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Unit 2: Fuels Classification

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Distribution & Behavior   ::   Characteristics   ::  Timelag & Life Cycle  ::   Availability   ::   Models   ::   Exercises


We also know that the moisture content in fine, dead fuels can change very rapidly, depending on the relative humidity of the air and precipitation. Moisture content changes in larger fuels, but at a much slower rate. How much slower? How do we predict what fuel moisture changes will occur in various fuels over periods of time? Well, fire scientists have determined drying times for different size fuels and have designed a system to determine and record fuel moisture percents. They use the term time lag and have placed various sizes of fuels into convenient time lag categories or classes.
Timelag is a measure of the rate at which a given dead fuel gains or loses moisture. The timelag categories are:

  1. 1 hour timelag fuels: less than 1/4 inch diameter
  2. 10 hour timelag fuels: 1/4 to 1 inch diameter
  3. 100 hour timelag fuels: 1 to 3 inch diameter
  4. 1000 hour timelag fuels: greater than 3 inch diameter

Do these fuel sizes look familiar? Our four size classes of fuels correspond to the four time lag categories for fuels. In Unit 5 of this course, we will study the time lag concept more and make estimates of fuel moisture percents from tables.

Stages in the Life Cycle of Herbaceous Vegetation

We noted earlier the wide range in fuel moisture percents from dead to live fuels. Since the fine fuels are the primary carrier of fire, we should be concerned with the amount of dead versus live fine fuels. We know that this proportion changes throughout the year with the seasonal growth and then with the curing of herbaceous vegetation.

Copyright 2008, by the Contributing Authors. Cite/attribute Resource . admin. (2005, October 20). Unit 2: Fuels Classification. Retrieved January 07, 2011, from Free Online Course Materials — USU OpenCourseWare Web site: This work is licensed under a Creative Commons License Creative Commons License