The main factors determining fire behaviour
Серяков П. С., Овчинникова И. С., Кобзева Н. А. The main factors determining fire behaviour // Молодой ученый. 2015. №11. С. 558-560.
The paper is devoted to the main factors determining fire behaviour: fuel, topography, weather.
Key words: fuel, topography, weather, fire behavior, heavy forest.
Fire is a fundamental aspect of the environment. Fires occur in enterprises, in hospitals, hotels, department stores, homes, trains, ships, planes. The fires destroy the museums and libraries, churches and cultural monuments, theaters and palaces. Fires resulting from accidents and disasters in industrial plants, highways, elevators and multistory buildings have the most serious consequences.
The total number of fires on the planet has reached 6.5 million a year: every 5 seconds somewhere fire breaks out. For example, in 2013, public fire departments responded to 1,240,000 fires in the United States .
The aim of this paper is to consider the main factors determining fire on the basis of authentic Internet resources, which may be used in the process of engineering students’ English Foreign Language learning.
There are three main factors determining fire behaviour and affecting fire hazard:
- fuel (material such as coal, gas, or oil that is burned to produce heat or power);
- topography (the arrangement of the natural and artificial physical features of an area);
- weather (the state of the atmosphere at a place and time as regards heat, cloudiness, dryness, sunshine, wind, rain, etc.) .
The aim of this paper is to consider each of these factors.
The hazardous fuels are related to fire behavior characteristics and topographic influences such as slope, aspect and elevation.
The potential for loss of valuable assets to fire is strongly related to its surrounding fuels. The assessment tool can provide the ability to compare asset location to landscape fuel hazards and fire risk factors .
Forest fires are strictly related to land use and vegetation characteristics of the area where ignition can occur. Forest fires are fuelled by vegetation. How hot the fire becomes or how fast it spreads depends on the vegetation or fuels: the amount, type, condition and arrangement.
For example, long dry grass, twigs and leaves will burn very quickly; while heavy forest and scrub will burn slowly, but at a much higher temperature and at greater intensity.
In Tomsk region heavy fuels are branches, trees and logs can burn for hours or days after the bushfire front has passed.
How different fuels burn?
- Grasses respond rapidly to changing air humidity. Very dry grass absorbs humidity from damp air overnight; however, humidity is lost to wind and dry air very early on high fire risk days. Grass fires can spread very rapidly.
- Scrub vegetation and trees drop leaves and twigs (fine fuel) on the ground around them. These fuels can accumulate in large quantities. This fuel burns slower than grasses, but it gives off far more heat.
- When the bark on trees is fibrous and dry, flames from a surface fire can pre-heat and ignite the bark. This helps a fire climb higher up the tree, adding to both the height of the flames and the heat of the fire.
- When shrubs, branches and bark provide a continuous ladder of fuel up into the tree canopy, a bushfire can burn high in the trees and give off very large amounts of heat. This is called a crown fire .
Topography affects both the intensity and spread of wildfires. Wildfires exhibit different types of fire behavior, depending on the degree/percent of slope, the slope’s aspect, and in some cases, the elevation of where the fire is burning. Slope and aspect also acts to partially determine the fuel/vegetation variety and loading, as well as having a significant affect on fuel temperature .
The shape of the land has a strong effect on fire behaviour. A fire will burn faster uphill because the flames can reach more unburnt fuel in front of the fire. The heat radiating from the fire pre-heats fuel on the slope ahead of the fire, causing the fuel to start burning more quickly.
As a general rule, the fire will increase its speed as it spreads uphill by four times for a 20 degree slope. The opposite applies to a fire travelling downhill: because the flames reach less fuel, there is less radiant heat to pre-heat the fuel ahead of the fire, so the fire spreads slower.
Weather and climate have a profound influence on wildland fire ignition potential, fire behavior, and fire severity. Local weather and climate are affected by large-scale patterns of winds over the hemispheres that predispose wildland fuels to fire. The characteristics of wildland fuels, especially the humidity content, ultimately determine fire behavior and the impact of fire on the landscape. The physical processes related to combustion, fire, and plume behavior are largely affected by both daily weather and long-term climate .
Effect of humidity on bushfires.
Humidity is the amount of water vapour in the air:
- low humidity means the air is very dry:
- when humidity decreases to less than 30 per cent the fire danger increases;
- low humidity evaporates moisture from vegetation and flammable materials, making them easier to ignite.
Effect of wind on bushfires.
Strong winds are normally present during bushfires, which makes it harder for firefighters to bring the fire under control. The wind pushes flames closer to unburnt fuel and causes the fire to travel quicker.
Wind influences the:
- speed at which a fire spreads, he higher the wind speed, the greater the fire danger;
- direction in which a fire travels and the size of the fire front, a change in wind direction will rapidly change the fire front and fire direction;
- intensity of a fire by providing more oxygen;
- likelihood of spotting, burning pieces of leaves, twigs and bark (embers) are carried ahead of the fire by winds, causing spot fires to ignite .
It is easy to recognize days when fuels are at their driest. This is more common in summer, on hot and windy days. The strong winds dry out the bush and fire can start. On a typical summer day the air may contain very little humidity. This means that vegetation cannot absorb much humidity from the air. When the air is dry, the bush or grasslands are also dry from very early in the day, adding to the fire danger.
Hot and dry weather is causing a sharp increase in the number of forest fires in the Tomsk region. The air temperature has the greatest influence on the fire hazard with other meteorological factors, such as low humidity and high wind speed.
Fire danger in the forests of Tomsk region is determined, above all, the natural features of the territory: a significant share of coniferous forests (53.7 %) developed burn ground cover and hot dry summers, especially in the first half .
Thereby, there are three main factors determining fire behaviour: fuel, topography and weather, that take place in the Tomsk region. They may be learned and discussed by engineering students within English Foreign Language classes.
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