Climate and Forest Fires in Montana and Northern Idaho, 1909 to 1919

MONTHLY WEATHER REVIEW ALFBED J. REBYBY, Editor. -. ______- Vot. 49 No. 2. 1922. GLOBED APRIL3,1922 W. B. No. 766. FEBRUARY, IBSUBD MAY 1.1922

CLIMATE AND FOREST FIRES IN MONTANA AND NORTHERN IDAHO, 1909 TO 1919. By I.A. LARBEN,Forest Examiner, and C. C. DELAVAN,Fire Assistant. [Priest River Forest Expcrimnent Station, Idnho, 1920.1

INTRODUCTION. four and one-half million dollars. This great destruction is anawing at the vitals of the national timber suppl . The present report is a result of the study of the rela- by far the greater percentage of this damage haa 53 een tion between climate and forest fires in Montana .aid visited upon Idaho and western Montana. This has been northern Idaho. This region is designated as District I somewhat difficult to understand, especial1 since Idaho of the United States Forest Service. The data used are and western Montana show a greater annuar precipitation

Fro l.-CliWc secWrms of Montnun and north Idaho. the weather records of the United States Weather Bureau than the sections farther east. The reasons underlying for the regular and cooperative stations, and the detail this should be understood so that methods of sup ression fire reports of the United States Forest Service for the and means of revention may be guided by w?I atever years 1909 to 1919, inclusive. Mr. C. C. Delavan has information is o7l tainable. compiled the fire records and Mr. J. A. Larsen the weather It is the plan of this work to investi ate the topographic data. and climatic causes for forest fires in &e district enerally The splendid forests of western Montana and northern and in Idaho in articular; to analyze the recorf s of fhes Idaho have, according to oral and visible records, always and climate for $fierent years for a better understanding .been subject to destructive fires. During the last 11 of the kind of season whch results in bad forest fhes; to ears the seriousness of the forest fire situation has been study the records of fires and weather for individual grought home by the fact that near1 5,000,000 acres of months to see how much rainfall is needed to allay these land burned over in District I, witi! a total estimated fires: to set forth especially dangerous fire weather, the damage to standing timber of over $38,000,000md a total length and intensity of the fire season in the different outlay in fire prevention and suppression. of about sections under consideration; to examine the climatlc 981S6-22-1 65 Unauthenticated | Downloaded 10/02/21 02:42 PM UTC (To face p. 55 ) M. W. R., February, 1922.

PLATE1 -Dense forest of western white pine western red cedar and western hemlock in northern Idaho.’ The dense under story of hemlock a&dcedar burns readily and carries the 6&es into the crowns of the trees.

PLATE 2.-The forest devasted by fire.

Unauthenticated | Downloaded 10/02/21 02:42 PM UTC 56 MONTHLY WEATHER REVIEW. FEBRUARY,1928 records for the last 40 years in order to discover whether TABLEl.--CZimatic sectdons of Hontana and north Idaho National or not the last three years are usual or unusual, and to dis- Forests and weatha. seetioirs used-continned. cuss the chances of predicting very dangerous fire weather. 3roups of national Eleva- Years BROAD TOPOGRAPHIC CONTROLS CUMATE AND Divisions. Weather stations. tion recorded OF FOREST forests. (feet.) to 1918. DISTRIBUTION. - _- - - -__ .- The region under discussion embraces three broad (3) Northwest Kootenai...... Kalispell...... 20 Montana. Blackfeat...... Columbia Falls ...... 22 top0 aphic divisions, as follows: (1) Idaho, north of Cabinet...... Fortine...... 5 975 12 Lolo...... Dapon ...... 2 925 13 the !T almon River, where the land rises gradually from Flathead...... 19 3,000 feet elevation of the Columbia River Plateau to the 6 az crests of the Bitterroot Divide, at from 6,000 to 7,000 Troy:...... ,I6 to 1910 feet elevation. The lower arts of the forest are com- Eaugan and St. Regis.. ... I 9 (4) Southwest Bitterroot...... Missqula...... 38 posed mainly of western yelP ow pine and Douglas fir, the Montana. msoula...... Edton...... I ...... mtermediate slope has western white pine, western larch, Deerlodge...... lo ...... 20 Douglas fir, western hemlock, western red cedar, and 14 and fir; (2) western Montana, which includes the land ~~ ~-~~ .~ 18 Butte...... 23 gtween the Bitterroot and the Continental Divides. In Deerlodge...... l6 (5) North cen- Helena...... Babb...... 12 this art the elevations vary from 3,000 to 10,000 feet, tral Montana. Lewis and Clark. Helena...... I d 110 38 and f rom prairies and forests of western yellow pine in Jefferson...... Adel ...... d!200 19 WOLlC reek ...... d4Go 14 the vaIIey throu h foresh of Douglas fir, western larch, Cascade ...... 3.361 13 Great Falls...... 28 lod epole pine, 8ngelman.n spruce, and alpine fir at the Fort Benton...... 2E 33 to 1912 higfer points; (3) that part of Montana which lies east Fort Shaw ...... 3,500 23 Augusta...... 4071 18 of the Continental Divide. This embraces the eastern Choteau...... 3,810 19 (6) South cen- Madison...... Bmman...Harlowton...... 23 slopes of the Rock Mountains, the elevated country tral Montana. GaUatin...... $E m surrounding the Ye1f owstone Park region and the broken Absaroka...... Yellowstone..Redlodge...... 33 Beaverhead...... 59 17 (18) topography in southeastern Montana. The forests con- Beartooth...... Blllines.-~ ...... 3,115 21 Bpt$ ...... sist mady of Douglas fir, lodgepole pine, Engelmann V nmClty ...... 29 spruce. In southeast Montana there is only western ETe gen Dam ...... 12 Norris ...... 4,845 11 . yellow ine. Dillon...... 5,143 20 The orests of Idaho contain a reater variety of species Renova...... 4m 19 P Bowen...... 11 more understory of inflammabf e cedar, hemlock, and Three Forks...... 4 % 12 to 1915 Bi Timber ...... 4094 12 white fur and much more dead and down material than (7) Southeast Sioux...... desCity...... 2,378 26 Montana. Cuter...... EkalakaI...... 17 those in the other divisions. In fact, the forests east of the Graham...... 10 Continental Divide are, except for dense young lodgepole Crow Agency...... 38 thickets and Alpine fir, quite free from understory, and those in southeastern Montana are of pure yellow pine 1 About. and, except for patches of dense advance growth, quite o en with the characteristic grasses and sedges. Central It should be stated that most of the cooperative dah0 northwest Monta.na, and the forests surrounding weather stations listed in Table 1 do not, as a rule, lie the yellowstone Park region in south central Montana within the forests. Since they are the on1 ones avail- are most mountainous and difficult of access. able, they must nevertheless be used anB the records In this study the data for weather and forest fires have will be of sufficient value for comparisons OI the seven been averaged by groups of national forests. These sections. groups are seven in number and conform roughly to Within this territory we are fortunate in having several separate watersheds. (See Fig. 1 and Table 1.) quite com lete regular United States Weather Bureau,stations whicg may be used as control stations for various sec- TABLEl.-Climatic sectwns of Montana and north Idaho Natwnal tions. Of these Spokane, Wash., is used for north Idaho Foresta and weathm stations ued. - (Lewiston, Idaho, being too low and too far removed I I I Eleva- Years from the forests), Kalispell for western Montana, Helena Didsiona. cImpsafnstional Weatherstation. I tion 1 morded for north central Montana, Yellowstone Park for south forests. (feet). . to1918. central Montana, and Miles City for southeast Montana. I- I-I--- Records of sunshine, humidity, tern erature, wind and (1) North Idaho.. PendOreille ...... Porthill ...... 1 665 29 Kanh...... Bonners Ferry...... 2' 1139 8 recipitation are also available from $riest River Forest Coeurd'Alene ..... Sand nt...... 2'100 8 eriment Station, within the timbered belt of northern St. Joe...... Prlesf!&ver...... 2'3W St01906 Ex Priest River Experiment $360 7 IdAo. -Station. Lakeview...... &.as0 18to 1915 In presenting the data on climate for use in this study, Coeur d[Alene...... a, 157 24 the year is begun October 1, for in this manner it is possi- St. Manes ...... I 2 155 19 Murray ...... 1'750 15to1908 ble to review at a glance all of the factors which may or Kellogg...... 2' 3Q5 14 , Burh...... 4'0S?. 9 may not contribute toward a bad fire season. Wallace ...... 81770 10 Avery...... 5 (2) Can- Idaho. Clsarwater...... Yoscow ...... 28(191Z15) FIRES AND CLXMATE BY THE SEVEN SECTIONS. selway ...... Lewiston...... 2%151 25 Nwpews...... De ary ...... 2854 4 oronno...... 1' 027 14 French Qulch...... 4' ooo 5 The data on forest fires, 1909 to 1919 b area, damage, E~...... 1: as1 10 Mwselshell...... a 171 5 cost, causes and classes are given in Tab9 es 2 and 3 and Nez em...... %OS2 9 are shown graphically in Figures 2, 3, 4, 5, and G. The 02d ...... a#735 4 ...... a 550 weather records for the same years are presented in St. Yiehsels Priory...... 4000 Elk City ...... 4 758 Tables 4.,. 5. and 6.

Unauthenticated | Downloaded 10/02/21 02:42 PM UTC FEBRUARY,1922. ’ MONTHLY WEATHER REVIEW. 57 TABLE2.--Fires aaording to the sewn seeti5n.9, 1909-1919, inelusive. In respect to the number of fires southwest Montane may be classed with the western sections, but from a standpoint of area per flre it resembles the eastern parts. Section. The outstanding oauses of forest fires in this dmtrict are lightning, railroads, campers, and slash burning, of which lightmg is the cause of the greatest number. By individual sections it ap ears that are the most Am.% railroads North Idaho ...... 3 476 380 22 725 234 proli6c source in north P daho, northwest Montana and C!entralIdaho...... 4’375’925 45’750 1,154 Northwest Montana ...... 8:986:718 19’332 281 north central Montana. Central Idaho and southeast louthweat Montana...... 3 688 059 6:12l 130 Montana are free from railroads and railrodcaused ha. North central bfontana...... 2’922’5% 8,g 410 South central Montana...... 4:979’930 97 Central Idaho has comparatively few fires from any Southeast Montana...... 710:619 I 1,077 48 Total...... 26,940,158 ...... Average ...... I...... 1 17,237 I 363 I 2,236

1 The damage is the estimated value of timber and young foreat destroyed. Avemy Ana per Fre TABLE3.-Numbers of fires’cn the seven sections, by classes and cawes, 1909-1919, inchwive.

Number per lO0,WO acres. - I 1 , To- tal. I- -I - - North Idaho...... 3 381 59.3 20.9 17.1 I 32.2 23.0 10.8 10.0 21.3 97.3 CentralIdaho...... (735 19.4 9.9 10.4 .O 33.1 7.3 2.0 3.7 39.6 Northwest Mon- . tan8 ...... 4,813 38.2 17.6 13.1131.3 13.8 7.5 5.6 10.7 68.9 SWthw& MOn- I tan8 ...... 1,818 24.4 12.9 9.9 9.8 13.0 4.0 11.7 8.8 47.2 NorthcentralMon- tana ...... 10.8 4.7 4.3 8.0 5.2 1.4 1.8 3.1 19.5 Southcentral Mon- I tana...... 5.2 2.1 2.3 1.2 2.0 .7 2.7 2.9 9.5 Southeast Montana ------7.0 8.0 7.51 .O 10.8 .1 3.7 7.9 -22.5 Total ...... ------...... I ...... I- ---.. B3.l 11.6 9.7’14.7 14.8 4.2 6.4 8.3 47.4 I I - The data presented in Tables 2 and 3 show an avera e of close to 17,250 acres burned per each 100,000 for t% e entire district from 1909 to 1919. North and central Idaho and northwest Montma suffered most. These three sections have about 89 er cent of the total area which was burned; northwest 8ontana 19 per cent, north Idaho 17 per cent, and central Idaho 43 per cent. This great difference between the western and eastern sections a pears to be due both to the relative number of fires and $e areas per fire in each section. (See Table 3 and fig. 3.) Montana, east of the Continental Divide, shows an aver- a e number from 9.5 to 22.5 of fires per 100,000 acres, wbe the western parts show 39.6 to 97.3. The eastern sections show an average area per fire from 48 to 410 acres; north Montana and north Idaho 281 and 234, re- s ectively,’ and central Idaho 1,154 acres per fire. In tE e last mstance the high acreage is due chiefly to large fires in 1910 and 1919. The damage per fire, given in Table 2, is based on esti- mates of merchantable-sized timber (stumpage) de- FIG.2.-Avemgo area per fire lor the seven section8 of Montana and north Idaho. stroyed and of additional values of reproduction and young growth. If watershed values, pg,and other cause exce t lightning, which causes 84 per cent of all hases were included, the figures woul be muck greater. fires. Slasg burning is the came of many fiea getting hey are mentioned here mainly for the purpose of beyond control in north Idaho and northwest Montana. bringing out the relations between the sections in this Many of the fires listed under this cause have, no doubt, respect. Damage Sgures for southeast Montana are the been set in slash by parties who desire either to clear lowest, as is also the average area per fire. land or to comply with the State law compelling burning Fires which cover more than 10 acres are classed as C of slash. fires, and those which burn less than + acre as A fires. The essential climatic elements, such as sunshine, air Those between % and 10 acres are B fwes. Large fires temperature, relative humidity, wind, and preci itation, are most numerous in the west and relatively few in the including snowfall, are given in Tables 4, 5, an?i 6. In east, except in southeast Montana, where there are many lookin over these figures there does not appear to be any surface fires. remarki able difference in the air movement, somewhat

Unauthenticated | Downloaded 10/02/21 02:42 PM UTC 58 MONTHLY WIL\THER REVIEW. FEBRUARY,1922 reater, to be sure, at Helena and Yellowstone Park. %he average annual air temperatures do not differ so very much, but eastern Montana has a much colder winter and southeastern Montana a warmer sumnier than the western sections. Of the latter, cent-ralIdaho shows the warnier suiiimer. Precipitation, however, differs in a very striking manner; north Idaho has an annual average of 30.37 inches and eastern Montana only 14.22 inches. Central Idaho has only 1.40 inches for July and August, and eastern Montana nearly 3 inches. Central and western Montana sections are again intermediate. These differences are due to the fact that Idaho partakes of the Pacific coast type of precipitation snd eastern Montana the continental type. The grea.ter amount of sunshine at Spokane in summer than at the other control stdons, and the lower relative

NORMAL. PR€CIPf TA TION Inchw

&&bu +o April

Fra 3.-Average amper flre hy seasons for the seven sections 01Montana and north Idaho.

4 1-\ 4-

fo m I I I I I I I 11.1 I I I I

FIG.S.-Prwipitation by months for the seven climatle sections of Montana and narth Idaho. humidity reflect the dry summers west of the Bitter- root Range. The light snowfall in central and eastern Montana illustrates the lighter- winter precipitation in these arts. In Pdah0 this low rainfall in summer, which is accom- panied by much sunny, warm weather &d low humidity, produces a critical forest fire situation almost every year. The reason for the greater devastation of forest by fire in Idaho would seem to be in the fact that, particularly in central Idaho, the summer rainfall and relative humidity are so much less than in the other sections. The heavier annual precipitation in Idaho, whch 18 also usually abundant in spring, gives rise to a dense forest, often with a niass of undergrowth and much dead and down material, which by virtue of the low summer rain- FIQ. 4.-Avemge predpltation for each of the sevm sections of Montana and north full and low humidity become highly. inflmable. "he Idaho. Based on published data of 0. S. Weather Bureau for stations listed ill Table 1.1 primary cause of this dry condition is found in the pre-

Unauthenticated | Downloaded 10/02/21 02:42 PM UTC FEBRUARY,1989. 50 vailin westerly winds, which lose moisture in assing the Casca% es, become heated by compression in tR e descent on the eastern slo es, and create desert conditions over a Wind Relative humidity Sunshine reent movement stretch of 100 mits of flat land, where they are further (per cent). of pmR~e. (alesper heated without receiving any addit,ional moisture. When i these winds strike the forest.s. therefore, they eaert a ...... - -. powerful drying influence. Aver-! Maxi-

#em +dh/y Pe/uhrc uum,.f+ =Am. -c?/.l SLh ...... * ~ ..... I 1- -. <- F * a .._ -. - 5 c -'__I__( Spokane...... 66 74 89 6.2 5.3 halispell ...... 36 64.5 69 87 4.8 4.4 Helena...... 37. ti i3 88 7.1 7.8 63 36 62.6 72.6 I ...... 7.4 6.9 MilesCity...... 42 ,._...... l ...... 6.0 5.1

I -4pril to September, ineliisive.

;, s i.

...... -- -.I .. -.- - Xorth Idaho...... _I 0.5 (1.1 iy.4 I 2i.a 18. R 10.8 I 1.2 0.2 93.3 Central Idaho...... : 1. (I 5.3 11.5 18.9 111.7 8.5! 2.0 .9 64.7 Northwest Moutam...... ' .8 7.9 16.4 19.7 15.5 8.51 1.2 .4 19.4 Sout.hwest Montana...... I 1.7 7.0 9.1 15.2 13. R North central hfmtsna ...... 8. U 4. x 5.9 10.3 9. 0 Snuth rentral Montana...... : 5. .9 1;. R 7.0 I 12.0 9. ti Southcwtcrn ACa)ntr~iia...... 1.Y 4. li 6.Y I111.3 1. 8

That the eastern sections of the district, the areas east of the Continental Divide, are ?articdarlyess exposed to this dry atmos heric condition is shown by the fact that the average re7 ative humidity for Auoust at Kalispell is 36 per cent., at Helena 30 per cent,, at &les City 42 per cent, and at Yellowstone Park 36 per cent. As to secondary causes contributing toward more forest 0 Mn.7 p n++,nn I s fires' in Idaho than elsewhere in the district it may be FIG.6.-Relative humidity by months for the seven sections of Montana aud north mentioned that the wind always strikes the sunny slopes Idaho. and blows hardest at the time of the day when the air TABLEk-.i rerage pircipitntioii. t.eniperature and moisture deficit are greatest. Over the Clearwater region in centrd Idaho the wind blows parallel [Data from stations listed Table 1, U. S. Weather Bureau.] wit,h the ridges in summer whereby its dryin , and fa.nning effect is incremed. In northern f daho, an over the greater parts of Montana, on the other hand, t.he trend of topogra.phy is transverse to the prevailin wind, so that each pronounced ridge checks the wind an% 3O.X resents natural breaks which retard the sprea.d of large 21. F1 Forest fires. 19.30 11.76 Since the moisture content of both dead' and living 15.85 leaves is lowered by a decrease in the relative humidity 16.55 14.33 of the air, and since all woody tissue becomes drier-under its influence, it is easy to understand howJhese climatic AVERAGE AIR TEMPERATURE (F.'). and topographic relations produce the critical conditions which result in the bad forest fires in Idaho. 44.7 47. i I I%ythe inw4i.gation.J nl S. B. Show. c'limate and Forest Fires in California, Jqurnal 428 01 Forestry, Dctember 1919, p. 815, and by the Priest River Experiment Station It has 42.2 been found that the $cad materhl on the forest floor receives and dves up modsture 41.8 arrnrdin to the relative amount of moisture II~the atmowhere. 42 4 2 *'In t%e caw of spwializ+ water tissue, the water which forms In, the bulk of the 4% 7 re11 roprweuta a store which is drasn upon by other living tissue and 1n particular by - the photosynthetic cells, in time of drought."-(Plal Anolomy, by Haberlandt.)

Unauthenticated | Downloaded 10/02/21 02:42 PM UTC 60 MONTHLY WEATHER REVIEW. FEBRUARY,1999

FIRES AND CUMATE ACCORDING TO YEARS. control stations in the district showed air tem erature, wind movement, and moisture deficit greater tgan nor- The data on fies b years are iven in Table 7 and the mal. The fire season came to a close with snow about weather conditions Jor north 19dah0 and north-central August 19. Montana) appear in Tables 9 and 10. TABLE7.-Fires according to years. A com dation of fires which are caused by lighhing is given in $-able 8. These data may be taken as fair indi- Number oIscrw burned per each lW,ooO acres. -- - -- cations of the hazard from this cause. It is of interest to I note that 56 per cent of the total number is confined to North- South- North south South- North 'Ei west wwt central Centrel east the two Idaho sections. Lightnin activity is greatest in Idaho. Idaho. Mon- Mon- hfon- Mon- Mon- Idaho, intermediate in western ontana and relatively tana. tam. tam. tans. tana. aB - ' ---- -.. 1- I- smdl in the central Montana sections but somewhat _- . I-.-- 1909 ...... 22,392 19 400 85 29 42 0 increased again in southeastern Montana. 1910 __.._..2,725,796 1,721 17,246 22,627 12,731 1,64619 5,7550 368 390 1911 ...... 6 m 18 e445 3 23 10 158 The fire records for the entire district for the last 11 1912...... 2'974 11 59 1 11 011 11 0 ears show years comparatively free from bad forest 1913 ...... doe0 6 344 8 2!a 10 12 8 six 1914 ...... 114:133 55 321 9D3 624 145 171 111 100 Les. These are 1911, 1912, 1913, 1915, and 1916. 1915 ...... 14,480 13 49 188 50 222 7 0 1916 ...... 9 134 15 7 159 2 7918 62 These years show one and sometimes two of the summer 1917 ...... 171'907 106 728766 1,098 874 116 19 248 months with deficient rainfall, but the summer averages 1918 ...... 58'087 49 260 649 217 25 137 12 1 1919...... 1,514:555 671 3.562 3,338 l,7W 335 109 were above normal. The beginnin of 1915 occasioned m.235 -4,662 -- - - Total...... I 22,728 45,750 19,332 6,121 924 1,077 considerable anxiety by an unusualy9 early and warm - --8,aoC - - s rin , but the late spring and summer rains were 1,757 '656 728 84 9s at!ii un ant. In marked contrast to these favorable seasons are the TABLE8.-Liqhtninq .fires. mum ber ; 7 end 100,000 acres. .I five years of .bad forest fires, 1910, 1914, 1917, 1918, and South- North South south- 1919. west central central east Aver- Mon- Mon- Mon- Mon- am. The year 1910 will always be remembered as one of the tans. tana. tana. tam. tana. worst fire ears in the Pacific Northwest. More I-. i---- I-.- __ _..- .- -- veTtim er land was zurned over in that year than during all 1909...... 0.9 0.2 0.4 0.3 0.3 0.3 1910...... &I 1.6 .8 .6 2.7 .R of the 10 years following. The damage was particularly 1911...... 3 1.0 .3 .4 .6 .4 heavy in northern Idaho. 1912...... 3 .3 .3 .I .a .I a .P 1913...... 4 .9 .3 .a .1 0 1.1 .3 In oing over the weather records for this year it is 1914...... 3.1 a7 1.8 1.9 .5 .3 .7 2.7 1915 ...... 7.8 6.2 1.8 1. 1 .1 2.7 seen t%at the snowfall and the winter reci itation were 1916...... 9 3.2 1.0 1.6 :: d1 1.4 1.2 about average. Even the month of ay s owed a fair 1917...... 7 1.2 .8 1.0 .8 .3 22 .u 81 1918 ...... 4.5 4.0 2.3 1.7 .4 .2 .4 2.1 amount of rain, but from the of June until the 1919 ...... 3.3 5,9 4.0 4.0 2.2 .5 1.3 3.3 1atter.part of August the intense. During ------Total ...... 23.0 33.1 ,13.8 13.0 10.8 I 14.8 this summer Spokane and Weather Bureau -~ TABLE9.-W'ther conditione by years for north Idaho and control station, &'poh?ie, Il-ash.

Sunshine ( er cent Air movement (miles Relative humidity Precipitation (inches). Air temperature, OF. possihpe). per hour). (per cent). I - - i [I

B .a 8 Y ai L a 3 Ei m z CI B - _-- - - 1910...... 102 11.6810.14 272 255 0.81 0.33 0.18 1.56 54.5 73 R'? 74 7.0 7.4 6.4 5.7 31 3 1912 ...... 89 7.02 6.89 1.96 292 2.10 1.70 238 1.46 53.2 62.0, 62.0 60.0 64 66 60 6.4 6.0 5.6 5.9 40 34 1914...... M 7.39 9.53 1.22 1.77 2.m 1.24 .32 2.48 54.5 57 5 I 67.0 04.5 62 m 79 6.1 6.9 5.s 5.7 34 33 ims ...... 143 10.60 13.20 1.91 2.37 3.23 ?.os 1.24 2.33 46.8 55:6 161.2 62.0 43 73 81 81 6.1 7.1 5.0 41 38 1917 ...... m aos an 289 2.05 1.30 .04 .os .a 51.3 56.6 66.0 64.6 51 R6 90 6.2 7. 8 6.8 5.3 40 81 1918 ...... 80 1255 11.16 .75 .1 .70 L25 .a5 .12 49.5 63.5 67.0 61.2 ' 55 74 70 8.4 6.2 5.6 6.0 3322 ieie ...... 69 ami2.36 1.07 1.98 .a .os .78 -.88 527 59.3 67.2 65.2 -2 72 ---89 81 83 -6.6 -5.9 5.6 35 33

1.94 51.9 55 77 176 6.8 6.9 5.9 - - -. - 1 73 ln Aprll. 78 in April. 1-1910. TABLElO.-Weathm conditions by yeare and monthfor north central Honktna and for the Helena, the control, sfaiiotr.

Wiid movement (miles Relative humidity per hour). (per cent). - __ -~__ - YSU. I.. ai A ai 5 - 5 1 z CII 22 s '-. -_ - - _. 1910...... 46 ...... 67.3 61.1 57 72 so 7.7 6.9 6.8 6.3 34 36 1912 ...... 60.9 61.0 57 77 62 9.4 a6 8.0 R2 52 43 1914 ...... 69.0 63.5 56 71 69 a3 a. 8 a3 a$ 45 49 1816 ...... 65.5 63.0 44 68 81 9.9 9.2 86 81 48 ' 47 1917 ...... 68.3 64.0 55 SS 78 a3 9.1 fL7 7.9 50 37 1918 ...... 64.7 62.4 53 64 66 9.2 8.2 7.2 a5 36 31 1919 ...... ------ea8 66.9 '61 --71 -80 -9.2 -R3 --a7 ai -'34 -34 I-- I--

4 6.8 4 7. 2 4 7.7 7.8 42 41 ,...... - - 1dzInApril. a April a8 per eent. * 19 years, 1916. Unauthenticated | Downloaded 10/02/21 02:42 PM UTC FEBRUARY,1928. MONTHLY WEATHER REVIEW. 61 The season of 1914 was preceded by a normal fall, and a very warm summer; it was, moreover, the thud winter, and spring recipitation, but there was a marked consecutive % season. Sunshine, wind movement, and deficiency of snoJdl and an early melting. The May mositure deficit were much above the average for all and June rainfall was about average, but somewhat low sections. It should be noted that the average afternoon in May and h' h in June. The drop in July was most July and August relative humidity for Spokane was only pronounced in'gh t e northwestern and the central Montana 18.5 per cent as com ared with a normal of 25.2 per sections; the August rainfall was considerabl below cent. The deficit at gelena for the same period was 9 normal everywhere, almost nothing in centra Idaho. er cent; for Yellowstone Park, 18 per cent; and for The summer temperature, 1914, summer sunshine, wind hes Cip 8.5 per cent. The average sunshine at . movement, and moisture deficit were in most cases Spokane or July and Au st, 1919, wm 80 per cent and greater than nornial. The result of these conditions are 85 per cent, com ared witT averages of 62.5 per cent and seen in the large number of fires in July and August, 70.5 per cent. Pt was also well above the average at and the comparatively large area per fire. As in 1918 the season was most severe in Idaho, and lightning fires were numerous and difficult to control. In central Idaho occurred the largest number of lightning fires on record for any one year. The winter of 1916-17 was not.ahle for its great quan- tities of snow and precipitation well above nornial until the month of May. From that time on the rainfall was much below the average and reached unusually low points in July and August. On account of the abundant snowfall and late meltmg everyone predicted a favorable season, but the sudden and unusual drought which followed reci itated a bad fire seaaon. As in case of all of the ad re years the sunshine, wind movement, summer temperature,Ef! and moisture deficit were greater than normal. The most noticeable feature of the 1917 fires was the low percentage caused by lightning. On this account central Idaho showed no greater area burned than north Idaho. The year 1918, which is the least severe of the five bad fire years, began with a heavy snowfall in the early art of the winter, followed by abundant January anB February rain, which caused floods and early melting of the snow, particularly in the Idaho sections. This was followed by an unusually dry spring; so dr that the height growth of young trees was noticeabyi? below the average. The season would, no doubt, have been very bad had it not been for the abundant rains in the middle of July. In all esceptF the central Montana sections the May and June temperature, sunshine, wind movmient, a.nd moisture deficit were greater than normal. The fire season, based on number and area of fires, was much more severe in the Idaho than in the Montana sections and began excep- tionally early. The number of lightning fires was in most sections well above the average. The year 1919 had an unusual1 large number of fires in all parts of the district. It ha the dryes! spring and summer on record since 1883 onB the Pacific slope and the dryest on record for the eastern slope (observation begun 1880, see @. 13 and Table XI). The deficit in total annual precipitation from October 1, 1918, to FIG.'I.-Deflcit in rainfall and forest fires for north Idaho, 1910,1914,1917,1918, and 1919. September 30, 1919, is as follows: North Idaho 5 per cent, northwest Montana 27 er cent, southwest Montana the other control stations. Air movement at Spokane, 42 per cent, north central lfontana 46 per cent, and in Kalispell, and Yellowstone Park were considerably above southeast Montana 51 er cent, with correspondin de- normal and greater than in any previous fire year. ductions elsewhere. dedeficit for May, June, %ly, "he light snowfall, lack of spring rain, and s ring and August combined in each section are, in the order temperatures above normal were the cause8 OF the usually given: 42 per cent, 50 per cent, 50 per cent, 63 unusually early start of the fires in 1919. The May fires er cent, 66 per cent, 72 per cent, 54 er cent. Central in nearly every grou exceeded both the 11-year average kontana shows the greatest summer %eficit. The more and those of any ot% er single year. Fires m June wem outstandm facts for the whole district for 1919 are: numerous in all parts of the district, and, while they did Et snowfall everywhere; frozen ground in the not burn with the same fierceness that the fires did later &Tzu!%melting of snow in late December, 1918; most in the season, considerable difficulty was ex erienced in of winter precipitation in the form of rain; compara- controlling them, principally, because of t e fact that tively little spring rain, followed by an unusually dry the protective force was just being put on. B

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FIRES AND WEATHER BY MONTHS. tral Montana is about 2 inches per month from June through September. This is shown very clearly for A comparison of fires and weather by individual May, June, and July in nort-hern Idaho, 1914, and for months brings out many ointa worth noting; but since the entire summer 1918 for north central Montana. the presentation of the Bata in tabular form leads to It is seen further that the quantit.y of winter snowfall such a great mass of detail a diagrammatic presentation is or total precipitation bears little relation to fires during given. This requlres less space and can be more readily the following summer. The great quantity of snow visualized. chin winter of 1917 and a precipitation about normal The correlation which is given in figures 7 and 8 was until%ay did not stave off a very bad season, which carried out for all of the seven groups in the district. be an as soon as the rainfall fell below 2 inches. Only two of these are given here and one which shows the ft map be well to &s our attention to the months of average relations for 1910, 1914, 1917, 1918, and 1919 May and June for a little closer consideration. June for four sections. does not show a critical fire situation, provided May has

Flu. 9.--Relation of rsinfall deRcit to forest fires for the years 1910, 1914 1917, 1918, and 1919, in north Idaho, central Idaho, northeentral and southwest dontana. FIG. 5.4omparison of precipitation and forest fiws for iiort.hanlra1 Monbaria, 1910. 1914, 1917. 1918, and 1918. &out 3 inches of rainfall. The year 1919 shows some- In these diagrams the difference between 2 inches of rainfall and the actual amount which fell has been used, mainly because the rainfall data for the safe years 1911, 1912,1913, 1915, and 1916 show that 2 inches per month comes close to the safety limit in July and Au ust. ized, however, that it is difficuK t to estab- lishIt these is recT re ations definitely, mainly, because the origin of the fires, and especially lightning fires, vary widely from year to ear. However, by averaging the number bring on many fires. of fires and deprecipitation for each month these rela- In Figure 9 are shown the relations between the deficit tions may be approximated as closely as it is possible in precipitation and the number of fires per month. from the present data. According to averages for 1910, 1914, 1917, 1918, and Referring to the diagrams it is seen that the safety 1919, the relations are quite re ular. From the data it line of precipitation in northern Idaho and north cen- is possible to derive a figure w% ich, when multiplied by

Unauthenticated | Downloaded 10/02/21 02:42 PM UTC FEBRUARY,1929. MOYTHLY IVEATHER REVIEW. 63 the dierence expressed by (2- actud rainfall for thr esperiment sttition and covered 1.300 acres in one after- month), a roximates the number of fires fnr tlint month. noon. On Sugust 19, rer serious forest fires occurred on For north PPdah0 this multiplier is 10 for &In>-, 30 for June, most of the north Idaho Porests. 100 for July and Au-ust, and 30 for September: e. g., It, is of interest to note that bhe records for Spokane, in north Idaho the fune deficit (Fig. 9) is 1.60 inches. Wash., also show unusually high temperature and low This multiplied by 30 equals 48. The average number of humidity and somewhat increased ressure at these times, fires for that month is 53. The factor 10, 30, 100. etc., hut. the wincl there and at the 3orest Service lookout is obtained from t,he actual records of the number of fires by formula _I.... -..*1-1.1 > . nnniber of fires ...I. 2- actual precipitntion------.r present very JULY /s/s I/ I2 I.? rrg 15 /6 17 18 6 /2 6 Wt 6 12 6 hldt 6 I2 6- Mdt 6 /2 6 MdL 8 /2 6 Mdt 8 /2 6 Mdf 6 /2 6 Mdt 6

R€LR 7/Y€ HUM/D/ T Y

WIND MOVEMENZ; MI. PER HR.

AUGUST /9/9 N /5 /6 /7 I8 /9 PO

FIG.10.-Daily rise in alr temperoture and lowering of relative humidity culminal.ing in critical acst.her for forest fires, Priest River Forat Experiment Station, 191%

ESPECIALLY DANGEROUS WEATHER FOR YOXERT HILEP.:' LENCTII .4N1) INTICNSLTT OF THE FIRE SEASON. The geogra hic and to ographic re1iit)ions which liavc Thc! iiuinbcrs of fires in each section of the district been discusseB above, ancp the prevalence of high-pressure have beeii averaged by inonths to show relatively the meas brin about clear, warm, dry wea.ther in sumnier in time of beginning and ending of forest fires, the month northern Bdaho during July and August. At certain e- of greatest intensity, and the relations of air tern erature riods during these months, the weather conditions Ze- and precipitation to the fires. (See Table I1 and gig. 11.) quently approach a climax of high nir temperature and North central Montana a pears to have the lon est low relative humidit.y. Estimples of this are available fire season. It begins in A)pril and ends in Octo% er. from weather records at the Priest River Forest Esperi- April fires occur also in the two western Montana grou s. ment Station and fire records, July 11 to 19, 1919, and In central Montana these early fires occur .chiefl in t?l e from August 13 to 20, 1919. The t.emperature curves in dry grass or the slash left over from the preceCT ing fall Figure 10 show this very clearly. On July 18, a serious or wmter. There is iio appreciable difference between fire broke out in a slashing five niiles away from the the groups in air temperature. Figure 11, however, a Mr. Edw. A. Beals has given a vory Interesting dlsciiwicm on weather ccni+tiaa shows that the -4pril rainfall and thc annual snow€all in attading thwe mat forest 5m Baudette Columbia, 1902, and Idaho, 1910, in the Febnury Yo. ~EATREBREV. hb., 1014, b: 111-119. these partslis lower than elsewhere.

Unauthenticated | Downloaded 10/02/21 02:42 PM UTC 64 MONTHLY WEATHER REVIEW. FEBRUARY,1929 Other significant points brought out by this cornpar- TABLE IS.-Prer@dntwn on the Pni~[lieslope and castcrn slop, 1880 ison is that the eriod of low summer raidall usually to 1920. corresponds close y with that of the hqhest air tempere - P ~aei~cslope.4 Easlern slope.4 tures, that a rainfall of less than 2 mches per month 1 (Averages.) (Arerams.) after March or April brings up the fire cme in every Tear. I-- section, and that there is an almost direct relation be- JUly- May- July- hnld. May- LllIlUl. tween the number of fires curve and the rainfall deficit June. .4ugust. June. Allgust. under 2 inches per month. -- I------a closer comparison of the different climatic ac- 1880...... 117-711 4.53 2.80 13.41 5.13 3.W For f 1851...... 2.50 3.91 5% 15.64 5.46 4.88 tors and forest fires the climatic data for the Priest ips? ...... 20.03 4.75 -3. 12 11.07 4.71 3.42 3.21 [' 0.1q 12. ns 3.70 2.69 River Experiment Station and the fire data for Kaniksu ...... ::: 4.62 I' 2.901 la 33 5.99 4.94 National Forest, within which the station is located, 5.7% 1.03 9. SI 6.M 2.52 ...... ;;:;f 2.95 1.29 12.42 5.60 2. m have been shown in Figure 13. This also reveals the 1~7...... 17.87 5.37 2.81 14.94 7.89 3.99

4.53. ~ 0.32 12.37 6.04 2.68 ra id rise of the fire curve as soon as the rainfall drops ...... 4.32 0.59 &so 3.07 2.57 bJow 2 inches per month. Other interestinF relations Ism.- ...... 13.91 4.33 1.45 10.03 L2.5 2.43 1691...... 1841 531 2. $4 16.6 s.m 4.55 are the clqse correlatiqn of precipitation and soil moisture 1m...... 15.6s 5.1G 1.44 13.05 6.53 2.80 1135 5.m 3.93 and the time of maxlmum forest fires at a time when :~:::::::::::::::::::: 3.9s6.59 2.3s2.13 13. S4 6.36 2.03 these two factors and the relative humidity are lowest. lfiS5...... 12.68 3.00 2.48 11.46 4.80 2.85 lm...... 17.75 4.62 246 14.53 6.20 3.17 Ism...... 21.m 5.30 4.00 13.49 5.59 1.90 3.01 &e8 100,000 acres. lws...... 14.79 5.47 1.7Y 13. w 6.23 . TABLEIl.-Avemge number of by ,munth pcr 1899 ...... 19.79 3.63 3.57 14.97 6.27 3.54 19W...... 17.66 6.02 3.M 11. m 3.4 4.80 mecord 1809-1919, inclusive.] 1W)l...... 15.15 4.42 2.10 14.55 7.11 4.42 lW...... 18.19 4.99 2.w 11.21 6.38 2.50 lrn...... 15.49 3.47 3.69 12. .Is 4.27 5.s ...... 13.62 3.09 0.w s. 45 3.81 1.57 ...... l5.W 6.43 1.91 9.75 4.79 3.5s ...... 17.67 5.45 1.42 15. Cn 7.60 3.m ...... 16.27 4.02 3.3s 13.59 7.21 3.04 14.0% 4.77 3.33 16.90 876 3.51 Nmlh Idaho...... 0.00 ...... 17.10 3.50 3.54 15.35 6.81 4.64 Central Idaho...... OO ...... 15.16 3.n5 2.34 11.61 4.02 3.09 Northwest Montana 12.41 4.29 1.98 16.39 5.29 5.94 sollthwest biontana...... 1s. 5% 15.68 6.67 1.93 North central Montens...... 17.06 ::: ;fi 13.7s 5.53 3.73 south mntd Montana...... 13.55 4. 2R 1.s 14.29 5.83 3.51 Southeast Montana...... 17.20 bR2 3.01 17.60 6.92 7.13 - 1916 ...... 17.80 5.67 2.00 1s. 50 7.13 5.90 1917...... 14..M 4.35 0.77 13.71 4. fM 2.80 191s...... 12.67 1..50 2.139 11.31 2.92 4.75 PRECIPITATION RECORDS 1880 TO 1919. 1919...... 14. 20 2.07 1. M 9.1x a.Bo 1.78 Man...... 16.83 13.50 5.61 The most noticeable thing about the climatic records 1-

for~ -~ the last 10 vears is the unusuallv low summer Drecipi- tation for the"three consecutive iears, 1917, 19iS, cnd The total annual precipitation, which appears to have 1919. In order to learn whether this was an unusual oc- reached the lowest points at periods of 15- ear intervals, currence or a thing to be expected periodically the writers has no influence upon the fire season unf ess associated examined precipitation records for the last 40 years from with summer drought. The three-year seesaw shows a stations east and west of the Continental Divide. These low downpour every third season, the only exception data are presented in Table 13 and figure 13. being an int,erval of three wet yews, 1905, 1906, and 1907. !I%e data for the 40-year rainfall west of t,he Conti- This three-year fluctuation is in accord with those found nental Divide (Table 12) show' a very low preci itation at Greenwich, England, by Dr. H. H. Hildebrandsson.b 1917, 1918, and 1919. The May and June rain rall was In the eastern and the southern parts of the United also quite scanty during the last two yeam and the July- States this fluctuation t~ ears dso, but is less regular August rain for 1917 and 1919 below nornial. When than in Europe and the # &ern States. The fluctmua- we consider that the summer rainfall 1918 did not come tions are ver like1 induced by the oceanic-pressure until after the middle of July and that the May and areas, on whic%l muc has been written. The relations of June rain of that year was the lowest on record, there these pressure areas over the Pacific to the climate of is no wonder that 1918 was a bad year for forest fire:. Northwest America has not been worked out. During 1888 and 1889 the Jul and August rain was also very low, but the records si ow at that time con- CHANCES OF PREDICTING DANGEROUS WEATHER siderable May and June rain. The year 1889 is remem- CONDITIONS. bered as one of the very worst fire seasons in the Pacific Northwest. It should be remembered that since the atmospheric Records for 1910 do not indicate such very critical ressure areas re but one day to travel from the conditions aa the fire records would have us believe. bacific coast to Mountains, and since there During that ear there was almost no June and July are no permanent weather stations to the west of the rain, and the &e season, thou h very intense, was limited Pacific coast, it is evident that the basis for forecasting to the first three weeks in Iugust, during a period of weather in the Northwestern States is largely wantin . hi h wind and air temperature. The data presented should have their value in deci2 ing !bast of the Continental Divide the annual recipitation upon distribution of men and funds for fire protection. for 1919 was the lowest since 1889 and t ge combined MayJune and July-August rainfall the lowest on record. *Cycles of Bun and Weather: Nu'ohrre April 1912. 147. e Secular Variations of Precipitation& the dnited gates hy Alfred J. IIenry Bulldin Ammkvrn Gragra mal Socidfi. Vol. XLVI, March, 1914. bee also the artielea'h C. F. 4 The avera 8 for the PacIBc slop6 are made from the records of B nknnr, MLssoula, Broats Eer k Etowsk Nansen, H. Hildehrandsm, E. F. Blanford. E. &&In- and Walla &la and for the eastern slope from Helena, Hovre, an!Mlles City. ton, & A. Luster, on Pressure, sunspots, and Climatic Controls.

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RB.ll.-l%elation between forest W, precipitation, and air temperature lor the seven climatic sectionsUnauthenticated of Montana and | Downloaded north Idaho. 10/02/21 02:42 PM UTC The averages of reci itation antl temperature may be ration per day takes place from the Livin ston porous- used as a basis %iy w ich to compare current weather c11 atmometer a dangerous condition wilK soon result. conditions from month to month for the different groups $he relation between evaporation and precipitation of District 1. ‘In this way an estimate ma E of the intensity of the drought which is to 9ollow- be in the niap he expressed thus: p, where E is the evaporation summer. and P the recipitation. For this comparison it is best One difficulty of applying these data in fire rotertion to use weeR lg or 10-day reconls. With a ratio H+ on lies in the fact that our records do not extend iack over flat land from July 27 to September 2, 1016, there was a sufficiently long period, and that summer rainfall bears no danger of forest fires, but durin the period July 20 a closer relation to fires than does the annual recipita- and -4ugust 3 following rain the ref ation on the south- tion. We may say with Robert DeC. Ward PSdentijc west slope was during which time the dufF dried Monthlg, September, 1919) that actual rainfall records out to below 10 per cent July 31 and became highly in this country are too short to give any definite indica- in fl amm ab1e.

I 4M:L .v,/ UT .m FIG.I2.-Rdation betwtmi weather and forest firs on the Kanihau National Forest In north Idaho. 1911-1918, incluslve. tion of secular variations. The longest records cover By sampling duff and litter of the forest floor during 100 years, and a few stations have records of more than the summer months and applying burning tests it was 50 years in length. While a study of these records shows found that this material would burn and the fire travel a more or less definite and reco nimble long-period fluc- if the moisture content was below 10 per cent measured tuation, they appear to be large5 y localized, and no clefi- by its dry weight. Though this .is a direct method, it nite conclusions can be drawn for extended areas. re uires much work in sampling, drying, weighing, antl There are several ways of keeping informed regarding ca? culation, which can not be condurted easily or without the weather conditions. From records of evaporation at considerable e uipment. the Priest River Forest Ex eriment Station for several Another method, which has been suggested by Mi. E. years it appeared that if tR e evaporation from n free II. Finlyson, of the Canadian Forest Service, holds much water surface is more than five times the amount of prc- promise es ecially because of its simplicity, ease of obssr- cipitation, dangerous conditions will soon follow, or if vntion, ImcP current daily application. It consists in giv- an average of more than 25 cubic centimeters of evapo- ing certain positive and negative values to all of the

Unauthenticated | Downloaded 10/02/21 02:42 PM UTC FEBRUARY,1932. MONTHTJY WEATHER. REVIEW. 67 climatic elements, such as sunshine, wind, humidity, and Since it has been learned from closer studies that the rain. By plotting these values for each day, say from the duff and other material on the forest floor absorb and beginning of June, beginning at a red line, designated as lose moisture readily, according to the relative humidity neutral, current condit.ions will be indicated either above of the air, and since the moisture content of this material or below this line durin the summer by giving positive affects its inflammability very materially, it would seem values to sunshine, hi& temperature, and wind, and feasible to prepare standard samples of duff which could negative values to rain or low temperature. The test for be weighed in different parts of the district simultane- accuracy and the giving of correct values to each clim.st.ic ously. The moisture content and inflammability of this

24 -

- .- SPRhV6 rrND 9UAt#€R PAC/F/C SLOPE

.- a- ,-

FIG.13.-Predpitation records for three P3ciBc slope stations and three continental slope stations, 1-1919. factor is in returnin4 to the neutral line as soon as there niaterial may readily he indicated by tabular data pre- is no fire danger. Several years' record will he needed pared at a forest experiment station. before the correct values are obtained. In any event it will be necessary, in order to have a To make this method more effective, one could meas- good idea of the current climatic status! to keep close ure evaporation from free water surfaces and, if desired, watch of the conditions from month to month, both by measure the intensity of solar radiation by the black and instruments, by general observation, and b reports from whit.e orous porcelain atmometer cups designed by Dr. the field, and to em loy some instrumenta9 method'for a B. E.%ivingston. It should be stated that maximum closer record from cP ay to day and week to week. air temperatures, the relative humidity at 5 or 6 p. m., and the mount of evaporation will undoubtedly come SUMMARY AND CONCLUSIONS. closer than any other factors of climat,e in pointing out the seriousness of the situation. Montana md north Idaho were divided into seven Current records for any one place are of no value unless smaller climatic and to ographic sections for close com- compared with data for the same place (luring previous parison between forest Rres and climatic records, 1909 to seasons. 1919, inclusive. Unauthenticated | Downloaded 10/02/21 02:42 PM UTC 68 MONTHLY WEATHER REVIEW. FEBRUARY,1922 The com mison brings out that in this region an aver- A comparison of climate and fireg by months brings age of nearP y 15 per cent of the area of the national for- out the fact that it is necessary to have at least 2 inches ests has burned over since 1909. The sections west of of rainfall for each month in summer to allay forest fires. the Continental Divide have suffered much eater dam- The average rainfall in the Idaho and western Montana - a e than the eastern parts. On the north Fdah0 forests sections are lower than this amount; central Idaho, June, th e fires covered 17 per cent of the area, in northwest 1.56; Julv, 0.10; August, 0.70; north-central Montana Montana 19 per cent, while the eastern sections are all section shows June, 3; July, 1.73: August, 1.20; south- below 8 per cent, some less than 1 per cent. The un- central Montana, June, 3.46; July, 1.37; August, 1.06 usually large area burned over in central Idaho, 43 per inches. cent, is due mainly to the fact that the 1919 fires covered The most dangerous weather for forest fires occurs at the same ground as the 1910 fires. the time of a succession of hqh-pressure areas over the The average area er fire in the different sections shows Pacific Northwest. At such times the air tem erature, eat variations. 2or north Idaho it is 234 acres, centrad both maximum and minimum, climbs a little hig% er each fdah0 1,154 acres, northwest Montana 281 acres, south- day and the relative humidity reaches very low points. west Montana 130 acres, north central Montana 410 Climatic records for the last 40 years show a tendency acres, and the rest of Montana less than 100 acres. toward periods of very low rainfall each 15 years; the This greater fire hazard in the western sections, pm- ].ow points were reached 1889, 1904, and 1919; the ticularly in Idaho, is due to differences in climatic condi- show also a quite re ar dro in the annual rainfalr tions, pmticularly rainfall, on the east and west of the every three years. PRowever, t .e total annual precipita- Continental Divide. The Idaho sections show high nn- tion does not necessarily mean a bad fire seasqn! for this mal and ver low summer rainfall, in wluch respect it depends almost entirely upon the summer conditions. It conforms to tT e Pacific coast type of precipitation, while is the coinbination of low annual, low spring and summer the eastern seetions show low annual and comparatively downpour which brings aBout the unusually bad fire high summer rainfall. In this respect the latter shows seasons. its relation to the continental type. Thus the annuill The matter of forecasting fire weather in the PacSc rainfall in Idaho averages up to 30 inches, in eastern Northwest is rendered difficult because there are no fixed Montana less than 16 inches; but the July and August points of weather obsemation to the west of the Pacific rainfall in central Idaho is only 1.40 inches. In eastern coast,. Current records, together with ast records of Montana it is from 2.43 to 2.93 inches. fire and climate, must therefore be rer? led on for the The heavy annual precipitation in Idaho gives rise to present. luxurious forests, with much cedar, hemlock, nnd fir The classihtion and nnalvsis of the natural conditions understory, also much dead and down material. This set forth in this report are offered, not with the idea that becomes ver dry and hivhly inflammable in summer on they will help ward off forest fires, but lead to a closer account of tE e low rainfa1 and the drying, warm winds understanding of the natural factors which operate to the coming from the desert re 'on east of the Cascades. destruction of so much valuable timber wealth, dispel1 These winds blow against tT e sunny slopes and reach certain misleading ideas regarding the relation of climate their maximum velocity during the hottest part of the to forest fires in this region, and serve as a groundwork for later investigations. da!he drier air in the western than eastern sections in summer, and consequently more critical conditions for flres, is shown by an-average relative humidity of 85 per cent for Spokane during August, 36 per cent for Kalispell SPIRAL NEAR FLAGSTAFF, ARIZONA. and Yellowstone Park, and 42 per cent at Miles City. DUST The record shows five years, 1910, 1914, 1917, 1918, By FERDINANDW. HAASIS. and 1919, with serious forest iires, and the remaining five ears, 1911, 1912, 1913, 1915, and 1916, fairly free from (Fort Valley Experiment Statim, Ariz., March 6.19!22.] &ea. The bad fire seasons show subnormal spring and At 1.15 or 1.20 p. m., on June 19, 1920, the writer's summer rainfall, greater than usual amount of sunshine, attention was attracted b a peculiar sound suggestin sogewhat higher air temperatures' and wind velocities. an automobile motor. dard out of doors, the soun% The average per cent-of sunshine at Spokane for July is resembled the tearin of coarse aper. 77. In July, 1910, Jt was 88, and in 1919, 59. The Above the forest, 5 ue west oP the Fort Valley Experi- average relative humdity at Spokane in August is 35 per ment Station, 9 miles northwest of Flagstaff, a column cent. In 1910 it was 22 per cent, and in 1919 it was only of tawny dust, the color of the dry spil at that time, 16 er cent. was seen traveling in an easterly direction, thou h with gore lightnin fires have occurred in central and north minor deviations, and forming a somewhat ung; dating Idaho than in t% e other sections; central Idaho shows a band in tho aeneral direction of the sun. From below it total of 33 per each 100,000 acres in 10 years, north appeared to6e a nearly vertical spiral so close to the Idaho 23, western Montana 13 each, south-central Mon- sun that the upper part could be seen only with great tana 2, and southeast Montana 10.8. difficulty. The column was intermittent, sometunes The causes of the unusual forest-fire situation during almost wholly disa pearing, at which time the charac- the summer 1919, the worst since 1910, are most likely teristic noise subsi2 ed also. The height was difficult to due to a combination of unusual weather conditions- estimate; erhaps 500 feet, perhaps 1,000. frozen ground in the fall, so that little moisture went into When a% out 500 feet west of the west fence of the the ound Gtthe time of melting in spring; much melting station grounds it broke off to the southeast downhill, of 8e snow in late December; light winter precipitation; and whirled around near the back corral. It oscillated early spring rains most likely fell on the snow in the back on this flat for a time, apparently moving first Southeast, woods and therefore did not soak into the ground; very then northwest, possibly in a circle, or in other directions. light spring and summer rains. "his seaaon, moreover, At one time it seemed to bo starting to move north or was the brdin a succession of dry summers. northeast directly toward the station buildings. The

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