Dead Fuel Moisture and Dry Times

The past month in Pennsylvania has been a historically dry spell, and with that, we would like to take the time to discuss something called dead fuel moisture content.

First and foremost, what really plays a part in the fire danger ratings as well as fuel moisture is relative humidity. Now, if you’ve been here a while you probably know what that is, but just in case let’s review. Relative humidity (commonly referred to as RH) is “the ratio of the amount of moisture in the air to the amount of moisture necessary to saturate the air at the same temperature and pressure.” It is expressed as a percent. The RH measurements that go into our daily fire danger reports are generated from various remote automated weather stations (RAWS) across the state.

Dead fuels and the air are always exchanging moisture - when the RH is low moisture is removed from the fuels, and when the RH is high moisture is gained by the fuels. See where we’re going with this? When that humidity drops, fire behavior increases because the fuels become drier – at different rates.

Fuel moisture is measured for live herbaceous, woody, and dry (dead) fuels. Their moisture is a calculated value that represents an approximate moisture content. The live fuel moisture varies through the growing seasons and between different climate classes, we look at the different fuel models for the areas as well (did you know there are 20 different fuel models?).

Dead fuel moisture, however, is “the moisture content of dead organic fuels, expressed as a percentage of the oven-dry weight of the sample.” This moisture is controlled solely by exposure to environmental conditions. This is absolutely critical in determining fire potential. So, dead fuel moistures are classified by time lag – which is “the time necessary for a fuel particle of a particular size to reach 63% of equilibrium between its initial moisture content and its current environment.”

Within the National Fire Danger Rating System (NFDRS) there are four different time lag classes:

• 1 Hour (Fine Flashy Fuels) – This is typically grass, leaves, mulch, and litter as well as anything ¼” in diameter or less. These fuels respond very quickly to weather changes. “Moisture in these fuels varies greatly throughout the calendar day and is principally responsible for diurnal changes in fire danger.” Moisture for the fine flashy fuels is computed from observation, time, temperature, humidity, and cloudiness.

• 10 Hour – This consists of round wood 1/4” to 1” diameter, and the litter layer that extends 3”-4” below the surface. The moisture in this class is measured from observation, time, temperature, humidity, and cloudiness.

• 100 Hour – 1” to 3” diameter dead fuels. “Moisture in these fuels is computed from 24-hour average boundary condition composed of day length, hours of rain, and daily temperature and humidity ranges.”

• 1000 Hour – 3” to 6” diameter dead fuels. “Moisture in these fuels is computed from a 7-day average boundary condition composed of day length, hours of rain, and daily temperature and humidity ranges.”

Our main concern through this dry spell is those 100- and 1000-hour fuels – they have been abnormally dry for this time of year coupled with the low RH and higher temps. A fire started right now could easily get into these larger fuels and result in a wildfire that is much more difficult to fully suppress. When we do see drought conditions, it's usually later in the summer but we also experience higher RHs which keep the fine fuels from spreading wildfire rapidly.

Oddly enough, as we were working on this entry a Fuels and Fire Behavior Advisory came through in an email – while it is initially for Northeastern Minnesota, Northern Wisconsin, and Michigan, our area is experiencing similar conditions (which it’s entirely possible that we may see an advisory of our own at some point because of this).

So, let’s share the concerns to firefighters and the public that were highlighted in this advisory:

• Entire surface area, including leaves and grasses (which appear vibrant green), is available to burn.
• The effect of rainfall is short-lived. Resources must be aware of the long-term impact of drought and expect a rapid increase in the potential for fire behavior immediately after any rain event.
• Expect increasing ignitions from lightning in forest fuels. Human-caused ignitions from fireworks and equipment are likely as grass fuels typically to human habitation are cured and receptive.
• Intensive monitoring and mop-up will be necessary to secure the fire line in lowland grasses where deep fires burn in layers of forest fuels and organic soils. Fires that smolder may cross non-mineral soil breaks and reignite on the other side.
• Water from the air (aircraft or rainfall) will do little other than slow the forward spread of fires.
• Existing build-up, hot and dry conditions, and an extended amount of summer remaining will bring a very high to extreme risk of large catastrophic fires to the advisory area.
• Extreme fire behavior, common under record-setting conditions, will occur where fires, fuels, and weather elements (namely wind) align to create the worst conditions.