this region.  He cannormally expect only two or three days of class 2, or very lowdanger.  Forty-five days or nearly 75 percent of this period willbe class 4 or class 5 danger:  average or a little higher.  He mustbe prepared to build up his organization for four or five dayseach July  1  to August 31  to  meet class  6  danger when  greatdamage may be expected unless fires are discovered almost im-mediately after origin, and extinguished with exceptional  dis-patch.   Fortunately, he has a probability  of  only  3  in 1,000  ofexperiencing  class  7  danger,  when  every  conceivable  precau-tion must be taken to prevent fires from starting.3,0 Nineteen Thirty-eLghtAt present in Region  One  of the  Forest  Service,  the  man-power needed on a million-acre National Forest would be pro-vided for each class of danger about as follows:  Class 1, no men,class 2-4 men, class 3-12 men, class 4-82 men, class 5-128men, class 6-220 men, class 7-22-0 men plus prayers.   Whencolnditions are such that fires can spread at 1,500 to 2,000 acresper hour, the fire control executive is in about the same positionas the football coach who has used his best punts and passes andstill has not definitely won the game;  the best that he can dofor the remainder of the game, or fire season, is to pray for alucky break.It is apparent, nevertheless, that in order to benefit by me-thodical,  dependable  planning the  executive  mt,st  know  howhis fire danger is likely to vary throughout a season.  This canbe approximated by records of number o£ fires over a period ofyears,  but  such  an  approximation  has  two  basic  weaknesses.First, the causative agencies-man and lightning-are not uni-formly active from year to year.  Man may be persuaded to de-crease his activities;  lightning is uncontrollable, unpredictableexcept from day to day and may-actually has-overloaded aForest with more fires in one day than the records for the pre-vious  10  years  had  indicated  as probable  for  a  whole  season.Second, the causative agencies may vary their activity as theywill,  but  the  weather  may  either  increase  or  decrease  theireffectiveness.  For example, an influx of careless tourists mayresult in double the number o£ campfires left burning, cigarettesnipes thrown glowing into the forest, matches carelessly flippedwithout being extinguished, yet the number of man-caused firesmay, decrecLSe, SOlely because abundant and frequent rains keptthe forest fuels too wet to burn.  Consequently, the old criteriaof  number  of  fires  is  a  poor  basis  for  judging  the  fire  load.Burned area, of course, is even less dependable because an effi-cient  fire  control  executive  will  hold  down  his  burned  areaeven during critical seasons, while an inefficient executive mayallow  a  large  acreage  to  be burned  even  in  a  season  of  onlyaverage danger.The results of forest fire research therefore now offer to theforest executive accurate methods of determining current firedanger, of judging past danger, and for estimating the normaldanger that must be known in order to provide adequate facili-ties and manpower.Ames Forester 3I

Determination of Normal Fire Danger

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Determination of Normal Fire Danger

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