ch 5. Fire Environment Flashcards
wildland fire environment consists of three major components
- topography
- fuels
- weather
of the three environmental factors, which is the most variable
weather
what are the elements of weather that will affect fire behavior
- wind speed and direction
- relative humidity
- temperature
- atmospheric stability
- precipitation
some areas of the Country, what is the 6th important factor in other areas of the Country, not a major concern in Southern California
Lightening
a wind of 15 MPH exerts about ___ pound of pressure on a one foot square surface
one
30 MPH wind will exert __ pounds of pressure
4.5
60 mph wind will exert __ pounds of pressure
18
doubling wind velocity will approximately ___ the rate of fire spread
quadruple
most severe weather condition for brush fires in our area is
dry Foehn wind (Santa Ana), blows from the north
Santa Anas are most common in cool months such as
September through April
how long do Santa Anas usually last
two to three days, getting weaker each day
average speed of up canyon, daytime winds are about
10-15 MPH
Down canyon, night flows average ___ mph
5-8
Common thunderstorm wind speeds
20 to 30 MPH
60 to 75 MPH are possible
what are the most common indications of intense local heating and unstable air conditions
whirlwinds or dust devils
relative humidity doubles with each __ degree decrease in temperature and is halved with each __ degree increase in temperature within an air mass
20
20
ignition temp of brush various between ___ and ___ F
400 and 700
Practical average temperature ignition temp for brush is considered to be about ___ F
540
Practical average temperature ignition temp for brush is considered to be about ___ F
540
What is the most constant of the three major components of the fire environment
Topography
What is the most important factor under topography
steepness of slop
Fuels on what side of slopes have more fires because they cure out earlier in the year, and typically smaller and drier. Relative humidity is lowest on these slopes
South and West
What side of slopes typically have the heaviest fuels, highest fuel moisture, lowest average temperature, lowest rate of spread of fires and lowest fire starts
North slopes
What part of the environment carry the fire
Fuels
fuels can be classified in three basic categories
- Grasses and weeds (Light fuels)
- Coastal sage scrub (moderate fuels)
- Mixed chaparral (Heavy Fuels)
Dead branches and twigs frequently make up ____ to ____ of the fuel volume in chaparral stands more than 30 years old
1/3 to 1/2
What is generally considered the most important influence on flammability of fuels
Moisture content
Grasses and weeds are light fuels and compose what we call them as
flash fuels
Coastal sage scrub only grows about ___ feet in height
four
Mixed chaparral grows up to __ feet and found mostly on north facing slopes
20 feet
criteria for fuel model “B” is
Woody plants 6 ft or greater which occupy 2/3 or more of a particular brush site. 1/4 of the site must be dead fuel
