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Unformatted text preview: UNIT III--TOPOGRAPHY AND FIRE BEHAVIOR This unit is on topography and fire behavior. Carefully read the instructions on page 1 of your workbook. On page 2, you will find the unit objectives on which you will be tested at the end of this unit. Please study these objectives. Return to this text when you have finished. Predicting fire behavior is a difficult job because of the many variables in nature. Burning conditions, relating to weather and fuels, are constantly changing as a fire spreads over time and space. Although the terrain usually does not change over time, it can change considerably over space. See figure 1 on page 3. All of the topographic features illustrated here are important in predicting the behavior of fire in mountainous terrain. A common method used to depict these various land features is the topographic map. We will be using topographic maps in the unit exercises. If you are not familiar with these maps and the interpretation of features from contours, you should seek help in map reading. Generally we can say that topography affects the fire environment by altering the normal heat transfer processes and by modifying general weather patterns, thus producing localized weather conditions that influence the types of vegetation or fuels. These, in turn, result in micro-climates with localized moisture conditions. When we consider all of these ways together, we can state that topography directly or indirectly affects fire intensity and the direction and rate of spread of a fire. In rare cases of mass ignition or fire storms over which large areas of moderate to heavy fuels are consumed in short periods of time, topography will have the least influence on the fire. The objectives of this unit are in-tended to give you a better understanding of these direct and indirect affects on fire behavior. We have stated that topography modifies general weather and produces localized weather conditions. Under item A, on page 4, we want you to list the following mechanical effects of topography on weather: Friction layer modifies general winds, induces slope and valley winds, creates thermal belt conditions, produces orographic thunderstorms, and contributes to foehn or chinook winds. We'll discuss each of these briefly in this unit, but they will be covered in more detail in later units on weather. Figure 2 illustrates how general winds coming in contact with the irregular surface of the terrain are modified and help to produce local surface winds. Under item B, please list the following ways general winds are modified: Channeling and changing wind directions, increasing wind velocities through constricted areas, producing eddies on lee sides of mountains, and mixing with locally induced winds. Although these processes are not thought to occur in flat, level terrain, we find that even certain patterns of vegetative cover and structures can produce the same effects on a smaller scale. cover and structures can produce the same effects on a smaller scale....
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- Spring '04