Wildland Fire Basics

Question 1

What are the two overall types of wildfire?

Answer 1

1. Flaming: bright red flames, heat and smoke; oxidised gases from heat release
2. Smouldering: heat confined to ground, lots of smoke

Question 2

Describe the fire triangle.

Answer 2

FUEL: veg dry enough to burn
OXYGEN: unlimited source, winds add more, removed from smothering
HEAT: once ignition occurs, heat from combustion ignites nearby fuel

Question 3

Outline the four phases of fire.

Answer 3

Phase 1: PRE-IGNITION - dehydration (need 600F to ignite, more heat as water reduces) and pyrolysis

Phase 2: IGNITION

Phase 3: COMBUSTION

Phase 4: EXTINCTION - when one of three elements of triangle removed

Question 4

What is pyrolysis? Explain its relation to the second phase of fire.

Answer 4

Thermal decomposition (when material heated above temp at which chemically decomposes i.e. chemical bonds/cellulose break down), leaving a residue, often char. This occurs after light gases and tar have been released from the (carbonaceous) material. Endothermic.

-> IGNITION: The gases released are ignited given there is enough pilot heat, forming flames. When it is emitting more heat than it is absorbing, it becomes a heat source and can ignite adjacent materials. Exothermic.

Question 5

What is combustion? Note there are three kind of types possible.

Answer 5

Combustion is the process of burning, an equation that is the opposite of photosynthesis:

C6H10O6 + ignition source = CO2 + H20 + heat

1. Flaming - fast pyrolysis, resulting in long flames, high temps of 1300-1800F
2. Smouldering - lower temps of 500-1100F, burning more slowly with longer residence times, deep into soils, incomplete combustion = smoke (combustible vapours above the fuel are too small to support a persistent flame, so gases condense, appearing as smoke).
3. Glowing - gases and smoke have gone, but still producing heat, tar combusts, embers remain.

Question 6

What are the basic mechanisms of heat transfer that result in the movement of fire?

Answer 6

Conduction - by molecular activity; directly contacting substances. The primary form of heat transfer in a wildland fire.

Convection - by movement of liquid or gas. Any flame has moved by this as it is the result of ignited gas. Can reheat crowns. Cause of fast-moving fires up slopes.

Radiation - electromagnetic waves like the sun, playing a role in preheating fuels ahead of fire and can cause injury despite not being close.

NB. Spotting is where sparks/embers from flames are carried ahead and ignite fuels.

Question 7

Name and describe the implications of the three basic fire types.

Answer 7

Ground fires - litter layer, soils. Not particularly dangerous but can be exceedingly damaging if they get underground, with long residence times e.g. Saddleworth Moor.

Surface fires - ALL WILDFIRES START WITH SMALL ONE OF THESE. Easiest to fight but impacts vary due to the influence of fuel type.

Crown fires - passive (torching, individual trees) and active (wall of flames, engulf everything); very difficult to fight, highly dangerous, virtually impossible to fight whilst near. Have to rely on fuel-removal in fire’s path.

Question 8

What are the four basic descriptors of fire? Out of these, which is the hardest to measure?

Answer 8

Rate of spread, heat per unit area, fireline intensity, and flame length.

Heat per unit area is hardest to measure because you can’t get close enough to accurately measure temperatures, and they can vary.

Question 9

Define rate of spread and fireline intensity.

Answer 9

Rate of spread is the speed at which fire travels through surface fuels, in ‘chains’ per hectare; 80 chains = 1 mile.

Fireline intensity is the heat energy release per unit time from a 1ft/1m wide section of fuel bed extending from the front to rear, the MOST IMPORTANT. Flame length is a proxy for intensity; an almost exponential relationship between them i.e. intensity rapidly increases per increase in length.

Question 10

What six overall aspects of fuel are important to consider?

Answer 10

Quantity: amount of available fuel i.e. fuel load. Larger = more fire.

Type: species, form or size (small surface area:volume easy to ignite, deep fuel beds result in higher flames, compact fuels have less oxygen, flammable oils in plants)

Size: fine, medium, coarse; dead matter fuels of 1-hr, 10-hr, and 100-hr, or even 1000-hr fuels (i.e. the amount of time taken for atmospheric changes to influence moisture content of fuels)

Arrangement: horizontal and vertical distribution of all combustible materials within a particular fuel size i.e. vertical shrubs result in longer flames compared to horizontal logs; fuel beds can be continuous or patchy.

Distribution: continuity of fuels over an area

Moisture content: amount of water in fire; high moisture content won’t burn.

Question 11

What is the difference between fine, medium and coarse fuels? Give examples.

Answer 11

Fine e.g. grasses, needles, leaves, small twigs. 1-hr lag so react quickly to changes in precipitation and relative humidity. Primary fire carriers. Ignite readily and are consumed rapidly.

Medium e.g. large sticks. 10-hr fuels. Relatively hard to ignite with conduction and convection. Small enough though to be completely consumed if in fire front.

Coarse e.g. tree branches, deep peat. 100-hr fuel. Difficult to ignite, burn slowly, often left smouldering post-fire.

Question 12

Besides fuel, what two other environmental conditions are essential to consider when dealing with fire? (Like another fire triangle, but regarding influencing factors).

Answer 12

Weather and Topography

Question 13

How can weather influence fires i.e. what aspects of weather are measured by fire ecologists?

Answer 13

Relative humidity: ratio between actual water vapour and the max moisture that the air can hold (%). Cold air holds less moisture than warm; the less humidity, the larger the wildfire.

Precipitation: influences moisture content of fuels, determines relative humidity.

Temperature: influences humidity and moisture content i.e. higher temps dry out fuels, making them easier to ignite (water vapour more quickly driven off).

Wind: direction and speed have important influlence on rate of spread, moist air movement, chance of spotting, convective column likelihood, oxygen supply.

NB. a doubling in wind speed results in rate of spread more than doubling!

Question 14

How is topography relevant to wildfire management?

Answer 14

The steeper the slope, the more rapidly a fire will spread because of the changes caused in the convective column; will always be tilted.

Slope angle: for every 10 degree increase, rate of spread will double.

Slope aspect: affects moisture, microclimatic differences, so fuel type affected e.g. S-facing has more fuel, receives more sunlight than N-facing, barren, in NH at least.

Question 15

What is wildfire anatomy? What are the key terms?

Answer 15

Origin

Fire perimeter

Back: the rear of a fire, moves slowest

Head: hottest, most dangerous part, most damage, largest flames.

Flanks: sides of fire

Islands: non-burnt patches within burnt area

Question 16

How does extreme fire behaviour occur? x7

Answer 16

High fuel load (fine in particular)
Ladder fuels
Strong winds
Steep slopes
High temps 
Low humidity
Drought conditions

Question 17

What is the 30-30-30 rule?

Answer 17

When met, fires almost inevitable.

Relative humidity at or below 30%. Wind speed at or above 30km/hr. Temperatures at or above 30degC.

Question 18

What are the global wildfire trends and their impacts according to Doerr and Santín (2016)?

Answer 18

Fires in the wild for >350M yrs. But a growing problem in media and academia. Quantitative evidence of an ‘increase’ opposes the media. Global area brned has declined in past decade. But data limited - hard to know exact no. of fatalities, economic losses, health problems etc.

Evidence of global decline from charcoal record.

Satellite evidence suggests general trend of fewer, but larger wildfires.

Have become ‘huge’ problem bcos urbanisation, expanding human occupation.

Economically, the largest change has been the increase in suppression efforts.

Conclusion: not questioning that fire season length and area burned has increased in some regions and over past decades, or that climate and land use change could lead to major shifts in future fire consequences. But no support for any general increase in area burned or in fire severity for many regions.

Question 19

How do mountains and meteorology effect fire; Sharples (2009)?

Answer 19

Potential for unexpected behaviours, fire sizes and severity.

Upper winds and rugged terrain resulting in turbulent winds, variations on temp and humidity. Highly dicontinuous properties found for atmospheric flows.

Temp/humidity vary with topography - role of radiation, microclimates, fuel moisture, veg types, S-facing 3degC warmer than N. Humidity tends to increase with height. BUT doesn’t guarantee fires not occurring at higher elevations because of cold-air drainage at night; diurnal changes…

Wind-terrain interactions e.g. dynamic channelling, lee-slope eddies = highly variable wind patterns in rugged terrain -> fire whirls and embers.

Foehn effect = elevate local fire danger levels through influx of warm, dry, accelerated air whilst regional fire danger may be much lower.

Low-level jets (nocturnal) = more severe fire behaviour in early morning, unexpected time.

Mountains SHOULD NOT be considered a part of low-land fire environment.

Question 20

How do wildfires create their own weather and what is a convective column?

Answer 20

Fire -> landscape dries out -> more fires, embers, spreading for miles = very unstable PYROCUMULOUS clouds that produce lighting and hail.

Fire tornadoes can occur through convective columns. These are simply strongly vertical movements of gas/smoke/wind

Question 21

What are the most common techniques used by wildland fire fighters? Name some equipment.

Answer 21

Prescribed burning - using special torches...
Lighting a backing fire
Lighting a head fire
Creating a 'backburn'
Ditch/trench digging
Defensible space - private landowners

Question 22

Describe the dehydration process as part of the first phase of combustion.

Answer 22

Where water is driven out by a heat source, into gas. Need 600F for ignition of materials. Water has higher capacity to absorb heat that wood, so until water is driven out of fuel, temperature of wood will not rise to a point where it can be ignited. The more water in the fuel, the longer dehydration stage, more pilot heat needed.

Question 23

What is the definition of ‘wildland fuel management’?

Answer 23

The modification of potential fire behaviour or fire effects to achieve a desired outcome