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S290 Unit 6 part 2

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S290 Unit 6 part 2

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S290 Unit 6 part 2

  1. 1. 6-1-S290-EPUnit 6 Atmospheric Stability Four Lifting Processes That Can Cause Thunderstorms 6-1-S290-EP
  2. 2. 6-2-S290-EPUnit 6 Atmospheric Stability 06-2-S290-EP ThermalsThermals UpdraftsUpdrafts Strong Surface Heating Unstable Atmosphere Thermal or Convective LiftThermal or Convective Lift Strong heating of air near the ground produces thermal updrafts and convective currents. It is most common in the summer. Strong heating of air near the ground produces thermal updrafts and convective currents. It is most common in the summer. 6-2-S290-EP
  3. 3. 6-3-S290-EPUnit 6 Atmospheric Stability Orographic Lift Lifting air over mountains. Caused by air being forced up a slope or valley by daytime heating, or by the upward deflection of air after colliding with a mountain barrier. heating of slopes upslope and upvalley flow 6-3-S290-EP
  4. 4. 6-4-S290-EPUnit 6 Atmospheric Stability Frontal Lift Converging Air Masses When two air masses converge, the lighter and warmer air mass is forced up and over the other air mass. The amount of lift depends on the depth and speed of the advancing air mass, and how unstable the air mass is.
  5. 5. 6-5-S290-EPUnit 6 Atmospheric Stability Convergence Over Mountain Ridges Occurs during the day over mountain ridges as thermally or heat driven winds on opposing slopes come together. Convective clouds are an indication that it is occurring. Clouds produced by this lift are normally tall and vertically developed cumulus type.
  6. 6. 6-6-S290-EPUnit 6 Atmospheric Stability Jet Stream Lift - First, What is the Jet Stream? It is simply a river of swiftly moving air most often at high altitudes. It is the change in wind speed and direction, or wind shear that is largely responsible for producing the lifting of air associated with the jet stream. Varies in speed and direction as it rounds the top of high pressure ridges and low pressure troughs.
  7. 7. 6-7-S290-EPUnit 6 Atmospheric Stability As the jet stream passes overhead, wind shear produces divergence (the horizontal spreading out of winds) and lowers air density and air pressure aloft. Jet Stream Low PressureLow Pressure Jet Stream Lift This pressure reduction causes air from lower levels to rise and fill in the “void” left behind by divergence. 6-7-S290-EP
  8. 8. 6-8-S290-EPUnit 6 Atmospheric Stability If this occurs near a wildfire or thunderstorm, explosive growth is possible. Jet Stream Divergence and Low Pressure Divergence and Low Pressure Jet Stream Lift This chimney-effect caused by divergence and a lowering of air pressure aloft can be enhanced by strong heating and convergence (the coming together of air) at lower levels. ConvergenceConvergence 6-8-S290-EP
  9. 9. 6-9-S290-EPUnit 6 Atmospheric Stability Jet Stream Winds A wildland fire can quickly intensify and become plume dominated with the strong lift produced by a passing jet stream. Wispy cirrus clouds high in the sky may be your only indication that jet stream winds are passing overhead. Cirrus 6-9-S290-EP
  10. 10. 6-10-S290-EPUnit 6 Atmospheric Stability Fire fighters should watch for any visual signs of a jet stream nearby such as the tops of thunderstorm clouds being sheared off by strong winds aloft. Jet Stream Lift and Thunderstorm Growth Rapidly Growing Thunderstorm Jet Stream Wind Shear 6-10-S290-EP
  11. 11. 6-11-S290-EPUnit 6 Atmospheric Stability EXERCISE 5 Lifting Processes
  12. 12. 6-12-S290-EPUnit 6 Atmospheric Stability The Thunderstorm Local storm nearly always produced by a cumulonimbus cloud, and always accompanied by lightning and thunder.
  13. 13. 6-13-S290-EPUnit 6 Atmospheric Stability Lifting, instability and moisture are all necessary to produce a thunderstorm. 6-13-S290-EP
  14. 14. 6-14-S290-EPUnit 6 Atmospheric Stability Three Stages of a Thunderstorm
  15. 15. 6-15-S290-EPUnit 6 Atmospheric Stability The Cumulus Stage • Early stage of a thunderstorm. • The cloud grows vertically, often with only a single updraft. • Precipitation is not produced during this stage, but gusty winds are relatively common.
  16. 16. 6-16-S290-EPUnit 6 Atmospheric Stability The Mature Stage • Most active stage of the thunderstorm cycle • First lightning and thunder are observed • Strong updraft and downdraft • Cloud is called a cumulonimbus • Strong rain-cooled downdrafts reach the ground, can fan out producing strong, sometimes damaging winds
  17. 17. 6-17-S290-EPUnit 6 Atmospheric Stability The Dissipating Stage • During this final stage, downdrafts exist throughout the cumulonimbus cloud. • Without an updraft to supply the thunderstorm with a source of moisture and energy, the core of the thunderstorm collapses. • The collapsing cloud can still produce downdraft winds, gust fronts, rain and lightning.
  18. 18. 6-18-S290-EPUnit 6 Atmospheric Stability All three stages of the thunderstorm pose a hazard to the wildland firefighter. It is the mature stage, however, that poses the greatest risk to personal safety, and potentially the greatest impact on wildland fire behavior.
  19. 19. 6-19-S290-EPUnit 6 Atmospheric Stability EXERCISE 6 The Thunderstorm and Its Stages
  20. 20. 6-20-S290-EPUnit 6 Atmospheric Stability Visual Indicators of Stable Air
  21. 21. 6-21-S290-EPUnit 6 Atmospheric Stability 6-21-S290-EP
  22. 22. 6-22-S290-EPUnit 6 Atmospheric Stability 6-22-S290-EP
  23. 23. 6-23-S290-EPUnit 6 Atmospheric Stability EXERCISE 7 Visual Indicators of Stable and Unstable Air
  24. 24. 6-24-S290-EPUnit 6 Atmospheric Stability Exercise 7 6-24-S290-EP
  25. 25. 6-25-S290-EPUnit 6 Atmospheric Stability Exercise 8 From the following photographs of smoke columns you may be asked to provide short answer to one or more of the following questions: What Can Smoke Tell Us About the Stability of the Atmosphere? 1. Is the atmosphere stable or unstable? Where? 2. If there is wind, where and how strong? 3. If there is an inversion, what type?
  26. 26. 6-26-S290-EPUnit 6 Atmospheric Stability A B C D 6-26-S290-EP
  27. 27. 6-27-S290-EPUnit 6 Atmospheric Stability E F G H 6-27-S290-EP
  28. 28. 6-28-S290-EPUnit 6 Atmospheric Stability I J K L 6-28-S290-EP
  29. 29. 6-29-S290-EPUnit 6 Atmospheric Stability What Is a Cloud? A visible collection of billions of water droplets and/or ice particles suspended in the atmosphere. A cloud that touches the ground is called fog. The formation and appearance of a cloud is strongly influenced by the stability of the atmosphere.
  30. 30. 6-30-S290-EPUnit 6 Atmospheric Stability Clouds form either by: condensing water vapor in a column of rising air, or by cooling air to the point of saturation. Clouds form under: stable and unstable atmospheric conditions, and not all clouds produce precipitation.
  31. 31. 6-31-S290-EPUnit 6 Atmospheric Stability Clouds Can Be Classified: • by their appearance, and • by the height of their bases above the ground. There are basically three cloud groups: 1) High > 20,000 ft 2) Middle 10,000 to 20,000 ft 3) Low < 6,500 ft Vertically developed 1,500-10,000 ft
  32. 32. 6-32-S290-EPUnit 6 Atmospheric Stability The Cloud Family 6-32-S290-EP
  33. 33. 6-33-S290-EPUnit 6 Atmospheric Stability Clouds Preceding a Significant Weather Change Such as a Cold Front • High clouds will first fill the sky, sometimes up to 3 days in advance of a cold front. • Next to appear will be the middle clouds, usually one to two days in advance of a cold front. • Last to appear in the sky are the low and vertically developed clouds, typically the day of the cold front passage.
  34. 34. 6-34-S290-EPUnit 6 Atmospheric Stability Cloud Progression With An Advancing Cold Front 6-34-S290-EP
  35. 35. 6-35-S290-EPUnit 6 Atmospheric Stability Impact of Cloud Cover On Fire Behavior Less Than 50% Sky Cover Greater Than 50% Sky Cover Active Fire and High Flame Heights Inactive Fire and Low Flame Heights 6-35-S290-EP
  36. 36. 6-36-S290-EPUnit 6 Atmospheric Stability Critical Clouds to Firefighters • Cumulonimbus or thunderstorm cloud • Cirrostratus • Altocumulus castellanus • Altocumulus floccus • Altocumulus standing lenticularus • Stratus
  37. 37. 6-37-S290-EPUnit 6 Atmospheric Stability Cumulonimbus (Cb) 6-37-S290-EP
  38. 38. 6-38-S290-EPUnit 6 Atmospheric Stability Cirrostratus (Cs) 6-38-S290-EP
  39. 39. 6-39-S290-EPUnit 6 Atmospheric Stability Altocumulus Castellanus (Accas) 6-39-S290-EP
  40. 40. 6-40-S290-EPUnit 6 Atmospheric Stability Altocumulus Floccus (Ac) 6-40-S290-EP
  41. 41. 6-41-S290-EPUnit 6 Atmospheric Stability Altocumulus Standing Lenticularus (Acsl) Wave Cloud Wave Cloud Wave Cloud 6-41-S290-EP
  42. 42. 6-42-S290-EPUnit 6 Atmospheric Stability Stratus (St) (Fog – Stratus in contact with the ground) 6-42-S290-EP
  43. 43. 6-43-S290-EPUnit 6 Atmospheric Stability EXERCISE 9 Cloud Types and Descriptions
  44. 44. 6-44-S290-EPUnit 6 Atmospheric Stability Unit 6 Objectives 2. Describe temperature lapse rate and stability, and the different temperature lapse rates used to determine the stability of the atmosphere. 4. Name four types of temperature inversions and describe their influence on wildland fire behavior, including the thermal belt. 3. Describe the effects of atmospheric stability on wildland fire behavior. 1. Describe the relationship among atmospheric pressure, temperature, density and volume.
  45. 45. 6-45-S290-EPUnit 6 Atmospheric Stability 5. Name and describe the four lifting processes that can produce thunderstorms. 6. Describe the elements of a thunderstorm and its three stages of development. 7. Use visual indicators to describe the stability of the atmosphere. 8. Describe the four principle cloud groups, and identify the six clouds most often associated with critical wildland fire behavior. Unit 6 Objectives

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