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Unit 6: Local and General Winds

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Wind Effects   ::   General Winds   ::   Local Winds   ::   Winds of Concern   ::  Wind Input  ::   Exercises

Wind Input for Fire Behavior Calculations

The last portion of this unit is on wind inputs for fire behavior calculations. Wind is a crucial input into fire spread models and calculations, and we need to have the best possible estimates of wind to obtain reasonably accurate results. You on the fireline are often in the best position to provide estimates of windspeed and direction.

Wind direction is observed on a wind vane and is recorded to the nearest 8 cardinal points on a compass--N, NE, E, SE, etc. Unless you have a mobile weather unit on your fire, you probably will not have such instruments. A small belt weather kit, which should be available on any fire, contains all the instruments needed to take good weather observations. Here wind measurements will normally be taken at eye level. The use of the belt weather kit will be covered in a late unit of this course.

Midflame winds

The graphic above 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?

Midflame winds

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 above) 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.

Copyright 2008, by the Contributing Authors. Cite/attribute Resource . admin. (2005, November 08). Unit 6: Local and General Winds. Retrieved January 07, 2011, from Free Online Course Materials — USU OpenCourseWare Web site: http://ocw.usu.edu/Forest__Range__and_Wildlife_Sciences/Wildland_Fire_Management_and_Planning/Unit_6__Local_and_General_Winds_11.html. This work is licensed under a Creative Commons License Creative Commons License