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Fire Behavior Essay

Fire has been associated with mankind for a very long time. There was probably a time when mankind had no fire, and later became acquainted with fire, derived naturally. Man then learnt to preserve fire so as to be able to use it whenever required (Ed Semmelroth, 1998). Today man has come a long way in not only being able to generate fire, but also vary its properties according to his needs and requirements. Today, man has successfully learned to harness this fire power because of his better understanding of the properties of fire. The qualities of fire are various, ignited by different factors and sustained by many factors.

A fire which keeps spreading can be examined and estimated using various parameters. Generally people including fire fighters have less understanding on the spread of fire and concentrate only the closest fire front or that which look dangerous. However fire can be more effectively tackled when the science behind it is better understood. The attributes of a fire like the direction of spread, its strength etc. are determined by fire behavior. There are several types of fire based on their strength and spread. For instance, firestorms are high strength or high intensity fires, spreading rapidly.

Fires of low strength and rapid spread are referred to as flashy fires while fires with low spread and low intensity are called creep fires (Marco Morais, 2001). The behavior of fires depends on several factors like fuel, weather and topography. A change in any of these factors would cause an appropriate and immediate change in the behavior of fire. The fire behavior is also affected by the interaction of these factors. Fuel is an important determinant of fire behavior and most fuels don’t change rapidly. The behavior of fire is directly related to the amount of fuel present.

Lower quantities of fuel result in low strength, low spreading fires, like creep fires. Fuel in larger quantities could produce heavy fires that are most likely to get out of control. When the quantity of fuel is more, the intensity of fire is more and more heat is generated. The total quantity of fuel present in a given area is determined by fuel loading, which is the amount of fuel present in a given area that can burn under high or intense conditions. The fuel loading factor is almost constant and require a long period of time, like a year, to change.

On the other hand, the available fuel is the quantity of fuel that can burn at any particular time, under the existing circumstances relevant to that point of time. The size of fuel is important in deciding its ignition time. Smaller fuel can be ignited more rapidly than larger fuels. This is because the time required to heat and ignite fuel is directly proportional to the ratio of surface area to volume. The presence of small sized fuels is also essential for the spread of fire and constitute the fuel bed; and help in igniting larger fuels. The burnout periods of the fuel are determined by their diameter.

Fuels with a diameter of less than 0. 25 inches are classified as one hour fuel while fuels with a diameter of 1 to 3 inches are referred to as 100 hour fuel. Dry leaves, twigs and sticks are examples of small fuels while logs and big stumps are large fuels (PFMT). The behavior of fire is also influenced by the arrangement of fuels. Fuels that are more loosely arranged would quickly ignite and burn more rapidly than compactly arranged fuels, due to the availability of oxygen. There are several fuel models which are based on the main cause of fire propagation, like grass, shrub, litter, logging slash etc.

The fire behavior can be easily assessed when a single fuel model corresponds to a particular area. By observing the fires and analyzing their behavior, it is possible to assign a fuel model based on its characteristics (NIFC, 2006). Each model has distinct qualities and sometimes the fire may be attributed to the interaction of several models. The behavior of fire is also dependent on the amount of moisture content in the fuel. Combustion is slow in moist fuels as the heat is utilized in converting moisture to steam. The fuel can reach ignition temperature and undergo combustion only when the entire moisture is removed.

The behavior of fire is also dependent on the shape of the fuels present. Fuels having a flat shape have a higher surface-volume ratio and therefore burn more rapidly. Weather is another important factor of behavior determination. Winds of high speed, low humidity and absence of rains contribute to the acceleration of fire. High temperature and hot air can reduce the moisture content present in the fuel, contributing to the intensity and spread of fire. Wind is a source of oxygen required for fire sustenance and winds of higher speed provide more oxygen and thus more intensity fire.

It should be noted here that when the wind speed is doubled, the rate of fire spread is quadrupled. Winds can also carry with them small burning fuels which can start fresh fires on their own. During periods of drought, the shrubs and grass are dry and serve as an ideal fuel for fire. Under such intense dry conditions, even logs and greener leaves get dried quickly and become inflammable. Therefore, rains have a dampening effect on the fuels and suppress fires. (Australian Bureau of Metrology, 2008) The weather changes are rapid, changing even on an hourly basis, causing appropriate change in fire behavior.

Topography or the nature of the earth’s surface is an important determinant of fire behavior. The characteristics of a region are unique and is the resultant of existing natural and man made features. The topography of a given region doesn’t change much. In the mountainous regions, topographic factors contributing to fire behavior are crucial while most of these factors are not relevant to fire behavior in the coastal plains. These factors include slope, aspect and barriers. Fire is spread more rapidly up slope than down slope as fuels are preheated by the approaching flames which are closer to them on the uphill side than the downhill side.

Hot air from fire move upward drying up fuels which facilitate ignition and burning. Fire spread is doubled when a given slope is raise by 10% (Tropical Savannas CRC, 2008). The direction of the slope favorable for fire is indicated by the aspect factor, which determines the amount of solar radiation received by the slope face. South to southwest facing slopes receive more solar radiation than slopes that face the northern direction. These slopes which receive more radiation would have less humidity and moisture, be drier and burn easily. The barriers to fire in a topography are the presence of roads, lakes, wet swamps etc.

Such barriers prevent or retard the advance of fire. Complex terrain can result in fire behavior being very erratic.

REFERENCES Marco Morais. What is fire behavior? (2001) [Electronic Version]. Downloaded on 28th May 2008 from http://www. physics. ucsb. edu/~complex/research/hfire/fbehave Private Forest Management Team Fuel’s effect on fire behavior [Electronic Version].

Downloaded on 28th May 2008 from http://www. pfmt. org/fire/fuels_effect. htm Tropical Savannas CRC (2008) Topography [Electronic Version]. Downloaded on 28th May 2008 from http://learnline. cdu.edu. au/wip/fire2/fundamentals/topography. html Australian Bureau of Metrology (2008) Weather and fire Topography [Electronic Version]. Downloaded on 28th May 2008 from http://www. bom. gov. au/inside/services_policy/fire_ag/bushfire/wandfire. htm National Interagency Fire Center (2006) NWCG Fireline Handbook [Electronic Version]. Downloaded on 28th May 2008 from http://www. nwcg. gov/pms/pubs/410-2/appendixB. pdf Ed Semmelroth (1998) A brief history of fire and its uses [Electronic Version]. Downloaded on 28th May 2008 from http://hearth. com/what/historyfire. html


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