FOREST EAXGEES' CATECHISM 33 



by a dry north wind that contains little moisture, which will lower 

 the relative humidity and cause dangerous fire conditions. 



Forest Service officers are equipped with sling-psychrometers with 

 which they are able to determine the relative humidity during the 

 fire season at any time or place. 



11. What are the principal classes of forest fires? 



Fires are of three classes: (1) Ground fires, which burn the ma- 

 terials composing the forest floor or beneath the surface; (2) sur- 

 face fires, which run over the surface of the forest floor burning 

 the brush and smaller trees: and (3) crown fires, which run in the 

 tree tops consuming the foliage and branches. Crown fires occur 

 where there is heavy undergrowth in the forest and the wind is 

 strong enough to carry the surface fire into the tops of the trees. 

 In times of heavy wind a crown fire may run in the tree tops ahead 

 of the surface fire. 



12. How does the Forest Service fight fire? 



Ground fires are controlled by " trenching." which is digging 

 a deep trench around the fire down to mineral soil. 



Surface fires, which are the most common kind, are controlled by 

 clearing a narrow line of all inflammable material around the 

 fire and from this line fighting back the flames with earth, water, 

 or sometimes with back fire. The width and position of this fire 

 line depends on such conditions as strength of wind, time of day, 

 steepness of slope, and character of ground cover. These conditions 

 vary with every fire and must be given due weight in conducting 

 fire-suppression work. The successful fire fighter is the one whose 

 judgment on all these points is accurate. 



Crown fires in timber can not be successfully combated. It is 

 necessary to wait until the flames cease to spread through the crowns 

 and then surround the fire with a line and fight it in the same way 

 as a surface fire. Crown fires in heavy brush or chaparral are 

 fought by back-firing from a cleared line or firebreak. 



Forest fires are not put out in the sense that every spark is ex- 

 tinguished immediately. A fire is considered under control when 

 surrounded by a fire line adequate to prevent its spread, and all 

 dead snags, logs, and other possible hazards have been removed 

 from near the fire line. The burned area is then patrolled to 

 prevent any new outbreak until the fire dies out or is quenched by 

 rain. 



13. What tools and special equipment are used by the Forest Serv- 

 ice in fighting fires? 



The main fire-fighting tools in use by the Forest Service for many 

 years are shovel, ax. saw. rake. hoe. mattock, and brush hook. 



Because of the close relationship between fire hazard and weather 

 conditions, most of the forest supervisors' headquarters and pri- 

 mary lookouts and some ranger stations are equipped with meteor- 

 ological instruments for reporting the temperature, humidity, wind 

 velocity, and precipitation. Special reports are also received from 

 the United States Weather Bureau giving forecasts for the ensuing 



