# The illustrations of slope and valley winds to this

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The illustrations of slope and valley winds to this point might suggest that upslope and upvalley winds occur on all slopes at the same time. This is not usually the case. For one example, see figure 14 on page 13. Let's suppose we have a ridge line and canyon parallel to each other running north and south. In the morning, the east aspects will be heated by the sun, but the west aspects are shaded. Upslope winds can occur on the east slopes, while downslope winds occur on the west slopes. As the sun passes overhead and into the afternoon positions, the west slopes become heated and the east slopes become shaded. The slope winds can reverse from those of the morning. Now do question. 3 on page 13; mark your choice or choices. In question 3, you should have marked statements 1, 2, and 4. If you have problems understanding the diurnal wind processes in mountainous terrain, we suggest that you go to one of several weather references listed for this course, all of which give more details and background. Local slope winds are often influenced and modified by the general winds. See figure 15 on page 14. It gives three examples of how local winds and general winds may predominate and produce the resultant surface winds. In the upper example, the general wind at 1000 feet is west at 10 miles per hour. As this wind drops closer to the surface, its speed is reduced to 7 miles per hour by frictional drag. We will refer to this windspeed as the general wind component. The upslope wind or the local wind component is 5 miles per hour. We can add the two components together to arrive at the surface wind speed of 12 miles per hour. In the middle example, the general wind is blowing in the opposite direction and opposing the east slope wind. At the anemometer, the general wind component is stronger than the local slope wind component. The surface wind at that point would probably be the difference between the two opposing windspeeds, or a west wind at 8 miles per hour.

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The lower example shows a nighttime situation where an inversion layer has developed in the valley. Here the general wind is confined to levels above the inversion by the stable air and therefore affects surface winds only at higher elevations. The surface wind at the anemometer will be the same as the downslope wind component. From these examples, you can see how surface winds are dependent on time of day, position on slope, and the strength and direction of the various wind components. Move on to page 15 and do exercise 1 on slope and valley winds. It will refer you to the topographic map on page 16. Please do this exercise; then check your answers and return to the text. The wind conditions that we have covered so far may, or may not, be considered problem winds to the firefighter. These are normal, everyday winds with which the firefighter must deal.
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• Spring '04
• MIchealJenkins
• weather forecasts, windspeed, midflame windspeed

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