PHYSICAL CONTROLS OF FIRES 



By S. B. Show 

 Forest Examiner, U. S. Forest Service 



A previous paper (Notes on Climate and Forest Fires in California)^ 

 reported on the results of intensive experiments which aimed to de- 

 termine the efifect of certain climatic factors on rate of spread of fires. 

 This report brought out the fact that there was a very specific relation- 

 ship between rate of spread and wind velocity and moisture content 

 of the litter, and that by experimental methods these relationships 

 could be established. 



The present paper is based on work of an entirely different nature. 

 The data used were obtained from the individual fire reports of 

 twelve timber forests in California covering a period of six years, 

 from 1914 to 1919, inclusive. On each individual report, among other 

 things there is given the degree and direction of slope under which 

 the fire occurred. These data were tabulated in connection with an 

 extensive study of forest fires, and an effort has been made to work 

 out the general relation between degree of slope and aspect and rate 

 of spread of fires. The figures derived are therefore general rather 

 than specific in their nature, but it is believed that they represent 

 very closely the relative values which actually exist in general practice. 

 The quality of the data on the individual reports, generally speaking, 

 is good ; and it is to be expected that by using very large numbers 

 of individual fires the errors introduced by a few incorrectly reported 

 fires will be largely eliminated. The basis of data is 6,877 fires. 



On many of the individual reports the degree of slope is reported 

 as "gentle." "medium," "steep," or "precipitous," using a descriptive 

 term rather than actual degree or percentage of slope. Steepness of 

 slope has therefore been divided into five classes, as follows : level, 

 to 5 per cent; gentle, ."> to 15 per cent; medium, 15 to 30 per cent; 

 steep, 30 to 60 per cent ; precipitous, 60 per cent plus. 



In tabulating direction of slope, fires on northeast and northwest 

 aspects were grouped with those on north, and similarly fires on south- 

 east, southwest aspects with fires on the south slopes. 



^Journal of Forestry, December, 1919. 



917 



