Transcript Fire Weather - The School of Forest Resources and Conservation

WEATHER
Fire Weather
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CPBM Objectives
1.
2.
3.
4.
5.
Describe the various types of wind and how wind
influences fire behavior.
Understand the temp/humidity relationship.
Define stability, inversion, mixing height, dispersion
index and transport wind speed.
List the methods by which heat is transferred, and how
weather affects these.
Explain the sea breeze process and the possible
effects on Rx fires.
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CPBM Objectives
6.
Describe the passage of a typical cold front in Florida,
particularly as it relates to wind and temperature.
7. List the influences of thunderstorms on fire behavior.
8. Understand the differences and usages of the daily
planning, spot, red flag event and other weather
forecasts.
9. Use a Belt Weather Kit to take a representative
weather observation.
10. Discuss the steps in obtaining a spot weather forecast.
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Temperature
• The hotness/coldness of
a substance
• Reflects average
molecular activity
(thermal energy)
– Fahrenheit
– Celsius
– Kelvin (0 = no molecular
activity)
What influences air temperature?
•Time of day
•Season
•Elevation
•Topography
•Latitude
•Weather systems
•Bodies of water
Why does air temperature
matter in wildland fires?
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Heat vs. Temperature
• Heat Energy
– Can be converted to other energy forms
– Can be transferred from one substance to
another
• Temperature
– Determines direction of net heat transfer (hot
to cold)
• Heat Energy applied = rise in
temperature (if NO physical structural
change occurs)
Temperature change depends on
molecular structure (“Heat Capacity”)
Examples of Specific Heats (relative to water)
•
•
•
•
•
Water: 1.0
Forests: 0.45 - 0.65
Dry Air: 0.24
Dry Soil/Rock: 0.20
Gasoline: 0.50
Solar heating
effect of latitude
SUMMER
WINTER
Atmospheric Reaction
• Unequal heating of the earth.
• Energy absorbed and energy released are
not in balance at any one time and place.
The attempt to regain balance
causes atmospheric disturbance
• THIS IS WEATHER!
Atmospheric Layers
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Winds
Horizontal movement of air
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Winds
Wind’s Effect on Wildland Fire
• Wind impacts the fire environment by:
–
–
–
–
Increasing the supply of oxygen to the fire.
Determining the direction of fire spread.
Increasing the drying of the fuels.
Carrying sparks and firebrands ahead of the main fire causing
new spot fires.
– Bending flames, which results in the preheating of fuels ahead of
the fire.
– Influencing the amount of fuel consumed by affecting the
residence time of the flaming front of the fire. The stronger the
wind, the shorter the residence time and the less fuel is
consumed.
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General Winds
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Local Winds
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Local Winds
Slope Winds
• Upslope Winds
– A result of differential
heating and convective
processes along the slope
– Average speeds range from
3 to 8 mph
– Develop along east facing
slope first and south and
west facing slope by late
morning
• Downslope Winds
– Air along the slope cools
and sinks producing the
downslope wind
– Average speeds range from
2 to 5 mph
– Develop on east facing
aspects first and south and
west facing aspects after
sunset
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Local Winds
Valley Winds
• Upvalley Winds
– As the air warm, temperature
and pressure differences
within the valley or adjacent
valleys result in upvalley
wind flow.
– Strongest mid to late
afternoon.
– Average speeds range from
10 to 15 mph.
• Downvalley Winds
– As the valley loses solar
heating, the air in the valley
cools.
– The cool air drains
downvalley.
– Average downvalley wind
speeds range from 5 to 10
mph.
– Typically develop a few
hours after sunset.
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Local Winds
Sea/Land Breeze
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Sea Breeze
1998 Perry Fire
Sea breeze interacting with Perry fire. Fire activity
increases when sea breeze front penetrates fire.
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Seasonal Weather
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Winter Weather in FL
Cold Fronts
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Cold Front Passage
• Wind shifts as the front passes
– clockwise SW to W/NW
• Winds increase in speed
• Air becomes drier
Implications for fire?
-Rx fire
-wildfire
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Relative Humidity
For a given air temperature, relative humidity
(%) is the amount of moisture in the air divided
by the amount the air could hold when
saturated; usually expressed in percent.
•Ranges from 1-100%
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Temperature and RH Relationships
3 grams
3 grams
3 grams of H2O
50ºF
100%
(saturated)
70ºF
50%
(unsaturated)
90ºF
25%
(unsaturated)
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Thermograph
Temperature and RH Relationship
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RH and Fire
• Fuel Moisture
-fine versus large fuels?
• Why does this matter
– wildland fire suppression
– Rx burning
Small changes in RH that aren’t noticed can have
significant impacts
• Monitoring (belt weather kit) is crucial!!
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Precipitation
Liquid or solid water particles that
originate in the atmosphere, and
become large enough to fall to the
earth’s surface.
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Precipitation
Amount vs. Duration
• Fine Fuels
– gain and lose moisture
quickly
– react rapidly to precipitation
• Heavy Fuels
– gain and lose moisture
slowly
– react slowly to precipitation
• Duration vs. Amount
– Precipitation duration has
greater impact on fuel
moisture than precipitation
amount
Keetch/Byram Drought Index (KBDI: 0 to 800)
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Atmospheric Stability
The degree to which vertical motion in the atmosphere is
ENHANCED or SUPPRESSED.
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Atmospheric Stability
• Depends on the vertical temperature distribution
of the atmosphere
• Unstable air encourages vertical motion
Atmospheric Stability
• Environmental Lapse Rate
– Change in temperature with height within the air
column
– Varies widely depending on conditions (average
3.5˚F/1000ft)
– Determines atmospheric stability
• Adiabatic Lapse Rate
– Change in temperature solely due to change in air
pressure (when a parcel of air is physically moved)
– Dry (unsaturated): 5.5˚F/1000ft
– Moist (saturated): 2.5˚F/1000ft is typical
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Initial Lifting Forces
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Stable Atmosphere
Stable atmosphere resists upward vertical motion
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Stable Atmosphere
Visual Indicators
• Visual Indicators
– Clouds form in
layers
– Smoke drifts apart
after limited rise
– Poor visibility due
to smoke or haze
– Fog layers
– Steady winds
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Stable Atmosphere
Visual Indicator Examples
• Smoke drifts apart
after limited rise
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Stable Atmosphere
Inversion
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Stable Atmosphere
Inversion Types
• Four Inversion Types
–
–
–
–
Nighttime (Radiation)
Frontal
Marine
Subsidence
• Two most common
types
– Nighttime (Radiation)
– Subsidence
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Stable Atmosphere
Thermal Belt
• Thermal Belt
– Nighttime inversions in
mountainous regions
– The warm layer
typically found on the
middle third of the
slope
– Characterized by the
highest minimum
temperature and lowest
minimum RH
– Fire can be very active
within the thermal belt
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Unstable Atmosphere
Encouraging upward vertical motion
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Unstable Atmosphere
Visual Indicators
• Visual Indicators
– Clouds grow vertically
and smoke rises to
great heights
– Cumulus clouds
– Good visibility
– Gusty winds
– Dust devils and
firewhirls
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Unstable Atmosphere
Visual Indicator Examples
• Clouds grow
vertically and
smoke rises to
great heights
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Unstable Atmosphere
Visual Indicator Examples
• Cumulus Clouds
• Good Visibility
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Unstable Atmosphere
Visual Indicator Examples
• Gusty Winds
• Dust devils and
firewhirls
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Atmospheric Stability
• Mixing Height
(500m minimum!)
• Atmospheric Dispersion Index (ADI)
0
40
Poor to Fair
May be too
stagnant for Rx
Burning (smoke
issues)
70
Good
Very Good
Preferred range
for Rx Burning
May be too
hazardous for Rx
Burning (control
issues)
• LVORI (low visibility occurrence risk)
– Calculated from ADI and relative humidity
– Associated with risk of vehicle accidents
– Range 0-10
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Critical Fire Weather
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Critical Fire Weather
Cold Fronts
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Critical Fire Weather
Foehn Winds
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Critical Fire Weather
Foehn Winds
• Common Foehn
Winds
– Chinook
– Wasatch
– North
– East
– Santa Ana
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San Diego, Oct. 2007
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Critical Fire Weather
Thunderstorms
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•
•
•
•
•
Move with the general winds
Downdrafts
Erratic/gusty winds
Very common in FL
Virga (western US)
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Critical Fire Weather
Dust Devils and Firewhirls
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Planning and Implementation
• Forecasts
– ‘regular’ weather forecasts
– fire weather forecasts
– Planning (long and short term)
• On-Site Wx Observations
– Rx burning
– wildand fire use
– fire suppression
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Predictive Services
(national, regional websites)
– Season Fire Potential
– Extended Outlooks
• 3-14 days
• month, 3 months
– Daily
– Spot Weather
Forecast
• Specific to burn site
• On request
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Spot Weather Forecast– DOF in Florida–
must have local information and location
Florida Division of Forestry - Spot Forecast
Forecast ID: 1185873416
Date Issued: Tue Jul 31, 2007 at 05:16
Name: Leda Kobziar
Company: University of Florida
Location: 29.74 / -82.21
• Site specific
forecasts
that are
issued to fit
time,
topography
and weather
of a specific
location.
Forecast
Time
Temp
Humidity
Wind
Speed
Direction
Cloud
Cover
5:00
76-79
85-90%
3-6
WNW
BK
6:00
77-80
84-89%
3-6
WNW
BK
7:00
77-80
83-88%
4-7
WNW
BK
8:00
78-81
83-88%
4-7
WNW
BK
9:00
80-83
78-83%
4-7
WNW
BK
10:00
82-85
74-79%
4-7
WNW
BK
11:00
83-86
70-75%
4-7
NW
SC
12:00
84-87
68-73%
4-7
SSW
SC
Precip
13:00
85-88
66-71%
5-8
ESE
SC
12%
Chance
(Trace)
14:00
86-89
64-69%
5-8
N
BK
15%
Chance
(Trace)
BK
12%
Chance
(Trace)
15:00
85-88
65-70%
5-8
NNE
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NWS: Fire Weather
Watches / Red Flag
Warnings
• Issued when the combination of
dry fuels and weather conditions
support extreme fire behavior or
ignition is occurring or expected
to occur.
• Can prevent prescribed burning
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Fire Weather Watches
• Issued when there is a high potential for the
development of a Red Flag Event.
• Normally issued 24 to 72 hours in advance.
FIRE WEATHER WATCH NATIONAL WEATHER SERVICE
MIDLAND/ODESSA TX 946 AM CST FRI JAN 27 2006
...FIRE WEATHER WATCH IN EFFECT SATURDAY FOR THE
GUADALUPE MOUNTAINS FOR RELATIVE HUMIDITIES OF 15
PERCENT OR LESS...SUSTAINED 20 FOOT WINDS OF 20 MPH
OR GREATER AND HIGH FIRE DANGER...
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Red Flag Warnings
• Used to warn of an impending, or occurring Red
Flag event.
Red Flag Warning/Fire Weather Watch Information
...RED FLAG WARNING IN EFFECT ACROSS THE SUWANNEE VALLEY
REGION OF NORTH FLORIDA FROM NOON TO 5 PM EST THIS AFTERNOON
FOR LONG DURATIONS OF LOW RELATIVE HUMIDITY...
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• National Weather Service
• http://www.srh.noaa.gov/jax/
• Florida Division of Forestry
• http://www.fldof.com/wildfire/index.html
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