S190 Unit 2

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S-190 Unit 2
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• Unit 2 Objectives
• List the indicators of an approaching cold front
  describe expected wind changes.
• List the common foehn wind conditions areas
  they occur
• Identify a thunderstorm how when it is
  dangerous.

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• Describe the daily cycle of slope valley winds.
  
• the effect relative humidity has on wildland fire
  behavior
• 6. Identify the wildland fire environment
  indicators that can produce problem and extreme
  fire behavior.

Video-part 1
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General Winds Cold Fronts
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General Or Gradient Winds Large scale winds are
caused by high and low pressure systems, but are
influenced in the lower atmosphere by terrain
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Cold Front
A cold front is the boundary linebetween a cooler
air mass which is replacing a warmer air mass
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Cold Front Winds
Ahead of the front Southeast to Southwest
After the front Northwest to Westerly
Wind is referred to by the direction it is
coming from!!
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Fire - Before Cold Front
Southerly winds drive fire north or northeast.
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Fire - After Cold Front
West or northwest winds drive fire east or
southeast.
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Cold Front Indicators

• Lines of cumulus clouds approaching from the west
  or northwest
• Large clouds of dust can precede the arrival of a
  cold front
• Shift in winds from the southeast, south, to
  southwest, and increase in velocity
• Winds will be strongest, erratic, and gusty as
  the front approaches.

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Dangers Associated with Cold Fronts

• Wind direction will abruptly change
• Strong southerly winds ahead of front will drive
  the fire head to the north or northeast
• Winds will continue to shift as the front passes,
  generally resulting in strong, gusty, cool winds
  out of the west and northwest.
• Winds shifting to west or northwest after front
  passes will drive fire head to the east or south
  east.
• Rapid drop in relative humidity within 24 hours
  of front passage.

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Foehn Wind A dry wind with strong
downward components. Common in mountainous regions
Exercise 1
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Thunderstorms Local Winds
Video-part 2
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Thunderstorms
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Thunderstorm
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Visual Indicators of Thunderstorms

• Tall building cumulus clouds
• Dark flat base
• Virga or rain falling from cloud bottom
• Ice crystal top usually in anvil shape with fuzzy
  appearance.
• Cauliflower cloud appearance

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Why Thunderstorms are Dangerous

• Produces strong gusty surface winds affecting
  direction of fire spread
• Downdraft winds will be erratic but always from
  the thunderstorm
• Thunderstorms also produce lightning which can be
  dangerous

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Fire Whirls Generated by intense fires can pick
up large burning embers and toss them across
fire lines causing spot fires
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Local Winds

• Sea Land Breeze
• Sea breeze
• Cooler air from higher pressure over water moves
  inland and replaces warm air rising off the land.
• Sea breezes can reach up to 30 mph
• Daytime event
• Land Breeze
• Cool air over the landmass moves out to replace
  the warmer air over the water
• Nighttime event, gentle air flow

• Slope Valley Winds
• Generally- upslope during the heating of the day
  8-15 mph
• Downslope during the cooling of the night, gentle
  2-5 mph.
• Slope aspect effects the heating, and also the
  strength of upslope/downslope winds
• Valley winds come later in the day, and are
  generally stronger than slope winds.

Exercise 2
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Slope Winds
Day Upslope Night - Downslope
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Land Breeze- winds from cooling land move out to
warmer water. Gentle night time event due to
heat loss.
Sea Breeze- winds from cooler ocean move onto
warmer land. Gusty daytime event related to
solar heating.
The different heating rates is the cause of
these winds. Water heats and cools slower than
land.
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Relative Humidity Fuel Moisture
Video- part 3
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Relative HumidityThe amount of moisture in air
can be expressed in a number of ways but the most
common reference is relative humidity.  This
value, expressed as a percentage, represents the
amount of moisture in the air relative to the
amount of moisture air could hold at that
temperature. In other words, air at 70 degrees
Fahrenheit (70oF) and 50 relative humidity (50
RH) is holding half of what it could hold when
completely saturated (100 RH) as shown on the
right. Therefore, the relative humidity at a
given temperature will range from 0 (dry air) to
100 (fully saturated air).  (When you exceed
100, moisture starts falling out of the air and
it rains.
Exercise 3
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Temperature / RH Chart
24 hours
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Atmospheric Stability
Video- part 4
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Inversion
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Subsidence
Large scale sinking of air with high pressure
systems.
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Subsidence
Large scale sinking of air from higher elevations
in high pressure systems sinks to lower
elevations, warming and drying the air mass.
Although the air is stable, it can still cause
trouble with fires due to drier air and
increasing winds with the sinking.
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Thermal Belt
Region of warmer air on middle third of slope.
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Dust Devils Firewhirls
Exercise 4
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Indicators of Extreme Fire Behavior
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Fuel Indicators of Extreme Fire Behavior

• 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

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Topographic Indicators of Extreme Fire Behavior

• Steep slopes
• Chutes, saddles, and box canyons which provide
  conditions for chimney effect.
• Narrow canyons may increase fire spread by
  radiant heat and spotting
• Fire located on cape which can be impacted by sea
  breeze from three directions

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Weather Indicators of Extreme Fire Behavior

• Strong wind
• Sudden changes in wind direction and velocity due
  to weather
• High fast moving clouds may indicate unusual
  surface winds
• Unexpected calm may indicate wind shift
• Thunderstorms above or close to the fire
• Unusually high temperature and low relative
  humidity
• Dust devils and whirl winds developing
• Bent Smoke Column

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General Indicators of Extreme Fire Behavior

• Keep an eye on the smoke column. Indicates
  direction of fire spread, spot fire locations,
  changes in fire intensity
• Many simultaneous fires starting or smoldering
  fires beginning to pick-up in intensity
• Fire begins to torch small groups of trees or
  shrubs
• Frequent spot fires occurring
• Firewhirls beginning to develop inside main fire
• Crown fires

Exercise 5
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Review

• Unit 2 Objectives
• List the indicators of an approaching cold front
  describe expected wind changes.
• List the common foehn wind conditions areas
  they occur
• Identify a thunderstorm how when it is
  dangerous.

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Review

• 4. Describe the daily cycle of slope and valley
  winds.
• Describe the effect relative humidity has on
  wildland fire behavior.
• Identify the wildfire environment indicators that
  cause extreme fire behavior.

test
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