Fire Behavior

Relative Humidity has a dramatic affect on the behavior of a fire and the potential of a fire. The Relative Humidity is checked at least every half hour when fighting a forest fire using a sling psychrometer.

Temperature also plays a role to the fuel availability. High temperatures will increase in flammability of fine fuels. Fuels that are in direct sunlight will have accelerated drying rates in comparison to similar adjacent fuels that are mostly or partially shaded by a canopy cover. Fine fuel flammability will change as fuel temperature changes. These changes will occur when slopes undergo transition between shade and sunlight.

Wind is also an important factor when watching the movement and behavior of a forest fire. Wind is the primary factor that influences fire spread. This includes both the rate and direction of spread.

Wind Speed Ranges

• Frontal Winds -- Too broad a range to be specific.
• Foehn -- 40 to 60 mph common; up to 90 mph reported at 20 ft.
• Land Breeze -- 2 to 3 hours after sunset, 3 to 5 mph at 20 ft.
• Pacific Sea Breeze -- 10 to 15 mph at 20 ft.
• Up-Valley Winds -- 10 to 15 mph, early afternoon at midflame height.
• Upslope Winds -- As high as 4 to 8 mph at midflame height.
• Downslope Winds -- 3 to 6 mph at midflame height.

Wind Indicators

• A. Strong Surface Winds
• Strong surface winds provide a means for wind driven fire runs and the transport of firebrands.
• B. Lenticular Clouds
• Lenticular clouds indicate high winds aloft with potential to surface and produce strong downslope winds.
• C. High Fast Moving Clouds
• High fast moving clouds indicate potential wind shifts, particularly if the clouds are moving in a direction different from the surface winds.
• D. Approaching Cold Front
• Winds will shift and increase in speed as a cold front approaches. Look for a squall line of thunderstorms as a visual indicator.
• E. Cumulonimbus Development
• Thunderstorms present the potential for strong, erratic downdraft winds. Anticipate strong downdraft winds if thunderstorms begin to form from cumulus cloud.
• F. Battling Winds or Sudden Calm
• 1. When a gravity or foehn wind interacts with a local wind significant wind reversals are likely. Indicators would be a sudden calm or winds battling back and forth causing a wavering smoke column.
• 2. A decreasing foehn wind that allows a local wind to regain influence can be as dangerous as the foehn wind that overpowers a local wind.
• 3. A sudden calm can be an important indicator for other situations such as prior to thunderstorm downdrafts.

Stable/Unstable Air Mass Indicators

• -- Good Visibility
• -- Gusty Winds and Dust Devils
• -- Cumulus Clouds
• -- Castellatus Clouds
• -- Smoke Rising Straight Up

• -- Thermal Belt -- The zone of warm night time temperatures and lower night time RH's.
• Inversion Lifting or Breaking

PMS 426 Student Workbook, April 1992, NFES 2242.

FIRE ORDERS (10)

1. Fight fire aggressively but provide for safety first.
2. Initiate all action based on current and expected fire behavior.
3. Recognize current weather conditions and obtain forecasts.
4. Ensure that instructions are given and understood.
5. Obtain current information on fire status
6. Remain in communication with crew members, your supervisor, and adjoining forces.
7. Determine safety zones and escape routes.
8. Establish lookouts in potentially hazardous situations.
9. Retain control at all times.
10. Stay alert, keep calm, think clearly, and act decisively.

• Lookouts
• Communications
• Escape Routes
• Safety Zones

• Tragically, 14 firefighters died during the fire run on the afternoon of July 6, 1994, on the South Canyon Fire.
• U.S. Forest Service Records show the deaths of 79 firefighters during the 1910 fires.
• The Mann Gulch fire of August, 1949 that killed 13 firefighters burned the same month in which the Soviet Union Exploded its 1st Atomic bomb.