CLIMA'PE AND FORES'P FIRES IN NOR'PHERN CALIFORNIA. BY S. B. SHow, Forest Examiner, U.S. Forest Service

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1 CLIMA'PE AND FORES'P FIRES IN NOR'PHERN CALIFORNIA BY S. B. SHow, Forest Examiner, U.S. Forest Service In a broad way, the general relation between climate and fire is obvious. 'phe annual recurrence of the fire season is accepted as a matter of course; but we do not yet know when, in a specified locality, the potential fire season begins, or even when it ends, nor the weather conditions during the fire season which create emergency periods or perioda of no danger. Neither is there definite information regarding the relative importance of wind, slope, temperature, or humidity in determining the rate of spread. ß phe p. ossibility of fires Starting and the rate at which they will spread are largely dependent on the climate, using the term in a broad sense; but any study of the relation of climate to fires, to have much value, must analyze the effect of each of the several components before attempting to determine the effect of climate as a unit. 'Phis paper is but a bare start, and can claim to present only a few definite figures of perhaps merely local value to illustrate certain specific relationships between climatic factors and forest fires. 'Phe data used were obtained at the Feather River Experiment Station in 1915, 1916, an.d 1917, in connection with a study of the'rate of spread of fires as controlled by physical factors. IGNITION POINT Logically, the first point to be considered is the ignition point of litter, or that percentage of moisture, based on dry weight, at which the ground cover can catch fire and spread. The litter of the timber--composed of needles, small twigs, bits of bark, dead weeds, and grass---holds against gravity, when saturated with moisture, from 60 to 110 per cent of its dry weight. Capacity seems to depend on the age, degree of decomposition, and relative proportions of the variou substances. Table 1 shows the results of a series of tests on the saturation point. 965

2 966 JOURNAL of FORESTRY TABLE 1 [Weights are all in grammes.] Sample Air-dry Saturated Moisture Percentage of humbert weight. weight. loss. mosture. I , , , , , , Average , The ignition point was determined experimentally by mounting two-thirds of a square foot of undisturbed litter on sheets of tin, airdrying the samples to a constant weight, saturating, and drying in shade until the litter burned freely. The litter was dried slowly so that the moisture loss should be uniform throughout the satnple, and weight determinations were' made before each test for burning, s.ince, of course, after burning it would be impossible to obtain the moisture content. The data are shown in the table: TABLE 2 Sample Air-dry Weight when Weight of Percentage of Remarks. number. weight. burned. moisture. moisture , ,5 Failed to burn , , ,8 Burned Burned ,781 21, Failed to burn. It is clear that about 8 per cent of moisture is the critical point. Tests made at the Priest River Experiment'Station also show that 8 per cent, based on oven-dry weight, is the critical point in that region. It has been found by a series of tests here that the air-dry weight of litter at 80 ø to 90 ø Fahrenheit, and g0 per cent relative humidity is only 1 to g per cent greater than the oven-dry weight. The Potential fire season then begins as soon as the litter contains 8 per cent or less of moisture, or strictly speaking, when the top layers contain that amount, since fire can spread if only the top halfinch or so of a 3-inch layer is dry. SEASONAL CHANGES IN MOISTURE CONTENT OF LITTER The moisture content of litter during the fire season, 1916, at 4,000, 5,000, 6,000, and 7,000 feet elevations on north and south slopes near

3 CLIMATE AND FOREST FIRES 96 the station is shown in charts 1 and 2. Samples were taken at intervals of one week, and the percentage of moisture {s based on oven-dry weight. Precipitation recordeduring the period at the experiment station is shown on each chart. It is to be noted that local storms occurred at the higher elevations. Table 8 shows the number of days in each week and the period during which the litter at each station was above (-1-) and below (--) the critical point of 8 per cent. Perhaps

4 968 JOURNAL of FORESTRY the most interesting feature of the charts is the rapid drying out of the litter to the danger point on Mt. Hough (south slope) after the storms of early October, while on Claremont (north slope) the litter remained saturated. This may be considered in connection with the close of the fire season.

5 CLIMATE AND FOREST FIRES 969 RATE of MOISTURE LOSS FROM LITTER Litter loses tnoisture at a surprisingly rapid rate when exposed to sun and wind. Chart 3 shows the result of a series of tests made in June on samples of litter, first saturated with moisture, then exposed to sun and wind and weighed at intervals at first of one-half hour and later of Week ending Totats 4,000 5, [32 [86 [ TABLE 3 Hough {{ Claremont %{ , ' J I 7...,, ] i 7 7 I" I 7 ]...['t... SUMMARY Moisture Content Above 8 per cent Below 8 per cent Days Per cent Days Per cent Average, south slope Average, north slope For the period, litter was above danger point one-third of the time on the south slope, three-fifths of the time on the north slope. ; one hour until a constant air-dry weight was reached. The samples, each 6 inches by lg inches and from 1 to 11/2 inches thick, were started at intervals of one hour, beginning at 9 a.m. It is seen that in all cases the moisture loss is extremely rapid at first. As a matter of fact, the very top layer of each sample was dry enough to burn within one hour after exposure, as was proved by test, and later losses were from the lower layer, protected from excessively rapid dessication. Note that the earlier samples had practically ceased to lose moisture by 6

6 97O JOURNAL of FORESTRY r mm½ p.m. the first day, though still exposed to direct sunlight; thathey gained weight overnight, and had a comparatively small moisture loss the seconday, reaching air-dry weight about 2 p.m. The lateisamples, Nos. to 8, lost less moisture the first day and continued to lose during the night, and became air-dry late the seconday. All samples gained weight during the second night. It is perhaps a fair s v.

7 CLIMATE AND FOREST FIRES 971 deduction that during the summer the effect of any rain may be entirely lost in from one to two days, when the litter is exposed to direct sunlight, and there is even a slight circulation of air. The rate of moisture loss is lower and the period for drying out is longer when the litter is in the shade, with air, temperature, humidity, and wind movement the same as in the sun. It is probably true that the rate of drying out of small samples is somewhat more rapid than in nature, but the available evidence certainly indicates that even a soaking rain, which completely saturates the litter, may lose its effect in a very short time, and that an attitude of complacency and a let-down in protection alertness are not justified during the fire season. A study of the moisture relations on Mt. Hough also indicates that the end of the fire season comes when the content of.the litter is above the ignition point, and not when a good rain early in October apparently ends the danger for the year. Certainly the records show that fires can and do occur after the fall rains, and after the protective organizations have been disbanded. Such fires frequently are larger than those during the recognized fire season, as, for example, on the Shasta National Forest in 1911 and An interesting example of rapid dessication of litter occurred in September, On September 13, a rain of 0.14 inch fell between p.m. and 6 p.m. During the rain, brush piles were burned on the grounds of the Feather River Experiment Station, and in no case did the fire spread. The mornihg of the 14th was cloudy until 10 a.m., after which the sun shone, and more brush was burned between 8 and 10 a.m. without the fire spreading. On the afternoon of September 14 a series of ten fires was set on pine needles, each fire being allowed to burn 15 minutes. In every case the fire spread freely and it is apparenthat the effects of the rain were lost in a very few hours. PROPERTIl' $ of AIR-DRY LITTER Air-dry litter is markedly deliquescent, that is, it takes up moisture from the air, indepencbently of precipitation. Chart 3 shows this, and it may be said that even duri'ng the hot summer season (June to August) the litter takes up from 5 to 6 per cent of its air-dry weight every night. This explains to a very large extent the well-known fact that fires burn more slowly by night than by day, though other factors --wind movement and temperature--also play a part. In June, the litter begins to take up moisture at about 5 p.m., and starts to lose it at about 6 a.m. Later, or earlier in the season, this period is of course correspondingly longer.

8 97g JOURNAL of FORESTRY SEASONAL IKARCI-I OF EVAPORATION Chart 4: shows the average evaporation rate {or each month oi the fire season on the north and south slopes (elevation, 3,500 feet), measured by the U.S. Weather Bureau evaporation. pan and corrected {or precipitation. The rate on the north slope is uniformly about 25 per cent lower than that on the south slope or flat (which are nearly equal); the peak is reached {n July; the rate holds up fairly well in August, and then drops rapidly. The evaporating power o{ the air in July is 2 times as great as in April, 12 times as great as in May, 1 times as great as in June, and 1 /9 times as great as in September. Evaporation as a factor is chiefly important in reducing the moisture content of the litter to the danger point after a rain. SEASONAL VARIATIONS IN WIND VELOCITY Chart 5 shows the daily wind movement for each month at the experiment station, based on four years' average. The months, April to September, inclusive, or roughly, the fire season, show values above the average, while the others are below. Being based on one station only, the figures, of course, cannot be applied generally, but serve, perhaps, as a fair indication of the general trend of wind movement in the Sierras'. Values vary for individual years and months, but the general tendency of wind velocity to follow the same annual course as air temperatures is well marked. DAILY variations OF WIND VELOCITY During the period, June to November, 1916, an automatic-recording wind register was maintained on Mt. Hough at the lookout station. Chart 6 shows the average wind movement for different hours'of the day for this station, based on the entire period of observation. The fluctuations are very striking; the maximum velocity is reached about p.m., or the hottest part of the day, and the minimum early in the morning. The general tendency already noted, for wind velocity to follow temperature, is here very marked. It is perhaps unnecessary to say that individual day show an entirely different set of values, especially when wind direction is 'changing, but the data are sufficient to warrant the statement that wind velocity may be expected to be highest during the hottest part of the day and lowest at night. This record was made at an elevation of something Over 7,000 feet, and presumably represents the so-called "master winds," or the main..

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11 CLIMATE AND FOREST FIRES ß air currents. The high velocity, averaging 15.8 miles per hour, is typical of the higher elevations throughouthe Sierras and is in decided contrast to the average of four miles per hour at the Feather River Station, which is 3,500 feet lower and only about 6 miles distant by air line. A short record at the experiment station in June, 1917, shown on Chart 7, is also of considerable interest. Temperature, saturation deficit of the air, wind velocity, and evaporation in sun and in shade, are all found to follow the same course, exhibiting maxima at about

12 JOURNAL OF FORESTRY,3 p.m. and minima at about 4 a.m. These days were typical clear, summer days, and the record, though short, is of considerable significance. RATE OF SPREAD of FIRES Three possible measures of rate of spread may be adopted: 1. Linear distance from start; g, Area burned; 8, Perimeter. The first is of little value; the second may be used, since damage varies as area; but for our present purpose the third is most signifi-

13 CLIMATE AND FOREST FIRES ' 975 cant. The suppression energy necessary to corral a fire varies directly as the perimeter, and for that reason it will be used in this study as the criterion of rate of spread. On level land, with no wind, and with uniform cover, a fire spreads in a circle and the perimeter varies directly with linear distance traveled. Wind and slope will, however, modify the shape o{ a fire, so that it tends to become longer in one axis than the other, though still retaining a generally oval shape. In other words, for a given geometrical figure, equal increments of time give equal increments o.f perimeter. In the actual experimental fires it is found that, with slow rate o{ spread, the perimeter time relation is approximately a straight line up to a period of two hours; with more rapid spread, under wind, the increments of perimeter increase from period to period and, instead of a simple arithmetical progression, perimeter on time tends toward a geometrical series. Two independent factors are active in the case of rapid spread: First, the ratio between linear spread and perimeter (which is, of course, 3.14 for a circle) tends to increase, so that per - meter increases more rapidly than distance traveled. Second, the release of a large amount of heat, in a short period, results in convectional currents of air, which increase the wind velocity and hence the rate of spread; or, to put it more simply, a fire creates its own draft. The extent of this increase is difficult to measure instrumentally. In one case, in which the wind velocity 300 feet from the fire was 4.4 miles per hour, the velocity at the front of the fire was 6., or roughly 40 per cent higher. Without many more data than are now available, it is impossible to formulate any definite law of the relation of perimeter to elapsed time. Table 4 shows, for 3 experimental fires, the average perimeter increases, by 5-minute intervals. TABLE 4 Periods Perimeter Perimeter alter start. linear feet. increment Average... 76

14 976 JO.URNAL OF FORESTRY Slowly spreading fires, on the other hand, exhibit a straight-line relation, as.table 5 shows. The intervals of elapsed time are 15 minutes. TABLE 5 Periods Perimeter Perimeier after start. linear feet. increments O IO Average It is seen that increases in perimeter are very nearly constant for all periods, except the last. INFLUENCE of WIND velocity ON RATE OF SPREAD No discussion is necessary to prove that wind has a profound influence on rate of spread o{ forest fires; the statement is sufficient. But to deduce a general law from experimental data is a very difficult problem, for wind is only one of several factors which combine to determine the rate of spread, and its isolation and evaluation is not easily accomplished. In the 33 experimental fires, however, enough measurements were secured so that at least some tentative conclusions can be offered regarding the effect of this factor. Three series of fires, 8, 10, and 15 in number, were available. For each series the same criterion of rate of spread, namely, perimeter 15 minutes after the start, was used. The other factors--slope, temperature, relative humidity, and elevation--were averaged for each series and were found to agree very closely, especially for Series I and III. The average figures only are shown in Table 6. TABLE 6 Relative Average No. of Series Slope humidity temperature fires I II 14 ø III 12 ø Average Series I and III w e burned during periods of the warm, dry, early fall weather, long enough after rains so that the litter was air dry.

15 CLIMATE AND FOREST FIRES 977 Series II, on the other hand, was burned on the day after a rain, as soon as the litter was dry enough to ignite. For each series the fires having the same wi d velocity were then averaged. Thus, velocities of 1.6 to g.5 miles per hour were thrown together, g.6 to 3.5 miles, etc., and the perimeter measurements were also averaged. After this had been done, it was found that the average values in Series I and III were practically identical, but the size of the perimeter for a given wind velocity was much lower in Series II than in the other two, due, of course, to greater moisture content of the litter. In order to compare all fires by the same standard, the basic rate of spread--that at 0.0 miles wind velocity--was taken as 100, and for each series the values at different velocities were referenced to this base. The values thus derived give, of course, not actual perimeters, but relative indices. Table 7 shows the final line-up of the average derived index figures. Wind velocity, miles per hour o '5 TABLE I 7 Relative Series I and II length of perimeter Series II loo oo 490 Average loo Average value, minus $ V2 Column 5 divided by column ' It will be seen that there are considerable differences in the values derived for the different series, and, indeed, there is no intention of claiming that this preliminary study is the final word on the subject. An examination of the last three columns is, however, ¾ery instructive 'in showing a possible mathematical law, which expresses rate of spread as governed by wind velocity. Column 5 gives the average relative perimeter for wind velocities of 1 to 5 miles an hour; column 6, the square of wind velocity; and the last column, the index figure secured by dividing column by column 6. The last figures are practically constant for the velocities tested and for the data. We may say that rate of spread in perimeter varies as the square of wind velocity. It is

16 978 JOURNAL of FORESTRY interesting and instructive to remember that vind pressure based on velocity also varies as the square of the velocity. It must be perfectly evidenthat the data here used are insufficient to justify the definite and final statement that the above law actually expresses the relation. Considering, ho vever, the care with which the observations were made and the rather close agreement with the formula of the empirically derived values, the law may at least serve as a working hypothesis, against which future data may well be checked. With the data presented, it may be of interest to compute the probable relative rates of spread of fires at different hours of the day, as governed by wind velocity. TABLE 8 Relative Hour Velocity rate of spread 12 Mt A.M A.M A.M M P.M P.M P.M Velocity data from Chart 6. Thus, the lower temperature and higher humidity, both of the air and of the litter, being left out of consideration, fires, as influenced by wind, are likely to spread only half as fast at night as during the early afternoon. SUM MARY This preliminary study shows: 1. Litter is capable of holding its own weight of moisture, but a,layer of litter from 1 to lx/ inches deep can, under normal conditions of summer weather, be reduced from the saturation point to air dry in from one to two days. g. Litter with over 8 per cent of moisture will not burn, and this is therefore the critical point. 3. During the fire season great fluctuations in moisture content occur on different exposures and at different elevations. On a north slope the moisture content may be above the danger point for threefifths of the season as compared with one-third on a south slope during the same period.

17 CLIMATE AND FOREST FIRES In fire protection, clear weather in late fall may result in the rapid drying out of litter, even at high elevations and after it has been saturated by over an inch of rain. 5. Air-dry litter has the property of taking up moisture from the air, chiefly at night, to the extent of 5 to 6 per cent of its own weight. 6. A study of the factors influencing the dryness of litter, namely, evaporation, mind movement, relative humidity, and temperatu're, shows that they all have the same seasonal and diurnal maxima and minima. 7. Rate of spread of fires is best measured by size of perimeter, rather than by linear distance traveled or by area covered. 8. For slowly spreading fires, size of perimeter based on elapsed time is an arithmetical progression, while with more rapid spread it tends to become a geometrical progression. This is due, in part, to changes in the shape of fast spreading fires and, in part, to the creation of a draft by the fire itself. 9. Rate of spread, as governed by wind velocity, may be stated to vary as the square of the velocity.