Chapter 5 Fire Environment

Question 1

Fire behavior can be defined as the manner in which fuels _____

Answer 1

ignite, flames develop, and fire spreads.

Question 2

The wildland fire environment consists of three major components, ______.

Answer 2

topography, fuels, and weather.

Question 3

Weather is the most ____ of the three environmental factors.

Answer 3

variable

Question 4

Weather will also play a ____ in determining how a fire will burn.

Answer 4

dominant role

Question 5

The elements of weather that affect the fire behavior are

Answer 5

Wind speed and direction / relative humidity / temperature / atmospheric stability / precipitation / and occurrence of lighting in some areas.

Question 6

Air is fluid and almost _____. Hot air rises and cool air settles. High-pressure areas force air toward ____ at the surface.

Answer 6

constantly in motion / low-pressure areas

Question 7

A high-pressure area is one where the air is ___, and a low-pressure area is one where air is ____.

Answer 7

falling / rising

Question 8

Fire spread is much faster when ____, as available oxygen is much greater when wind is present.

Answer 8

pushed by the wind

Question 9

A wind of ____ exerts about one pound of pressure on a one-foot-square surface.

Answer 9

15 MPH

Question 10

A 30-MPH wind will exert _____, and a 60-MPH wind will exert _____ on the same surface.

Answer 10

4.5 pounds of pressure / 18 pounds of pressure

Question 11

It is estimated that ____ the wind velocity will approximately _____ the rate of fire spread.

Answer 11

doubling / quadruple

Question 12

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.

Answer 12

different direction

Question 13

The most severe weather condition for brush fires in our area is a ____, called a Santa Ana, which blows from ____.

Answer 13

dry Foehn wind / the north

Question 14

Santa Anas are most common in the____, September through April.

Answer 14

cool months

Question 15

The Santa Anas usually last ____, getting weaker each day.

Answer 15

two to three days

Question 16

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.

Answer 16

poor visibility / smoke columns that drift apart

Question 17

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.

Answer 17

vertically-developing cumulus clouds

Question 18

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.

Answer 18

twenty feet

Question 19

Friction at the surface causes the wind to swirl and eddy on the ___ of obstructions, thus producing erratic fire behavior.

Answer 19

lee side

Question 20

Strong winds blowing over ridges will eddy on the ____ ridge causing the air to flow back toward the ridge.

Answer 20

lee side

Question 21

Local, ____ air movement at a fire scene is of great importance to firefighters.

Answer 21

diurnal

Question 22

If the air is relatively calm, it will flow____ in the daytime from about _____ evening.

Answer 22

up slopes, up-canyons / 1000 hours until 1 early

Question 23

Diurnal winds :

Then it will slow and begin flowing down-slope, down canyons at ____.

Answer 23

night.

Question 24

The average speed of up-canyon, daytime winds are about ____. Down-canyon, nighttime flows average ____.

Answer 24

10-15 MPH / 5-8 MPH

Question 25

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.

Answer 25

up-hill attack / reverses direction

Question 26

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.

Answer 26

updrafts /

Question 27

In mountainous terrain, a thunderstorm downdraft tends to continue its downward path into the principal drainages. Speeds of _____ are common and speeds of ______.

Answer 27

20 to 30 MPH / 60 to 75 MPH are possible.

Question 28

Whirlwinds, or dust devils, are one of the most common indications of ______. A whirlwind can sometimes rejuvenate an apparently dead fire.

Answer 28

intense local heating and unstable air conditions

Question 29

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.

Answer 29

amount of water vapor in a given volume of air

Question 30

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.

Answer 30

100%

Question 31

____ air has a much greater capacity for holding water vapor than ___ air.

Answer 31

Warm / cold

Question 32

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.

Answer 32

20 degree

Question 33

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.

Answer 33

high pressure / low pressure

Question 34

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.

Answer 34

vegetation that makes up wildland fuels / more moisture

Question 35

It is generally considered that at ____ relative humidity, conditions are becoming favorable for brush fires.

Answer 35

30%

Question 36

The ignition temperature of brush varies between_____ Fahrenheit. A practical average temperature is considered to be about ____ Fahrenheit.

Answer 36

400 and 700 degrees / 540 degrees

Question 37

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.

Answer 37

160 degrees /

Question 38

Atmospheric stability is defined as the resistance of the atmosphere to____.

Answer 38

vertical motion

Question 39

Unstable air encourages ___ of the air and increases fire activity.

Answer 39

vertical movement

Question 40

Stable air ____ vertical movement of the air and decreases fire activity. The vertical distribution of temperature in the atmosphere determines _____.

Answer 40

discourages / atmospheric stability.

Question 41

In a rising air mass, the temperature of the air mass drops about ____ with each ____ it increases in altitude.

Answer 41

5.5 degrees / 1000 feet

Question 42

When the temperature drop of the surrounding air ____ 5.5 degrees per 1000 feet elevation, the air is ____.

Answer 42

exceeds / unstable.

Question 43

Common indicators of unstable air are _____, smoke that rises to great heights, dust devils, good visibility and gusty, erratic winds.

Answer 43

cumulus clouds that develop to great altitudes

Question 44

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.

Answer 44

constant / steepness of slope