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.
7.
8.
Describe the passage of a typical cold front in Florida, particularly as it relates to wind and temperature.
List the influences of thunderstorms on fire behavior.
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?
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Time of day
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Season
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Elevation
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Topography
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Latitude
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Weather systems
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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. 2C-18-S190-EP
General Winds
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Local Winds
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Local Winds
Slope Winds • Upslope Winds
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A result of differential heating and convective processes along the slope
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Average speeds range from 3 to 8 mph
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Develop along east facing slope first and south and west facing slope by late morning • Downslope Winds
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Air along the slope cools and sinks producing the downslope wind
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Average speeds range from 2 to 5 mph
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Develop on east facing aspects first and south and west facing aspects after sunset
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Local Winds
Valley Winds • Upvalley Winds
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As the air warm, temperature and pressure differences within the valley or adjacent valleys result in upvalley wind flow.
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Strongest mid to late afternoon.
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Average speeds range from 10 to 15 mph. • Downvalley Winds
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As the valley loses solar heating, the air in the valley cools.
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The cool air drains downvalley.
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Average downvalley wind speeds range from 5 to 10 mph.
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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 2C-27-S190-EP
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.
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Ranges from 1-100%
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Temperature and RH Relationships
3 grams 3 grams of H2O 3 grams 50 ºF 100% (saturated) 70 ºF 50% (unsaturated) 90 ºF 25% (unsaturated)
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Thermograph
Temperature and RH Relationship
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RH and Fire
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Fuel Moisture -fine versus large fuels?
Why does this matter
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wildland fire suppression
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Rx burning Small changes in RH that aren’t noticed can have significant impacts
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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
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Fine Fuels
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gain and lose moisture quickly
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react rapidly to precipitation Heavy Fuels
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gain and lose moisture slowly
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react slowly to precipitation Duration vs. Amount
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Precipitation duration has greater impact on fuel moisture than precipitation amount
Keetch/Byram Drought Index (KBDI: 0 to 800) 2C-33-S190-EP
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 2C-36-S190-EP
Initial Lifting Forces
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Stable Atmosphere
Stable atmosphere resists upward vertical motion
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Stable Atmosphere
Visual Indicators
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Visual Indicators
– Clouds form in layers – Smoke drifts apart after limited rise – Poor visibility due to smoke or haze – Fog layers – Steady winds 2C-40-S190-EP
Stable Atmosphere
Visual Indicator Examples
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Smoke drifts apart after limited rise
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Stable Atmosphere
Inversion
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Stable Atmosphere
Inversion Types
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Four Inversion Types
– – – –
Nighttime (Radiation) Frontal Marine Subsidence Two most common types
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Nighttime (Radiation) Subsidence
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Stable Atmosphere
Thermal Belt
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Thermal Belt
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Nighttime inversions in mountainous regions
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The warm layer typically found on the middle third of the slope
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Characterized by the highest minimum temperature and lowest minimum RH
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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
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Visual Indicators
– Clouds grow vertically and smoke rises to great heights – Cumulus clouds – Good visibility – Gusty winds – Dust devils and firewhirls 2C-46-S190-EP
Unstable Atmosphere
Visual Indicator Examples
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Clouds grow vertically and smoke rises to great heights
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Unstable Atmosphere
Visual Indicator Examples
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Cumulus Clouds Good Visibility
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Unstable Atmosphere
Visual Indicator Examples
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Gusty Winds Dust devils and firewhirls
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Atmospheric Stability
• Mixing Height (500m minimum!) • Atmospheric Dispersion Index (ADI) 0 40 70
Poor to Fair
May be too stagnant for Rx Burning (smoke issues)
Good
Preferred range for Rx Burning • LVORI (low visibility occurrence risk) – Calculated from ADI and relative humidity – Associated with risk of vehicle accidents – Range 0-10
Very Good
May be too hazardous for Rx Burning (control issues) 2C-50-S190-EP
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
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Common Foehn Winds
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Chinook
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Wasatch
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North
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East
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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
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Forecasts
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‘regular’ weather forecasts fire weather forecasts Planning (long and short term) On-Site Wx Observations
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Rx burning wildand fire use fire suppression
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Predictive Services (national, regional websites)
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Season Fire Potential Extended Outlooks
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3-14 days month, 3 months
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Daily Spot Weather Forecast
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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
Name:
Leda Kobziar
Date Issued:
Tue Jul 31, 2007 at 05:16
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
5:00 6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00
Temp
76-79 77-80 77-80 78-81 80-83 82-85 83-86 84-87 85-88 86-89 85-88
Humidity
85-90% 84-89% 83-88% 83-88% 78-83% 74-79% 70-75% 68-73% 66-71% 64-69% 65-70%
Wind Speed
3-6 3-6 4-7 4-7 4-7 4-7 4-7 4-7 5-8 5-8 5-8
Direction
WNW WNW WNW WNW WNW WNW NW SSW ESE N NNE
Cloud Cover
BK BK BK BK BK BK SC SC
Precip
SC BK 12% Chance (Trace) 15% Chance (Trace) BK 12% Chance (Trace) 2C-61-S190-EP
NWS: Fire Weather Watches / Red Flag Warnings
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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
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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
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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.fl dof.com/wildfire/index.html
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