CFD Wildland Manual Flashcards
3 types of ground cover fire
ground fire
surface fire
crown fire
ground fire
burn in the layer of dead organic matter that generally covers the soil in forested areas
surface fire
most common type
it burns on the soil surface and uses low lying grass, shrubs and other vegetations as fuel
crown fire
are wind-driven, high-intensity fires that move the treetops of heavily forested areas
wildland fire triangle
fuels
weather
topography
fuels can be divided into two categories
dead and live
fine fuels
1 hour fuels, 1/4” or less in diameter
grass, small twigs, surface duff
Light Fuels
10 hour fuels
twigs and stems
Medium Fuels
100 hour fuels
branches, small trees and logs, medium depth duff
Heavy Fuels
1000 hour fuels
logs, trees, deep duff, peat
Factors causing variation in ignition temperature
Size (smaller will ignite easier)
shape of fuel
moisture content (heat of vaporization)
quantity of fuel being heated
velocity of air around the particle
the most important aspect of fire ignition temperatures
moisture content
Two categories of fuel continuity
uniform and patchy
uniform fuels
are distributed continuously over the area
patchy fuels
all fuels distributed unevenly over the area
Fuel moisture
defined as the percentage of the fuels weight that is water but is usually referred to in more general terms
two conditions influencing fuel moisture
greeness or curing stage
shade protection
Greeness or Curing stage
green (spring)
curing (summer)
Cured (fall and early spring)
Shade protection
Fuels under dense stands dry out slower
south slope fuels dry fast
The most important variable to be considered in evaluating fire behaviour
Weather
Four variables influence fire weather
relative humidity
temperature
wind
precipitation
relative humidity
is the actual amount of water vapour in the air compared to what it could hold at the same temperature and pressure, expressed as a percentage
An RH above 50% to 60% is considered a humid condition
greater than 60% - humid
less than 30% - dry
less than 20% - very dry
Crossover
when the increasing temperature in degrees celcius and the decreasing RH in percentage are equal or approaching the same value
fire problems are likely