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BASIC FIRE WEATHER

BASIC FIRE WEATHER. Objectives. Describe relationship among dry bulb temperature, dew point, wet bulb temperature, and relative humidity. Describe typical diurnal temperature and relative humidity changes. Determine relative humidity and dew point using appropriate psychometric tables.

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BASIC FIRE WEATHER

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  1. BASIC FIRE WEATHER

  2. Objectives • Describe relationship among dry bulb temperature, dew point, wet bulb temperature, and relative humidity. • Describe typical diurnal temperature and relative humidity changes. • Determine relative humidity and dew point using appropriate psychometric tables.

  3. Become familiar with the use of belt weather kits. • List the types and uses of weather forecasts available.

  4. Moisture in the atmosphere can appear in three stages: solid, liquid and gas. The amount of moisture in the air affects fine dead fuel moisture by either wetting or drying fuels. Low fuel moisture increases fire behavior while high fuel moisture decreases it.

  5. The main source of atmospheric moisture is the oceans, but transpiration from plants is also important. • When discussing atmospheric moisture, it’s desirable to have some point of reference or means of quantifying the amount of moisture present. The most common points of reference are dew point, relative humidity, and wet bulb temperature.

  6. Dew Point • The temperature to which air must be cooled to reach its saturation point. In other words, it’s a measure of how much water vapor is in the air. • If air is cooled to its dew point, condensation occurs, and dew, fog or clouds will form. • A rising dew point indicates increasing moisture which may lead to precipitation. • Air temperature and dew point accurately define atmospheric moisture at any time or place. If both are known, relative humidity can be calculated with tables.

  7. Rising dew point ----- Lowering dew point ---- Increasing moisture Decreasing moisture Dew Point and Moisture

  8. Relative Humidity (RH) • The ratio of the amount of moisture in the air to the amount which the air could hold at the same temperature if it were saturated. Always expressed as a percentage.

  9. Warm air can hold more moisture. • Air can hold more moisture as temperature increases. • If air is cooled to its dew point, it must lose some of its water vapor through condensation.

  10. Relative humidity changes with the temperature.

  11. Relationship of Temperature, Dew Point, and RH • With little or no change in dew point, RH will decrease as temperature increases (or increases as temperature decreases). • GUIDELINE: RH doubles with each 20 F decrease in temperature, or halves with each 20 F increase in temperature.

  12. Wet Bulb Temperature • The amount of moisture in the air can also be determined by the wet bulb temperature. • The wet bulb temperature is the lowest temperature to which the air can be cooled by evaporating water into it at a constant pressure.

  13. Diurnal Temperature and RH Changes • Maximum RH generally occurs near sunrise when temperature is lowest. • Minimum RH generally occurs in mid afternoon when temperature is highest.

  14. Determining RH From Tables • Select proper table for your elevation. • Get dry and wet bulb temperature readings from psychrometer. • Find wet bulb temperature at the top of the table, then move down that column. • Find dry bulb temperature at left side of table, them move across that row to right. • Find the intersection. • The lower number is RH, the upper number is dew point.

  15. Belt Weather Kits • Sling psychrometer • RH/Dew point tables • Compass • Distilled Water • Log Book

  16. Class Exercise

  17. Weather Forecasts • Help with planning daily fire management activities. • Determine potential hazards.

  18. Become familiar with your local forecasting services!

  19. Types of Forecasts • General fire weather forecast • Red Flag Warnings and Fire Weather Watches • Smoke management forecasts • Spot weather forecasts

  20. General Fire weather Forecast Grand Junction, CO 7/16/2002 • Daily fire management planning (RX, tactics) • Staffing Levels • Prevention • PROBLEMS: • Average conditions over large area • Issued for predetermined time period • May not stress important elements • May not fit local topography

  21. Red Flag Warnings and Fire Weather Watches To alert fire managers of critical fire weather conditions that will increase fire danger in a significant way • Dry Lightning • Unusually low RH • Very Dry and Unstable air • Strong and shifting winds with dry cold front

  22. SMOKE MANAGEMENT FORECAST Advise fire managers of atmospheric conditions that will affect the dispersal of pollutants from fire

  23. Spot Weather Forecast Site specific forecasts that are issued to fit time, topography and weather of a specific location

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