LESSON 3 - EFFECTS OF WEATHER, TOPOGRAPHY AND FUELS ON FIRE BEHAVIOR

3.1 Relative Humidity

Relative humidity is an indicator of the percentage of moisture in the air at the prevailing temperature.

There is a close relationship which exists between the occurrence and severity of wildland fire situations and periods of low relative humidity. Atmospheric moisture is a primary factor which determines fuel moisture content and therefore the fire potential on a given site. Fine fuels are extremely sensitive to the changes in atmospheric moisture and will absorb moisture from humid air as well as give up moisture to drier atmosphere. This process is constantly changing and should be monitored even on an hourly basis because fuels are that sensitive. Fire conditions will change dramatically, even if the RH values drop by a small percentage ( 5-10%).

3.1 Precipitation

Precipitation supplies fuels with moisture which will cool down the ignition temperature and slow down or eliminate the combustion process altogether. Precipitation influences the moisture content of fuels by cooling down the surface and helping to maintain moisture levels within the fuel itself.

3.1 Temperature

Air temperature can influence the susceptibility of fuels to increased fire behaviour. The interaction between fuels and air temperature consists of two basic effects. An increase in air temperature will result in raising fuel temperatures which will then require less energy to cause ignition ( warm fuel require less heat to burn). The second effect of increased air temperature on a parcel of air will result in a decrease in relativity humidity values(lower RH values result in easier ignition and greater intensity).

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