The potential for wildfires, forest and grass fires, depends on several weather elements acting together, over both short (hourly/daily) and long time scales (weeks/months) particularly those elements which influence the moisture content of potential fire fuels.
The prime weather factors affecting soil and fuel moisture are:
Precipitation (snow and rain)
Solar Radiation
Snow Melt
Air Temperature
Dew Formation
Relative Humidity
Fog
Wind
Other important fire factors are:
Thunderstorm Activity
Vegetation Type
Nature of Fuels
Amount of Fuels
Topography
Fog
Three types of fuel are:
Ground Fuels
All combustible materials lying beneath the ground surface including deep duff, roots, rotten buried logs, peat and other woody fuels.
Surface Fuels
All materials lying on, or immediately above the ground, including needles or leaves, duff, grass, small dead wood, downed logs, stumps, large limbs, low brush and reproduction.
Aerial Fuels
All live and dead vegetation located in the forest canopy or above the surface fuels, including tree branches and crowns, snags, moss, and high brush.
Each of these fuel types react over a number of time scales — from days to years — to changing weather conditions altering their moisture content.
Duff
A mat of partially decomposed organic matter immediately above the mineral soil, consisting primarily of fallen foliage, herbaceous vegetation and decaying wood.
Fine Fuels
Fuels such as grass, leaves, draped pine needles, fern, tree moss and some kinds of slash which, when dry, ignite readily and are consumed rapidly. Also called flash fuels.
As we know, most areas eventually experience periods when the moisture levels of the fuels fall to critical levels. Because of climatic studies of the conditions under which fires begin and how fiercely they will burn can be estimated for present conditions and future conditions given long-range weather forecasts.
Both Canada and the US have developed a series of Forest Fire Weather Indices for fire potential and monitor conditions across their vast forests using manned and remote observation networks.
Here is a block diagram which shows how the Canadian Fire Weather Index is determined.
First, readings of weather conditions are collected, usually at noon each day during the season.
From these data and the date, three sub-indices are calculated:
Fine Fuel Moisture Code (FFMC)
Represents the moisture content of litter and other cured fine fuels in a fores stand with a dry weight of about 0.05 lb/ft2
Duff Moisture Code (DMC)
Represents the moisture content of loosely compacted, decomposing organic material 5-10 cm (2-4 inches) deep and weighing about 1 lb/ft2 when dry.
Drought Code (DC)
Represents the moisture content of a deep layer of compact organic matter weighing about 10 lb/ft2 when dry.
Each of these sub-indices is a complex function of environmental conditions. If recent precipitation is to alter any of these codes, a minimum amount must fall in order to affect the code. The threshold values are:
FFMC
0.6 mm
DMC
1.5 mm
DC
2.9 mm
These are then used to calculate:
Initial Spread Index (ISI)
Combination of wind speed and FFMC that represents rate of spread alone without influence of variable quantities of fuel;
Buildup Index (BUI)
Combination of DMC and DC that represents the total amount of fuel available to the spreading fire
Fire Weather Index (FWI)
Combination of ISI and BUI that represents the intensity of the spreading fire as energy output rate per unit length of the fire front which can be converted into a unitless index scale and a Danger Class.
While the prime use of the Fire Weather Index is to assess current conditions, it can be used with historical data and long-range forecasts to assess potential danger areas, resource allocation and public information programs.
Written by Keith C. Heidorn, PhD, THE WEATHER DOCTOR,
July 1, 2004
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