Fire Weather and Fuel Moisture

Question 1

Fire behavior triangle

Answer 1

Weather, fuel, topography

Question 2

Wind

Answer 2

Movement of air relative to Earth’s surface
- most variable element in time and space, most difficult to predict, most critical factor in fire behavior (for weather)
- direction it blows (N wind blows from N → S)

Question 3

Effect of wind on fire behavior

Answer 3

• can carry away moist air and quicken drying of wildland fuels
• brings oxygen
• carries heat, increases fire spread
• affects residence time of flaming front

Question 4

General wind

Answer 4

Mid to upper levels of troposphere

Question 5

Local wind

Answer 5

Lower levels of troposphere
- influenced by terrain
- caused by small-scale differences in air temperature and pressure
- upslope and upvalley vs. downslope and downvalley

Question 6

Upslope and upvalley local winds

Answer 6

Faster spread rates

Question 7

Downslope and downvalley local winds

Answer 7

Can reverse fire direction

Question 8

Foehn winds

Answer 8

Strong, warm, dry winds generated by high pressure in mountainous regions
- extreme fire behavior
- Santa Ana winds (1971 windstorm, sudden increase in temperature = decrease in relative humidity)

Question 9

Effect of humidity on fire behavior

Answer 9

Moisture in atmosphere affects fuel moisture/fire behavior
- less moisture = drier fuels and more extreme fire behavior

Question 10

Wet bulb temperature

Answer 10

Lowest temperature to which air can be cooled by evaporation (measured using psychrometer)

Question 11

Dewpoint temperature

Answer 11

Temperature to which air must be cooled to reach saturation

Question 12

Saturation

Answer 12

When air temperature falls to dewpoint temperature
- when air is fully saturated dry bulb temperature, wet bulb temperature, and dewpoint temperature are the same
- air cannot hold more moisture

Question 13

Relative humidity

Answer 13

Ratio of amount of moisture in air to amount air could hold when saturated at same air temperature
- warmer air can hold more water

Question 14

Live fuel moisture

Answer 14

• varies with plant type/growth
• newest foliage = highest moisture content
• deciduous foliage

Question 15

Live fuel moisture content of evergreens

Answer 15

• Older than 1 year = 80-120%
• Current year = 250%

Question 16

Dead fuel moisture

Answer 16

• absorb water vapor from air
• equilibrium moisture content (EMC)
• 4 classes based on timelag (1-1000 hour fuels)
• diurnal changes in fuel moisture

Question 17

Equilibrium moisture content

Answer 17

Moisture content of fuels when they are in equilibrium with the environment, no exchange of water

Question 18

Effects of fuel properties on fuel moisture

Answer 18

1. Compactness (evaporation = slow in compound fuels)
2. Arrangement (continuous or patchy)