exam 1 Flashcards
oxidation
A chemical reaction that involves the combination of oxygen with a compound (common fire definition)
the fire triangle
heat, oxygen, fuel
heat (ignition)
human (anthropogenic) and
natural (lightning) sources of
ignition
fuels (biomass)
produced by photosynthesis and abundant
oxygen
our atmosphere is about 21% oxygen
heat
measure of energy flow from one object to another
temperature
measure of energy within an object–physical property of an object
heat capacity
the amount of heat required to raise the temperature of a substance by a given amount
heat capacity equation
q=Cdt
pyrolysis (combustion)
reaction in which solid materials, such as organic materials, are converted into gaseous (or liquid) state
combustion steps
ignition source (endothermic)
reactant (combustable) -is the fuel
reactant (oxidant) -usually oxygen
heat yield
heat of combustion minus heat lost to fuel moisture
conduction
transfer of heat through solid objects
convection
transfer of heat by movement of gas or liquid (can be movement of air)
radiation
transfer of heat by electromagnetic waves
four burning phases
- preignition
- ignition
- glowing phase
- extinction
preignition
heat is added to the system; volatile
components expelled; fuels dry ahead of flames
ignition
laming phase of combustion; heat is
generated by the process
glowing phase
with high temperatures, materials
continue to be consumed; smoldering
extinction
termination of combustion; caused by
consumption of fuels (no fuel), temperatures reduced (no heat), mechanical smothering of fire (no oxygen)
fire behavior triangle
topography, weather, fuel
rate of spread
-Movement of fire in the horizontal dimension
-Typically described for the head of the fire but applicable to any side (flank or tail)
fireline intensity
Measure of energy output per foot section of the fireline per second
i=hwr
flame length
distance from the flame tip to the midpoint of
the flame depth
ways to measure fire temp
-temp sensitive paints
-thermocouples (the wires)
-ir thermal camera
fuel size and shape
-surface area to volume ratio of fuels affects heat transfer to fuel (pre-ignition and combustion)
-affects the moisture levels and retention of fuels
fuel moisture content
-the quantity of moisture in fuel expressed as a percentage of the weight when thoroughly dried (oven-dried weight)
-affects rate of ignition and combustion
fuel loading
the amount of fuel present expressed quantitatively in terms of weight of fuel per unit area. this may be available fuel
(consumable fuel) or total fuel and is usually dry weight
fuel compactness
-spacing between fuel particles
-related to amount of available oxygen for combustion process, heat transfer, surface area available for combustion, etc.
horizonal continuity
-distribution of fuels at various levels along the horizontal plane
-largely affects how fast/where a fire can spread horizontally
-patchy versus continuous
vertical arrangement
the distribution and continuity of fuels above the ground in the vertical plane (understory versus crown fire)
chemical content
the compounds contributing to the fuels often exhibit different combustion properties
topography
the arrangement of the physical features
of an area
aspect
direction a slope faces
slope
steepness of a location
slope position
where along a hillside a fire occurs (bottom, middle, top)
shape of landscape
features create different expected behavior
elevation
affects curing of fuels, oxygen availability, precipitation, etc.
northern face
Heaviest snows
Highest Moisture
Lowest Temperature
Later Curing of Fuels
Late Snow Melt
southern face
Lightest Fuels
Lowest Fuel Moisture
Highest Temperature
Earlier Curing of Fuels
Earlier Snow Melt
eastern face
Fuels Transition
Earlier Heating
Earlier Cooling
Generally Lee Side
western side
Fuels Transition
Later Heating
Later Cooling
Generally Windward Side
slope and fire behavior
-rate of spread increases upslope
-upslope fuels pre-heated by radiation
narrow or box canyons
-enclosed space can retain radiant heat, preheating fuels on all aspects of the canyon at once.
-spotting across the canyon
elevation (oxygen)
abundance of oxygen decreases with
increasing elevation
elevation (fuels)
amount of biomass and characteristics,
including fuel moisture, changes
elevation (topography)
although more variable, narrow canyons and steepness are more common at high elevations
barriers
an obstruction to the spread of fire (lake, creek, road, ridge, etc.)
determining slope% on topo map
%slope=(rise/run)*100
air pressure
the amount of force exerted by the weight of air molecules on a surface area
water vapor
-water vapor is the gaseous form of water and is extremely important to weather patterns
-forms clouds
-evaporation and condensation
temperature and moisture
at a given air pressure, warm air can hold more water vapor than cold air
temperature
temperature is defined as the
measurement of the average kinetic
energy (the energy of motion of air
molecules) in an object or system
and can be measured using a
thermometer
dry bulb temp
standard measurement of air temp
wet bulb temp
the lowest temperature to which air can be cooled by evaporation
difference between wet and dry bulb temp
informs dewpoint and relative humidity
dewpoint
the temperature to which air must be cooled (at constant pressure) to reach saturation
relative humidity
the ratio of the amount of water vapor in the air to the amount the air could hold when saturated at the same temperature and pressure
dewpoint and relative humidity relationship
for a given dewpoint, relative humidity will decrease as temp. increases
saturated air
the dry bulb temperature, wet bulb temperature, and dewpoint will be the same; this will be 100% relative humidity
non saturated air
dry bulb temperature will ALWAYS be higher than wet bulb and the wet bulb higher than the dewpoint
most critical factor affecting fire behavior
wind
wind direction
direction in which the wind blows. ex. a north wind blows north to south
general winds
do not directly impact fire behavior
20ft winds and midflame winds
directly affect fire behavior
upslope wind
-forms in the morning with strong solar heating
-maximum strength and depth reached by late morning
upvalley wind
-begins to form when the valley floor becomes warmer than the valley walls
-greatest speeds at max heating
downslope wind
-forms when slopes begin to cool around sunset or under heavy daytime cloud cover
-strongest before midnight
downvalley wind
-transitions from the weaker downslope wind often by midnight
-strongest on cloudless nights.
atmospheric stability
the resistance of the atmosphere to vertical motion of air
stable atmosphere
can limit burning because smoke does not disperse and air movement is limited
unstable atmosphere
can have greater fire behavior and risk of extreme events
