Now do question 4 on page 18 mark your choice or

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Now do question 4 on page 18; mark your choice or choices. In question 4, you should have marked statements 2 and 3. In number 1, winds immediately after the frontal passage will usually be from the west. In number 4, winds immediately ahead of the front are usually from the southwest. It's -very important that you remember the winds associated with a cold front. This understanding could save your life on a fire some day. Another kind of wind that causes firefighters great concern is the foehn wind. Foehn winds represent a special type of local wind associated with mountain range systems. They occur as heavy, stable air pushes over a mountain range and then descends the slopes on the leeward side as warmer, drier air. Figure 17 on page 18 gives examples of two commonly known foehn winds. The two foehn winds shown are blowing in opposite directions, but they have similar origins. The chinook occurs on the east slopes of several large mountain ranges in the Western United States. Chinook winds are most prevalent on the east side of the Rocky Mountains during fall and winter. In the case of the chinook wind, air pushed up on the windward side is cooled adiabatically to the point that clouds and precipitation may occur. As that air passes over the mountains and descends on the lee side, it is warmed adiabatically at 5-1/2 degrees per 1,000 feet of fall. It also gains velocity as it passes through the constricted topography and accelerates as it flows downslope. The resulting foehn or gravity winds on the lee side of mountain ranges can be warm and dry, with moderate to high velocities. The Santa Ana creates the most critical fire weather situations in areas of Southern California during fall and winter. Foehn winds are produced when the large-scale circulation is sufficiently strong and deep to force air completely across a major mountain range in a short period of time. In many cases, there is a large stationary cell of high pressure over the land. Subsidence or heavy air lowering within the pressure cell may push up against the mountain range. This heavy or stable air speeds up as it flows through passes and saddles, then down the lee slopes by gravity and pressure gradients. On page 19, please do question 5. The true statements in question 5 are 1, 3, and 4. In number 2, foehn winds are associated with areas of high pressure, but their winds are flowing toward lower pressure. Figure 18 on page 19 shows the more common and better known foehn winds in the Western United States. All shown can cause serious fire control problems, as these winds often reach 25 to 50 miles per hour. Some have been measured in excess of 100 miles per hour.
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If you know of foehn or local gravity wind conditions existing in your locality, you should talk to a meteorologist or some other knowledgeable individual about them. There are likely to be peculiarities for each area, and knowing these can help you in recognizing or anticipating the effects of foehn winds.
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  • Spring '04
  • MIchealJenkins
  • weather forecasts, windspeed, midflame windspeed

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