3. By warning through long range forecasts, based on sunspot or other phenomena, of the approach of abnormal seasons comparable to 1889, 1910 and 1919 in the Northern Rocky Mountain region.

The present relatively high percentage of accuracy warrants the use and the further development of the regular thirty-six (36) hour daily forecasts in practical fire control work and such use is being made of them in the Northern Rocky Mountain region to a limited but increasing extent.

Since approximately thirty-five (35) per cent of the forest fires in the region are caused by lightning and since practically all of the fires in the more remote districts are due to this cause, it is important that the protection forces be warned in advance of impending lightning storms. That such storms can be forecasted is suggested by the fairly well established relation of the occurrence of lightning to disturbances in atmospheric pressure and by the effect of atmospheric electricity on radio instruments and ordinary telephones.

The hope that an abnormal season can be predicted from one to three months in advance has, perhaps, little or no scientific foundation at this time. It is presaged by the studies of Huntington, Douglas, Ricard and others and the continuation of thorough studies along these lines is worthy of ample support because of its value to the problem of fire protection alone. (author's summary.)

The CHAIRMAN appointed MR. WELLS and MR. SHOW as a committee to draft a resolution as to the fire control problem, to be submitted to the Society at the next meeting.

The CHAIRMAN then advised all in attendance that applications for membership in the American Meteorological Society could be secured from MR. BLAKE.

The meeting was then adjourned until 2.00 P. M.

FIFTH SESSION

Room 250, Old College, University of Southern California, Wednesday afternoon, September 19, 1923.

Chairman: MR. J. M. SHERIER.

The meeting was called to order at 2.06 P. M., there were twenty present.

The CHAIRMAN announced that while DR. J. V. HOFMAN, of the U. S. Forest Service, in charge of the Wind River Experiment Station, could not be present, on account of the pressure of his official duties, the subject of his paper, "Effect of Relative Humidity on Forest Fires," (copy attached), is so important in this and in the Rocky Mountain district, that he would ask Mr. Osborne, who is a collaborator of Dr. Hofman to some extent, to read the paper. This was done.

Meteorological factors and forest fires

By J. V. HOFMANN

A gathering of foresters with, and at the invitation of, meteorologists marks the realization of the long felt need of a close correlation of these natural sciences. This accomplishment leaves behind the cry of the

insistent few who were ever urging unity of purpose and co-operation and opens the door to a new era of development that will apply all of the scientific facts to the existing conditions. Meteorological factors and forest development are inseparable in nature and progress in the establishment of a forestry practice will be measured by the extent that these factors are made inseparable in the study of sciences. The correlation of the meteorological factors needs no discussion at this meeting, consequently this paper is confined to the individual or collective relation of these factors to the forest conditions.

Although all of the climatic factors are related to the development of the forest, those directly related to the forest fire problems are most important because the control of the fire situation is the greatest question in the conservation of the timber supply. The study of the influence of climatic factors on fire hazard has been continued for two years by the Wind River Experiment Station. These studies have included the effect of all of the meteorological factors on forest fire conditions as well as on the behavior of the fire. Temperature, evaporation, wind and other factors influence the fire hazard, but the relative humidity was found to be the most important factor in the development of a dangerous fire period as well as the most usable factor in actual fire control. . . .

Studies conducted by the Wind River Forest Experiment Station of the effect of relative humidity on forest fires showed that fires did not spread when the relative humidity was above 60 per cent. That they spread very slowly and only in very favorable material when the humidity was between 50 and 60 per cent. When the humidity was between 40 and 50 per cent fires picked up, varying from a few running fires to fires that merely smoked up and did not spread. With a humidity of 30 to 40 per cent fires gained some headway and some rapidly spreading fires occurred. A humidity below 30 per cent caused all fires that were in material that would allow spread at all to gain headway, or spread beyond control. Crown fires occurred when the humidity dropped to 25 per cent or lower.

Unquestionably one of the main causes of our enormous fire losses has been due to the failure to realize how very suddenly forest materials may change from a low degree of inflammability to an extremely high degree of inflammability and convert in a few hours, fires which have been smoldering harmlessly for days into raging conflagrations.

A realization of this situation can lead to only one conclusion, that the smoldering fires must be put out immediately while they are small, when they can be handled at small expense and before conditions change.

The correlation of the meteorological factors and the forest fire hazard that has been discussed in this paper emphasizes the importance of the relative humidity and shows that it is the principle factor that can be used as an index of the fire conditions as well as its direct use in fire control. With this fact established it is evident that the greatest need in forest fire prevention and control is a knowledge of changes in relative humidity as far in advance as possible.

Recognizing this point, the Wind River Experiment Station is now

conducting studies to determine the relation between relative humidity and static electricity. This study has progressed far enough to demonstrate a definite relation and furthermore that static can be used as a basis for the prediction of changes in humidity. (Excerpts.)

The CHAIRMAN asked for questions, stating, however, that as the relation between Dr. Hofman's paper, and that of Mr. Osborne, to follow immediately, was so close that it might be well to discuss them together.

FR. RICARD asked if, when the humidity is extremely low, say about twenty per cent, does fire ever arise spontaneously, or must there be a specific cause, such as a lighted match, a burning cigarette butt, or the like.

MR. OSBORNE said that there must be a specific cause.

FR. RICARD then explained that the late Maurice Connell, while at San Jose, had explained fires in the nearby ranges as possibly resulting from over-heated pieces of glass and tin cans thrown away by campers or picnickers during dry weather. He thought that in times of very low humidity enough heat might be concentrated to cause combustion.

MR. OSBORNE stated that he had heard of fires being caused by pieces of glass, such as the bottoms of bottles, focusing the sun's rays on dry material.

MR. SHOW informed Fr. Ricard that in the California District it has been known that malicious incendiaries have placed lenses in wire fences, with a view to starting a fire at a later hour, by the heat from the concentrated rays, while the incendiary could have traveled to a sufficient distance as to have a good alibi. A few such cases authenticated, but there were no authentic records of fires caused from casual glass.

were

MR. W. E. OSBORNE, JR., U. S. Forest Service, in charge of Fire Protection in Washington and Oregon, was re-introduced, and presented his own paper, "The Application of Relative Humidity Records to Fire Control." (no abstract). He prefaced the paper itself by remarking that conditions existing just prior to and during the recent fires that had been so destructive in northern California and the city of Berkeley were such as to strikingly corroborate the conclusions stated in Dr. Hofmann's paper. He presented an enlarged copy of the hygrograph trace obtained at the San Francisco office of the Weather Bureau, for September 16th and 17th, showing the unusually low relative humidity that obtained during the day and night immediately preceding and during the conflagrations, with the remarkably sudden rise in humidity that occurred just prior to the time the fires were brought under control. He also referred to humidity records at the time of some of the historic forest fires which, while not so detailed as some of the later records, seemed pertinent to the present line of study.

MR. BEALS Spoke of the spreading of forest fires due to increases in the velocity of the wind. The intense heat of the fires initiates convection currents which cause a strong indraft from all sides, and sometimes causes cloud formation over the fire area. He recalled that during the great Michigan fire, in the old Signal Corps days, the inrushing winds had overturned wagons near the burning area.

MR. OSBORNE, commenting on the strong winds recorded at Duluth during nearby fires, said that they may have been caused by the fire itself. He had seen reports of large trees broken off by the wind, just within the fire area, but of no such case outside of it.

MR. WELLS said that during the Idaho Pan-handle Fire the wind had snapped off trees at the edge of the burned area.

MR. OSBORNE stated that in Forestry District No. 6 they were satisfied that relative humidity was the best index of fire hazard. Sixty sling psychrometers had been purchased and distributed among the field forces, and the men taught to use them carefully. Formerly the excuse given for allowing a fire to get beyond control was "A Hellofawind came up;" now the excuse is "Low Humidity." The psychrometer is good for short time forecasts of hazard, but somewhat longer forecasts are needed, as those based on the weather map and other sources of information open to the Weather Bureau.

MR. SHOW spoke of the effects of low humidity and high winds in combination on causing a rapid spread of fires, he believed the latter to be a very important factor.

MR. OSBORNE said that he did not mean to give an erroneous impression, the two factors did go together and both were important. The effects of the winds may be observed-but fires do spread, and rapidly, at times when there is but little air movement, especially at the surface. A strong updraft may carry embers from a fire as much as two miles aloft into the upper currents which carry them over unburned areas.

MR. WELLS Spoke a word of appreciation of the work being done by the Forest Service as to this very important problem, stating that the meteorologists appreciate it, as do the various state fire wardens and the private owners of timber lands.

MR. SHERIER remarked that Relative Humidity was a new one-in his experience Lows, attended by high temperatures and high winds in certain quadrants, with a fall in the humidity, greatly increased the fire hazard, all of which conditions are swept forward by the winds.

The CHAIRMAN then introduced MR. GEORGE W. ALEXANDER, of the Weather Bureau Office, San Francisco, Cal., who read his paper:

Intensive studies of local conditions as an aid to forecasting

fire-weather

By GEORGE W. ALEXANDER

It is stated that analysis of data accumulated over a period of years has shown that the meteorological conditions known as "Fire-weather," may be divided into three classes; 1, those causative of fires, as lightning storms; 2, conditions favorable for the inception of fires, as periods of low humidity and high temperatures; 3, conditions favorable for the spreading of fires already in existence, marked by desiccating winds and other winds of from moderate to high velocities. The cumulative effects of summer conditions are not considered, except as they affect the general fire hazard.

The necessity for accurate forecasts of fire-weather has long been recognized, and such forecasts, quite general in character, have been issued. The difficulty of making specific forecasts for individual forest

areas, from the information at present available to the forecaster, is pointed out.

It is stated that intensive studies of local conditions have enabled the Weather Bureau to solve the problem of forecasting damagingly low temperatures, which leads the writer to the opinion that a similar course may assist materially in the production of reliable forecasts for fire-weather.

An outline of the course of investigation necessary, which includes intensive studies of air movements, humidity, and evaporation rates, as affected by the inequalities of local topography, with a statement of the instrumental equipment required, is given. A detailed knowledge of conditions in each section so surveyed would be placed at the disposition of the district forecaster, enabling him to localize his forecasts, while the local investigator should be able further to adapt the forecast as issued to the different sections of his district. With the accumulation of data the local observer should be able, combining his knowledge of local conditions with the information as to general conditions available from the daily synoptic charts, to predict local thunderstorms some hours in advance, and possibly indicate the relative precipitation to be expected; by forecasting expected maxima and winds produce a reliable curve of the expected humidity or evaporation rate, and give reliable information as to expected shifts of air movement.

These results are predicated on the complete co-operation of the district forecaster and the local observer on the part of the Weather Bureau, and of the local representatives of public and private forest organizations, so that all data available may be at the command of the investigator for study and correlation. The results expected are not to be attained at once, as time will be required for the collection and investigation of data, but a real improvement in localized forecasts can be expected within a comparatively short period.

MR. SHERIER stated that the whole problem was an extremely intricate one, from the forecasters' standpoint, its elements are highly localized and present many difficulties.

MR. BEALS then resumed the Chair. He said that Mr. Alexander's paper shows how the problem must be handled, from the standpoint of the forecaster. Some such correlation of local and general weather conditions, as outlined, must be attempted to properly deal with the intricacies of the situation. Fire weather warnings can and should be improved by such local surveys, as has been done in the case of frost warnings in various valleys of the west.

The CHAIRMAN then asked Mr. Show to read the paper by Dr. ROBERT H. WEIDMAN, Director of the Priest River Experiment Station, on

Relation of weather forecasts to the prediction of dangerous forest fire conditions

By R. H. WEIDMAN

The purpose of predicting dangerous forest fire conditions, of course, is to reduce the great cost and damage caused by forest fires. In the region of Montana and northern Idaho alone the average cost, to the U. S. Forest Service, of fire protection and suppression is over one