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Unit 5: Fuel Moisture

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Comparison of Fuel Moistures on Various Aspects

The graphic below illustrates the effects of aspect by time of day. Let's follow the curves through the 24-hour period shown. South slopes obviously receive more heating during the daytime than north slopes; thus temperatures are higher, relative humidity is lower, and fuel moisture ordinarily is lower on the south slopes. When darkness comes, temperature differences on various aspects diminish, and by early morning, temperature, relative humidity, and fine fuel moisture values have mostly stabilized.

You should also note that east aspects reach their lowest fuel moisture contents by early afternoon; whereas, southwest aspects have the lowest afternoon fuel moisture contents.

Aspects affects fuel moisture content
Aspect affects fuel moisture contents.

Solar heat affects fuel moisture contents
Solar heat affects fuel moisture contents

Here we see more topographic effects on fuel temperatures. Time of the year influences the amount of solar heating received, thus affecting ground surface and air temperatures. The graphic above gives the amount of solar heat received at Boise, Idaho, by average day of month. For example, during March, this area receives about 1,250 BTUs per average day per square foot of horizontal surface. July receives the highest amount of solar heating at almost 2500 BTUs per average day per square foot. The average day in July receives twice as much solar heat as an average day in March.

Now let's look at September. It receives about 1,700 BTUs per square foot on an average day. You may have noted that although solar equinoxes; that is, equal days and nights, occur during the months of March and September, September receives considerably more solar heating than March. Why is this? Well, the reason is that there are more cloudy days at Boise during March than September. Actually, when the sun is shining, March 21 should receive about the same amount of solar heating as September 21. The point that we want to make here is that time of year has considerable influence on fuel temperatures and fuel moisture contents.

Latitude, or distance north of the equator, also has some effect on the amount of solar heating received. Although heating values will vary by locality, the shape of the Boise curve is mostly representative of that of the contiguous states. The curve will be considerably different for Alaska and northern Canada.

Elevation affects fuel moisture
Elevation affects fuel moisture (daytime).

How about elevation? The graphic above illustrates the accepted "normal" temperature lapse rate of about 3-1/2° decrease per 1,000 feet of elevation rise. Note that as temperature decreases with elevation, the relative humidity increases. We have determined fuel moisture percents for the given temperatures and relative humidities as listed at the right. In this example, with an elevation rise of 5,000 feet, dead fuel moisture contents have increased from 4 percent to 8 percent. Together with later curing dates and higher green to dead fuel ratios at higher elevations, the overall fuel moisture differences can be very significant to fire ignition and spread-rates.

Slope Percent Affects Solar Heating

Slope Percent Affects Solar Heating
Slope Percent Affects Solar Heating

Surfaces perpendicular to incoming radiation receive considerably more heating than slopes that are almost parallel to these heat rays. The angle at which solar radiation hits various surfaces changes throughout the day and with the time of year.

The steepness of slopes is a factor in the amount of solar radiation received on various aspects, and this affects the fuel moisture content of fuels on various slopes. You should note that surfaces perpendicular to incoming radiation receive considerably more heating than slopes that are almost parallel to these heat rays. The angle at which solar radiation hits various surfaces changes throughout the day and with the time of year. The steepness or percent slope on north aspects is particularly important, as there may be times of the year when such slopes receive no direct solar heating at all.

Copyright 2008, by the Contributing Authors. Cite/attribute Resource . admin. (2005, November 07). Unit 5: Fuel Moisture. 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_5__Fuel_Moisture_4.html. This work is licensed under a Creative Commons License Creative Commons License