Chapter 5 Fire Environment Flashcards
Fire behavior can be defined as the manner in which fuels _____
ignite, flames develop, and fire spreads.
The wildland fire environment consists of three major components, ______.
topography, fuels, and weather.
Weather is the most ____ of the three environmental factors.
variable
Weather will also play a ____ in determining how a fire will burn.
dominant role
The elements of weather that affect the fire behavior are
Wind speed and direction / relative humidity / temperature / atmospheric stability / precipitation / and occurrence of lighting in some areas.
Air is fluid and almost _____. Hot air rises and cool air settles. High-pressure areas force air toward ____ at the surface.
constantly in motion / low-pressure areas
A high-pressure area is one where the air is ___, and a low-pressure area is one where air is ____.
falling / rising
Fire spread is much faster when ____, as available oxygen is much greater when wind is present.
pushed by the wind
A wind of ____ exerts about one pound of pressure on a one-foot-square surface.
15 MPH
A 30-MPH wind will exert _____, and a 60-MPH wind will exert _____ on the same surface.
4.5 pounds of pressure / 18 pounds of pressure
It is estimated that ____ the wind velocity will approximately _____ the rate of fire spread.
doubling / quadruple
If a strong wind is blowing in a ____ than the canyons, the surface air will be channeled by the canyons while the upper-level a continues in the direction of the wind.
different direction
The most severe weather condition for brush fires in our area is a ____, called a Santa Ana, which blows from ____.
dry Foehn wind / the north
Santa Anas are most common in the____, September through April.
cool months
The Santa Anas usually last ____, getting weaker each day.
two to three days
Stable air = Predictable fire behavior; stable air can be recognized by ____ at low levels (smog, fog), stratus-type clouds, steady winds, and _____ after limited rise.
poor visibility / smoke columns that drift apart
Unstable air = Danger in the form of unpredictable fire behavior, fire whirlwinds, etc. Unstable air can be identified by _____, tall smoke columns, dust devils, clear air, and gusty winds.
vertically-developing cumulus clouds
The wind speeds quoted in weather forecasts are wind speeds measured at ____above the surface in light fuels, or ____ above the mean cover height in heavier or taller fuels.
twenty feet
Friction at the surface causes the wind to swirl and eddy on the ___ of obstructions, thus producing erratic fire behavior.
lee side
Strong winds blowing over ridges will eddy on the ____ ridge causing the air to flow back toward the ridge.
lee side
Local, ____ air movement at a fire scene is of great importance to firefighters.
diurnal
If the air is relatively calm, it will flow____ in the daytime from about _____ evening.
up slopes, up-canyons / 1000 hours until 1 early
Diurnal winds :
Then it will slow and begin flowing down-slope, down canyons at ____.
night.
The average speed of up-canyon, daytime winds are about ____. Down-canyon, nighttime flows average ____.
10-15 MPH / 5-8 MPH
This change in local wind direction, right after nightfall, is important for all firefighters to understand. Especially if firefighters are making an ___ on a fire in the evening, and the fire ___ and pushes back toward the firefighters.
up-hill attack / reverses direction
Thunderstorm winds are the ___ that are found in and beneath growing cumulus clouds, downdrafts in the last stages of full thunderstorm development, and cold air outflow from decaying thunderstorm clouds.
updrafts /
In mountainous terrain, a thunderstorm downdraft tends to continue its downward path into the principal drainages. Speeds of _____ are common and speeds of ______.
20 to 30 MPH / 60 to 75 MPH are possible.
Whirlwinds, or dust devils, are one of the most common indications of ______. A whirlwind can sometimes rejuvenate an apparently dead fire.
intense local heating and unstable air conditions
Relative humidity is the ratio of the actual _____, to the amount of water vapor which could be present if the air were saturated at the same temperature.
amount of water vapor in a given volume of air
If a volume of air was completely saturated, the relative humidity would be ____. If the air mass only contained half of the water vapor that it could hold, relative humidity would be 50%, etc.
100%
____ air has a much greater capacity for holding water vapor than ___ air.
Warm / cold
As a rule of thumb, relative humidity doubles with each ___ decrease in temperature, and is halved with each ____ increase in temperature within an air mass.
20 degree
Wind is caused by air moving from an area of ____ to an area of _____ moisture in the atmosphere moves from areas of high concentration to areas of low concentration, as well.
high pressure / low pressure
Since dry, warm air has a tremendous capacity for holding additional moisture, it draws moisture from the _____. The drier the air, the ____ it takes from the fuels, and the faster the fuels ignite and burn.
vegetation that makes up wildland fuels / more moisture
It is generally considered that at ____ relative humidity, conditions are becoming favorable for brush fires.
30%
The ignition temperature of brush varies between_____ Fahrenheit. A practical average temperature is considered to be about ____ Fahrenheit.
400 and 700 degrees / 540 degrees
On very hot days, the temperature of the brush can be as high as ___; this is about 25% of the temperature needed to ignite the brush.
160 degrees /
Atmospheric stability is defined as the resistance of the atmosphere to____.
vertical motion
Unstable air encourages ___ of the air and increases fire activity.
vertical movement
Stable air ____ vertical movement of the air and decreases fire activity. The vertical distribution of temperature in the atmosphere determines _____.
discourages / atmospheric stability.
In a rising air mass, the temperature of the air mass drops about ____ with each ____ it increases in altitude.
5.5 degrees / 1000 feet
When the temperature drop of the surrounding air ____ 5.5 degrees per 1000 feet elevation, the air is ____.
exceeds / unstable.
Common indicators of unstable air are _____, smoke that rises to great heights, dust devils, good visibility and gusty, erratic winds.
cumulus clouds that develop to great altitudes
Topography is the most ____of the three major components of the fire environment. The most important factor under topography is _____, since changes in slope have very direct and profound effects on fire behavior.
constant / steepness of slope
