These are called midflame windspeeds the difference

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need to use windspeeds measured or estimated at the mid-height of the flames. These are called
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midflame windspeeds. The difference between the two is due to friction with the surface vegetative cover and topography. Figure 23 on page 26 illustrates some midflame winds in relation to the 20-foot winds. On the left, a fire is burning in brush and the midflame windspeed will be less than half of that at 20 feet. On the right, the surface fire under timber will experience midflame wind of only a small fraction of those at 20 feet above the canopy. A fire reaching into the canopy will still not receive the full velocity of the 20-foot winds. Why is this? Before continuing our discussion of determining midflame speeds, please do question 9 on page 26. In question 9, you should have marked statement 4 as being true. In number 1, wind measurements taken on fires are usually taken at eye level with a handheld wind meter. In number 2, generally midflame windspeeds will be less than the 20-foot windspeeds except in cases where downslope winds under a canopy occur when the 20-foot wind is calm. In number 3, some spot weather forecasts give midflame windspeeds. For number 4, eye level wind measurements can be used as midflame windspeeds if the measurements are taken under the same sheltering conditions as the fire. Go to page 27. The present state of the science requires that fire behavior calculations be confined to surface fires. If there are trees present, open or closed canopy, the midflame windspeeds will be considerably less than the 20-foot winds. This reduction in windspeed will depend on how sheltered the surface fuels are. See figure 24. It illustrates how surface fuels may be exposed, partially sheltered, or fully sheltered from the 20-foot winds based on tree canopy and position on the slope. We will be adjusting the 20-foot windspeed based on these conditions. Please study this illustration and note the descriptions below the horizon line. When you have finished, return to the text. On page 28 you will find a wind adjustment table. This table can be used to estimate midflame windspeeds from 20-foot windspeeds. The adjustment factor depends on how sheltered the surface fuels are from the wind. This sheltering depends on canopy closure and position on the slope. The adjustment factor for exposed fuels also depends on fuel model. The midflame wind is obtained by multiplying the 20-foot windspeed by the appropriate wind adjustment factor. Read the notations at the bottom of the page; then study the table until you become familiar with its use. The last exercise on page 29 will require that you use the wind adjustment table to determine midflame windspeeds. This will help you to meet a key skill objective for this course. After completing the exercise, check your answers; then prepare for the unit test.
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  • Spring '04
  • MIchealJenkins
  • weather forecasts, windspeed, midflame windspeed

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