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Unit 11: Fire Behavior Prediction Systems

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Effective Windspeed

The figures below give examples of effective windspeed. The fire on top is burning on level ground. The effective windspeed is the same as the mid-flame windspeed, 5 miles per hour. The fire below is burning on a steep slope. Although the midflame windspeed is 3 miles per hour, the effects of slope will increase rate of spread. The combined effects of wind and slope are added together to give an effective windspeed of 5 miles per hour. In this example, the rates of spread of both fires should be about the same.

Effective windspeed Effective windspeed

Fire patterns become elongated when wind drives the fire in one direction. Upslope usually has the same effect on burn patterns. When the two factors are combined and complement each other, the fire becomes even more elongated.

Size and shape

This deduction helps us to predict the shape, and area and perimeter growth of a fire. The graphic above gives some examples of fire sizes and shapes depending on effective windspeed. The first fire is spreading with a 5 miles per hour effective windspeed. After 1 hour, its area is 15 acres. The second fire has 8 miles per hour effective windspeed. After 1 hour, its area is 26 acres. The third fire has 11 miles per hour effective windspeed. After 1 hour, its area is 38 acres.

Fire shapes are dependent on effective windspeeds

Example 1 Example 2
Example 3 Example 4

The images above give approximate fire shapes for various effective windspeeds. The forward spread distances are constant here, so be concerned only with fire shapes. These elliptical shapes can be used to plot fire perimeters on maps by using the ratios between forward spread distance and side spread distance. Remember, these patterns apply to fires burning on slopes as well as level ground.

Copyright 2008, by the Contributing Authors. Cite/attribute Resource . admin. (2005, November 16). Unit 11: Fire Behavior Prediction Systems. 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_11__Fire_Behavior_Prediction_Systems_5.html. This work is licensed under a Creative Commons License Creative Commons License