EVAPORATION AND FOREST FIRES

Abstract

SYNOPSIS Hitherto apparently, little attempt has been made by foresters and meteorologists correlate the factors of climate and forest fires. The purpose of this paper is to show that the occurrence and spread of large forest fires are coincident with a greatly increased rate of evaporation or a decrease in vapor pressure. Since evaporation is a climatic complex dependent on the three major factors of temperature, humidity, and wind, the influence of any one of them may be offset by a pronounced change in either or both of the other two. The close relation between periods of high evaporation and forest fires is strikingly brought out in figures 1 and 2. These figures also show that the rate of evaporation does not follow constantly either temperature, humidity, or wind. In some cases it follows wind alone, in others temperature, while in still others it follows changes in relative humidity only. In southern California the wind direction is highly important. For example, an east wind blowing directly off the great deserts, brings excessively dry, hot air, resulting in extraordinary dryness in a short time. In examining the vapor pressure data far the period 1911–1920, it was found that in those years and months when the average vapor remained high a very small number of fires occurred, while in those years and months with a relatively low average vapor pressure there were uniformly periods of extreme hazard, and many bad fires occurred.—H. L.