32 



MISC. PUBLICATION 10 9, U. S. DEPT. OP AGRICULTURE 



Humiclitjr, temperature, velocity of wind, character of forest cover 

 and slope, time of day, and season of year. 



10. What is meant by "humidity?" 



Humidity, usuaUy spoken of as " relative humidity," is the ratio 

 between the amount of moisture in the atmosphere and the amount 

 which could be present at the same temperature and under the same 

 pressure. When the air has taken up all the moisture in the form 

 of water vapor that it can contain at a given temperature it is said 

 to be " saturated," or to have a relative humidity of 100 per cent. 



Carelessness is the cause of over 75 per cent of all fires 

 in California 



Low relative humidity, or the dry condition of the air, affects the 

 forest cover and renders the leaves, needles, dry wood, and ground 

 litter more inflammable and so creates increased fire hazard. Hot 

 days are not necessarily dry because the air may contain sufficient 

 moisture to raise the relative humidity and render the weather 

 sultry and oppressive. On the other hand, a cold day may be caused 



IF YOU DON'T KNOW, 



ASK A UNITED STATES FOREST 

 RANGER 



