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  3. Wildfires In just seconds, a spark or even the sun’s heat alone sets off an extremely large fire. The wildfire quickly sprea

Wildfires In just seconds, a spark or even the sun’s heat alone sets off an extremely large fire. The wildfire quickly sprea

admin2009-04-23  30
问题                                            Wildfires
    In just seconds, a spark or even the sun’s heat alone sets off an extremely large fire. The wildfire quickly spreads, consuming the thick, dried-out plants and almost everything else in its path. What was once a forest becomes a virtual powder keg of untapped fuel. In a seemingly instantaneous burst, the wildfire overtakes thousands of acres of surrounding land, threatening the homes and lives of many in the vicinity.
Fire Starters
    On a hot summer day, when drought conditions peak, something as small as a spark from a train car’s wheel striking the track can ignite a raging wildfire. Sometimes, fires occur naturally, ignited by heat from the sun or a lightning strike. However, the majority of Wildfires are the result of human carelessness.
    Common causes for wildfires include:
    Arson
    Campfires
    Discarding lit cigarettes
    Improperly burning debris
    Playing with matches or fireworks
    Prescribed fires
    Everything has a temperature at which it will burst into flames. This temperature is called a material’s flash point. Wood’s flash point is 572 degrees Fahrenheit(300 ℃). When wood is heated to this temperature, it releases hydrocarbon gases that mix with oxygen in the air, combust and create fire.
    There are three components needed for ignition and combustion to occur. A fire requires fuel to burn, air to supply oxygen, and a heat source to bring the fuel Up to ignition temperature. Heat, oxygen and fuel form the fire triangle. Firefighters often talk about the fire triangle when they are trying to put out a blaze. The idea is that if they can take away any one of the pillars of the triangle, they can control and ultimately extinguish the fire.
    After combustion occurs and a fire begins to burn, there are several factors that determine how the fire spreads. These three factors include fuel, weather and topography. Depending on these factors, a fire can quickly fizzle or turn into a raging blaze that scorches thousands of acres.
Fueling the Flames
    Wildfires spread based on the type and quantity of fuel that surrounds it. Fuel can include everything from trees, underbrush and dry grassy fields to homes. The amount of flammable material that surrounds a fire is referred to as the fuel load. Fuel load is measured by the amount of available fuel per unit area, usually tons per acre.
    A small fuel load will cause a fire to burn and spread slowly, with a low intensity. If there is a lot of fuel, the fire will burn more intensely, causing it to spread faster. The faster it heats up the material around it, the faster those materials can ignite. The dryness of the fuel can also affect the behavior of the fire. When the fuel is very dry, it is consumed much faster and creates a fire that is much more difficult to contain.
    Here are the basic fuel characteristics that decide how it affects a fire:
    Size and shape
    Arrangement
    Moisture content
    Small fuel materials, also called flashy fuels, such as dry grass, pine needles, dry leaves, twigs and other dead brush, burn faster than large logs or stumps(this is why you start a fire with kindling rather than logs). On a chemical level, different fuel materials take longer to ignite than others. But in a wildfire, where most of the fuel is made of the same sort of material, the main variable in ignition time is the ratio of the fuel’s total surface area to its volume. Since a twig’s surface area is much larger than its volume, it ignites quickly. By comparison, a tree’s surface area is much smaller than its volume, so it needs more time to heat up before it ignites.
    As the fire progresses, it dries out the material just beyond it—heat and smoke approaching potential fuel causes the fuel’s moisture to evaporate. This makes the fuel easier to ignite when the fire finally reaches it. Fuels that are somewhat spaced out will also dry out faster than fuels that are packed tightly together, because more oxygen is available to the thinned-out fuel. More tightly-packed fuels also retain more moisture, which absorbs the fire’s heat.
Wind and Rain
    Weather plays a major role in the birth, growth and death of a wildfire. Drought leads to extremely favorable conditions for wildfires, and winds aid a wildfire’s progress—weather can spur the fire to move faster and engulf more land. It can also make the job of fighting the fire even more difficult. There are three weather ingredients that can affect wildfires:
    Temperature
    Wind
    Moisture
    As mentioned before, temperature has a direct effect on the sparking of wildfires, because heat is one of the three pillars of the fire triangle. The sticks, trees and underbrush on the ground receive radiant heat from the sun, which heats and dries potential fuels. Warmer temperatures allow for fuels to ignite and burn faster, adding to the rate at which a wildfire spreads. For this reason, wildfires tend to rage in the afternoon, when temperatures are at their hottest.
    Wind probably has the biggest impact on a wildfire’s behavior. It’s also the most unpredictable factor. Winds supply the fire with additional oxygen, further dry potential fuel and push the fire across the land at a faster rate.
    The stronger the wind blows, the faster the fire spreads. The fire generates winds of its own that are as many as 10 times faster than the wind of surrounding area. It can even throw embers into the air and create additional fires, an occurrence called spotting. Wind can also change the direction of the fire, and gusts can raise the fire into the trees, creating a crown fire.
    While wind can help the fire to spread, moisture works against the fire. Moisture, in the form of humidity and precipitation, can slow the fire down and reduce its intensity. Potential fuels can be hard to ignite if they have high levels of moisture, because the moisture absorbs the fire’s heat. When the humidity is low, meaning that there is a low amount of water vapor in the air, wildfires are more likely to start. The higher the humidity, the less likely the fuel is to dry and ignite.
    Since moisture can lower the chances of a wildfire igniting, precipitation has a direct impact on fire prevention, when the air becomes saturated with moisture, it releases the moisture in the form of rain. Rain and other precipitation raise the amount of moisture in fuels, which suppresses any potential wildfires from breaking out.
Fire on the Mountain
    The third big influence on wildfire behavior is the lay of the land, or topography. Although it remains virtually unchanged, unlike fuel and weather, topography can either aid or hinder wildfire progression. The most important factor in topography as it relates to wildfire is slope.
    Unlike humans, fires usually travel uphill much faster than downhill. The steeper the slope, the faster the fire travels. Fires travel in the direction of the ambient wind, which usually flows uphill. Additionally, the fire is able to preheat the fuel further up the hill because the smoke and heat are rising in that direction. Conversely, once the fire has reached the top of a hill, it must struggle to come back down because it is not able to preheat the downhill fuel as well as the uphill.
    In addition to the damage that fires cause as they burn, they can also leave behind disastrous problems, the effects of which might not be felt for months after the fire burns out. When fires destroy all the vegetation on a hill or mountain, it can also weaken the organic material it the soil and prevent water from penetrating the soil. One problem that results from this is extremely dangerous erosion that can lead to debris flows.
    While we often look at wildfires as being destructive, many wildfires are actually beneficial. Some wildfires burn the underbrush of a forest, which can prevent a larger fire that might result if the brush were allowed to accumulate for a long time. Wildfires can also benefit plant growth by reducing disease spread, releasing nutrients from burned plants into the ground and encouraging new growth.
选项 A、Y
B、N
C、NG
答案A
解析 首先看文章标题Wildfires，再纵览各个小标题：Fire Starters(火引子)、Fueling the Flames(给火焰加燃料)Wind and Rain(风和雨)、Fire on the Mountain(山上的火灾)可知，本文主要讲的就是野火如何产生和持续的，故本题叙述是符合原文的。
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