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South Texas Fire Weather

South Texas Fire Weather. Jason Runyen Fire Weather Program Leader National Weather Service Corpus Christi. South Texas Fire Weather. Topics we will cover: Importance of weather to your prescribed burns Winds Relative Humidity 2. Stability Ventilation basics Mixing Heights

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South Texas Fire Weather

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  1. South Texas Fire Weather Jason Runyen Fire Weather Program Leader National Weather Service Corpus Christi

  2. South Texas Fire Weather • Topics we will cover: • Importance of weather to your prescribed burns • Winds • Relative Humidity • 2. Stability • Ventilation basics • Mixing Heights • Transport Winds • 4. Critical fire weather patterns to avoid • NWS products and services at your fingertips

  3. South Texas Fire Weather Section 1: Importance of weather to your prescribed burns

  4. Why is weather so important? • Success in prescribed burns is directly related to how well you understand and are able to predict fire behavior. • The safety of all personnel also depends on fire behavior knowledge.

  5. Wind • Stability WEATHER Wildland Fire Behavior TOPOGRAPHY FUELS • Fuel Moisture • Terrain • Fuel Temperature • Fuel Characteristics

  6. It can not be overemphasized: A basic knowledge and awareness of weather is essential for making critical fire management decisions.

  7. Why is weather so important? Two of the most critical weather elements: Wind and Relative Humidity

  8. Effects of Wind on Wildland Fire Behavior • Wind carries away moisture-laden air and thus hastens the drying of wildland fuels 2. Once a fire ignites, wind aids combustion by increasing the supply of oxygen 3. Wind increases fire spread by carrying heat and burning embers to new fuels - Spotting 4. Wind bends the flames closer to the unburned fuels, thus preheating the fuels ahead of the fire front

  9. Effects of Wind on Wildland Fire Behavior • The direction of the fire spread is determined mostly by direction of the wind • Wind influences 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.

  10. Why is weather so important? Wind Generally the wind prescription for a successful burn is: 5 to 15 mph and a Steady wind direction

  11. Why is weather so important? Wind • Light and variable winds create poor burning conditions and an unpredictable direction of spread • High wind speeds may reduce fuel consumption, increase chances of escape, and increase risk of spotting 5 to 15 mph and a Steady wind direction

  12. Why is weather so important? Wind • Conducting burns when winds are forecasted to change direction is not advised • A stable/consistent wind direction throughout the burn will avoid unpredictable fire behavior 5 to 15 mph and a Steady wind direction

  13. Critical WindsCold Front Winds • Fronts • Boundary between two dissimilar airmasses • Extend from the center of low pressure • Move at 20 to 30 mph • Cold Front • Boundary separating a cold airmass from a warm airmass • Can migrate west to east and north to south

  14. Critical WindsCold Front Winds • Pre-Frontal Conditions • Light southeast winds 150 miles ahead of the front • Winds shifting and increasing from the south as the front approaches • Winds shifting southwest just ahead of the front and becoming strong • Winds strongest along the front • Warm air ahead of the front • Sometimes but not always a dry airmass • Unstable airmass • Favorable burning environment

  15. Critical WindsCold Front Winds • Post-Frontal Conditions • Winds rapidly shift to the northwest as the front passes with speeds remaining strong and gusty • Temperatures cool rapidly • RH increases • Fire behavior typically decreases

  16. Critical Winds Thunderstorm Winds • Two Characteristics important to fire weather • Lightning • Indraft and downdraft winds are most important

  17. Critical Winds Thunderstorm Winds • Thunderstorm Winds • Indrafts and downdrafts can change both direction and speed suddenly • Result in sudden changes in rate and direction of fire as well as intensity • Indraft speeds range from 10 to 20 mph and gusty • Downdrafts speeds range from 25 to 35 mph with gusts over 60 mph

  18. Critical Winds Thunderstorm Winds • Gust Front • Leading edge of the downdraft • Boundary between two dissimilar airmasses, similar to a cold front • Most of the time, marked by a wind shift, decrease in temperature and increase in RH

  19. Critical Winds Low-Level Jets • Low-Level Jet • A jet stream 100 feet to several thousand feet above ground • Develop ahead of cold fronts or troughs • Wind speeds of 25 to 35 mph • Can increase lift and plume dominated fire • Can surface and significantly increase rates of spread

  20. Local WindsLand and Sea Breeze • Land-Sea Circulation • Temperature and pressure contrasts between land and sea • Little if any temperature change over the large body of water • Large temperature change over land from day to night • Sea breeze during the day • Land breeze at night • Strongest in spring and summer • Land-Sea breeze 10-20 mph

  21. Local WindsSea Breeze 1998 Perry Fire

  22. Effects of RH on Wildland Fire Behavior • Affects fuel moisture • - as RH increases, fuel moisture increases • Affects fire intensity • - fires of different intensity can be achieved by selecting different times of day or night as well as different weather conditions • Affects rate of spread • Some fuels will not burn adequately if RH is too high • Fire becomes difficult to control if RH is too low

  23. Why is weather so important? Relative Humidity Generally the RH prescription for a successful burn is: Above 30% and Below 60%

  24. South Texas Fire Weather Section 2: Stability

  25. Three Types of Stability Unstable Atmosphere – Enhances or encourages vertical movement of air Stable Atmosphere – Suppresses or resists vertical movement of air Neutral Atmosphere – Neither suppresses nor enhances vertical movement of air. This condition seldom exists for long periods of time.

  26. Unstable atmosphere Promotes the formation and growth of vertically developed clouds, thunderstorms and tall smoke columns Rising Air

  27. An unstable atmosphere is most often associated with critical or extreme wildland fire behavior.

  28. The Effects of Unstable Atmospheric Conditions on Wildland Fire Behavior • Increased likelihood of fire whirls and dust devils • (both indicators of very unstable conditions) B) Increased likelihood for gusty and erratic surface winds C) The height and strength of convection and smoke columns often increase significantly, and D) Increased likelihood of fire brands being lifted to great heights.

  29. Stable atmosphere Light winds and poor smoke dispersal from poor vertical mixing trapped smoke and haze

  30. A stable atmosphere will tend to suppress or reduce wildland fire behavior.

  31. The Effects of Stable Atmospheric Conditions on Wildland Fire Behavior • Limited rise of smoke columns, resulting in • poor smoke dispersion and visibility. B) Reduced inflow of fresh air, thereby limiting wildland fire growth and intensity. C) Lowers surface wind speeds and fire spread rates except in mountainous or hilly terrain.

  32. Weather balloons measure temperature, moisture, wind and atmospheric stability within a column of atmosphere above a point on the earth’s surface.

  33. Haines Index A combination of: stability and dryness Indicates potential for large plume-dominated fire growth.

  34. Haines Index Numbers The Potential for Large Plume Dominated Fire Growth 2 or 3 … Very low potential 4 … Low potential 5 … Moderate potential 6 … High potential

  35. Haines Index Bottom Line… The drier and more unstable the lower atmosphere, the HIGHER the Haines Index. The more humid and stable the lower atmosphere, the LOWER the Haines Index.

  36. South Texas Fire Weather Section 3: Ventilation Basics

  37. Ventilation Basics Two important weather elements that affect ventilation are: Mixing Height and Transport Wind

  38. Ventilation Basics What is Mixing Height? Maximum height in which rapid vertical mixing takes place in the atmosphere Typically is the height where a temperature inversion occurs

  39. The convective mixing layer is normally “capped” by a layer of very stable air, which limits the rise of vertically developed clouds and smoke columns. The tops of tall smoke columns and cumulonimbus clouds are often seen spreading out at the top of the mixed layer. Top of the Mixed Layer Top of the Mixed Layer

  40. Ventilation Basics Mixing Height • Visual indicators of unstable conditions and higher mixing heights • Clouds grow vertical and smoke rises to great heights • Cumulus type clouds with large vertical depth • Gusty winds • Good visibilities • Dust devils and firewhirls

  41. Ventilation Basics Mixing Height • Visual indicators of stable conditions and lower mixing heights • Low clouds, overcast conditions • Stratus type clouds • Light/steady winds or calm winds • Poor visibilities (fog or haze) • Low smoke column

  42. Ventilation Basics Mixing Height • Unstable atmosphere and higher mixing heights typically occur during afternoon • This is because the sun heats the earth’s surface during the day, which in turn heats the air just above the surface and causes it to rise • The more heating that takes place, the more unstable the atmosphere becomes and the higher the smoke will rise

  43. Ventilation Basics Mixing Height • Stable atmosphere and lower mixing heights typically form overnight and in the early morning • This is because a temperature inversion typically develops just off the surface overnight • Smoke still in the air will drop back down to the surface

  44. Dissipation of the Nighttime Inversion Late Morning Mid-Morning Early Morning Transport Wind Transport Wind Transport Wind Top of the Inversion Rises as it Weakens Surface Inversion Has Dissipated Top of Surface Based Inversion 20-foot Wind 20-ft wind 20-ft wind By mid to late morning, the inversion has weakened considerably with the air next to the ground becoming nearly as warm as the air above the inversion. Fire intensity slowly increases as light drainage winds add more oxygen to the fire. The smoke column also begins to tilt upward and smoke dispersal improves as the top of the inversion rises. When the surface inversion breaks, fire intensity may suddenly increase with a rush of fresh oxygen. The smoke plume may also rise suddenly and become well formed. After rising high enough, the plume will begin to tilt in the direction of the transport wind, potentially carrying small embers and fire brands down wind from the fire. During the early morning hours, fire intensity remains low with more smoke than open flame visible. Smoke dispersal also remains poor with the smoke column hanging low and spreading out in several directions as surfaces winds remain very light or calm.

  45. What to Expect When Nighttime Inversions Break • Winds often increase suddenly and • possibly become gusty and erratic 2. Air temperature increases suddenly 3. Relative humidity decreases suddenly.

  46. Ventilation Basics Mixing Height Inversions behind shallow cold front can result in low mixing heights

  47. Ventilation Basics Mixing Height Generally stagnant/stable conditions occur with mixing heights less than 1700 feet and prescribed burning should be reconsidered

  48. Ventilation Basics What are Transport Winds? Average wind between the surface and the mixing height This is the wind that will help move the smoke out of an area an help disperse it into the atmosphere

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