Introduction to Wildland Fire Behavior

1. Introduction to Wildland Fire Behavior S-190 Unit II

2. Unit 2 Objectives • List the indicators of an approaching cold front and describe what wind changes to expect • List three common foehn wind conditions and the areas in which they occur • Identify a thunderstorm and describe how and when it is dangerous 02-01(1 of 2)-S190

3. Unit 2 Objectives (cont.) • Describe the daily cycle of slope and valley winds • Describe the effect relative humidity has on wildland fire behavior • Identify the wildland fire environment indicators that can produce problem and extreme fire behavior 02-01(2 of 2)-S190

4. Differential Heating

5. Group 2 — Weather • Temperature • Wind • Increases supply of oxygen • Drives convective heat into adjacent fuels • Influences direction of spread and spotting • Carries away moist air replacing it with drier air • Dries fuels • Raises fuel moisture if the air contains moisture • Relative humidity • As RH increases, fuel moisture increases • Precipitation • Increases fuel moisture 01-14-S190

6. General or Gradient Winds • Large scale winds caused by high and low pressure systems, but generally influenced and modified in the lower atmosphere by terrain • Everyone is most familiar with them from the nightly news weather reports • In short & simple terms, classed into warm fronts and cold fronts 02-02-S190

7. Warm Front

8. Cold Front • A cold front is the boundary line between a cooler air mass which is replacing a warmer air mass • Cold air is more dense and will hug the ground more than a warm front 02-03-S190

9. Cold Front

10. Cold Front Winds 02-04-S190

11. Indicators of a Cold Front Passage • Line of cumulus clouds approaching from the west or northwest • Large clouds of dust can precede the front • Shift in winds from the southeast, south, to the southwest, and increase in velocity • Winds will be strongest, erratic, and gusty as the front reaches you • Winds will continue to shift as the front passes, generally resulting in strong, gusty, cool winds out of the west and northwest 02-05-S190

12. Cold Fronts are Potentially Dangerous • Wind direction will abruptly change • Strong southerly winds ahead of front will drive the fire head to the north or northeast • Winds shifting to west or northwest after front passes will drive fire head to the east or southeast • Rapid drop in relative humidity within 24 hours of front passage 02-06-S190

13. Fire Before Cold Front Passage 02-07-S190

14. Fire After Cold Front Passage 02-08-S190

15. Foehn Wind • Also called gravity winds, dry wind with strong downward components, characteristic of mountainous regions • As the wind moves over long distances, air friction warms the moving air up and loses even more moisture 02-09-S190

16. Subsidence 02-23-S190

17. Examples of Foehn Winds 02-10-S190

18. Thunderstorm • A storm produced by a cumulonimbus cloud • Always accompanied by: • Lightning • Thunder • Strong gusty winds 02-11-S190

19. Visual Indicators of Thunderstorm • Tall, building cumulus cloud • Cauliflower appearance of cloud • Dark, flat base • Virga or rain falling from bottom of cloud • Ice crystal top usually in anvil shape with fuzzy appearance 02-12-S190

20. Thunderstorm 02-13-S190

21. Why a Thunderstorm is Dangerous • Produces strong, gusty surface winds affecting direction of fire spread • Downward winds will be erratic, but always away from the thunderstorm • Thunderstorms also produce lightning, which can be dangerous 02-14-S190

22. Sea Breeze Land Breeze 02-15-S190

23. Slope Winds Day Night 02-16-S190

24. Valley Winds Day Night 02-17-S190

25. Temperature/Relative Humidity Chart Noon Midnight Noon Maximum Relative Humidity Temperature Minimum 02-18-S190

26. Visual Indicators of Unstable Air • Clouds grow vertically and smoke rises to great heights • Cumulus-type clouds • Gusty winds • Good visibility • Dust devils and firewhirls Relatively Cold Relatively Warm 02-19-S190

27. Cumulus Clouds

28. Dust Devil 02-24-S190

29. Obvious Unstable Air

30. Visual Indicators of Stable Air • Clouds in layers, no vertical motion • Stratus-type clouds • Smoke column drifts apart after limited rise • Poor visibility in lower levels due to accumulation of haze and smoke • Fog layers • Steady winds Relatively Cold Relatively Warm 02-20-S190

31. Stable Air Clouds

32. Inversion 02-21-S190

33. Thermal Belt 40° 45° 50° 45° 40° 02-22-S190

34. Indicators of Problem and Extreme Fire Behavior: Fuel Indicators • Unusually dry fuels • Large amount of light fuel • shrubs, grass, needles • Fuels exposed to direct sunlight • Fuels dried by prolonged drought • Ladder fuels that allow a surface fire to move into the crowns of shrubs or trees • Crown foliage dried by surface fire • Concentration of snags 02-26-S190

35. Fire in Timber Areas • Fire normally stays on the surface unless there is enough heat or ladder fuels to move into the aerial fuels • When the fire moves into the crowns, the situation changes dramatically

36. Torching vs. Crowning • The term “Torching” refers to a single tree or a group of trees that rapidly burn off • Torching stays in one spot; it’s isolated aerial fuels that cant jump to other aerial fuels

37. Torching vs. Crowning • “Crowning” or a “Crown Fire” is where the fire is able to take all of the aerial fuels in the area, and keep going • Moves very fast, extreme heat, and will continue until fuels, weather or topography make a change

38. Indicators of Problem and Extreme Fire Behavior: Topography indicators • Steep slopes • Chutes, saddles, and box canyons which provide conditions for “chimney effect” • Narrow canyons may increase fire spread by radiant heat and spotting 02-27-S190

39. Steep Slopes • Slopes with the wind direction going uphill with the direction of the slope is a recipe for a fast moving hot fire

40. Canyons and Valleys • Remember, narrow canyons + steep slopes = death traps!

41. Indicators of Problem and Extreme Fire Behavior: Weather Indicators • Strong Wind • Sudden changes in wind direction and velocity due to weather fronts • High, fast-moving clouds may indicate unusual surface winds • Unexpected calm may indicate wind shift • Thunderstorms above or close to the fire • Unusually high temperatures and low relative humidity • Dust devils and whirlwinds developing • Bent smoke column 02-28-S190

42. Aftermath of a wind-driven fire

43. Firewhirls • Generated by intense fires • Can pick up large burning embers and toss them far across the fireline causing spot fires 02-25-S190

44. Indicators of Problem and Extreme Fire Behavior: Fire Behavior Indicators • Keep an eye on the smoke column • Indicates direction of fire spread, locationof spot fires, and changes in fire intensity • Many simultaneous fires starting or smoldering fires beginning to pick-up in intensity • Fire begins to torch small groups of treesor shrubs • Frequent spot fires occurring • Firewhirls beginning to develop insidethe main fire • Crown fires 02-29-S190

45. Unit 2 Objectives • List the indicators of an approaching cold front and describe what wind changes to expect • List three common foehn wind conditions and the areas in which they occur • Identify a thunderstorm and describe how and when it is dangerous 02-029 (1 of 2)-S190

46. Unit 2 Objectives (cont.) • Describe the daily cycle of slope and valley winds • Describe the effect relative humidity has on wildland fire behavior • Identify the wildland fire environment indicators that can produce problem and extreme fire behavior 02-30 (2 of 2)-S190