Excerpt from Weather Conditions and Forest Fires in California

Comparing further the evaporation and the humidity records, it is found that days of equal evaporation may be either days of low relative humidity and low wind movement, or days of high relative humidit and high wind movement. Whether or not such days are of equa importance so far as fires are concerned is at least open to question. The evaporation records generally do not show as high peaks in September as in June, although the fire history demon strates conclusively that the September periods are incomparably worse than those earlier in the year. The investigations of evapora tion from water surface have shown conclusively the importance of sunshine in the evaporations formulae; and this may be the factor that is operative in our records since obviously the number of hours of sunshine per day is considerably less in September than in June. At any rate, it is by no means proved that evaporation is anything more than a general indicator of fire hazard.

However, it is undoubtedly true that within a short period, when evaporation is increasing, fire hazard likewise is increasing, but not necessarily in the same ratio in which the evaporation changes. If a longer record is considered, as for an entire fire season, it does not follow at all that days of equal evaporation represent days of equal fire hazard. In general, it may be fairly said of evaporation records that the days of highest evaporation do not always correspond to the days of greatest fire hazard. In many of the records, although days of great fire hazard correspond to peaks of evaporation, these peaks are not necessarily the highest of the season.

It was believed originally that it would be possible to establish absolute rates of evaporation as indicating different degrees of fire hazard. An examination of the data at hand does not permit any such distinction to be made. Some of the highest absolute rates are attained at points of considerable elevation and relatively low rates at points in the foothill and lower forest region. The value of the evaporation data would seem to lie purely in the direction and gen eral elevation of the curve. Any fine-drawn attempts to establish a mathematical basis of forecasting is going far beyond the capacities of the present instruments. The data are, however, of the highest value in post-season studies, and undoubtedly a series of records over a period of years may prove of decided advantage in deter mining the relative difficulty of handling fires in the different seasons and in deciding whether any long-time fluctuations, as demanded by the sun-spot theory, have taken place.

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