1/16/2024 0 Comments Favorable winds goldfish standardThus, an upslope or upcanyon wind is actually headed up the slope or up the canyon. Here it is common to express the wind direction as the direction toward which the wind is headed. In mountain country, though, surface wind direction with respect to the topography is often more important in fire control and provides a better description of local winds than the compass direction. The method of describing the direction of both surface winds and winds aloft, by the direction from which the wind blows, is ordinarily very practical. Direction is also described in degrees of azimuth from north-a northeast wind is 45°, a south wind 180°, and a northwest wind 315°.Ī wind vane indicates wind direction by pointing into the wind-the direction from which the wind blows. Thus, a north wind blows from the north toward the south, a northeast wind from the northeast, and so on around the points of the compass. Wind direction is ordinarily expressed as the direction from which the wind blows. The direction can be determined visually or, with more elaborate instruments, it can be indicated on a dial or recorded on a chart. In fire weather, however, we should remember that winds can also have an appreciable vertical component which will influence fire behavior, particularly in mountainous topography.Īt weather stations making regular weather observations, surface wind direction is determined by a wind vane mounted on a mast and pointing into the wind. Ordinarily only the horizontal components of direction and speed are measured and reported, and this is adequate for most purposes. Wind direction and speed are usually measured and expressed quantitatively, while in field practice turbulence is ordinarily expressed in qualitative or relative terms. Its principal characteristics are its direction, speed, and gustiness or turbulence. Wind is air in motion relative to the earth's surface. Certainly all winds are produced by pressure gradients, but the distinction here is that the pressure gradients produced by local temperature differences are of such a small scale that they cannot be detected and diagnosed on ordinary synoptic-scale weather charts. In the next chapter, under the heading of convective winds, we will consider local winds produced by local temperature differences. They vary in speed and direction as the synoptic-scale Highs and Lows develop, move, and decay. In this chapter we will consider local winds that are produced by the broadscale pressure gradients which are shown on synoptic weather maps, but may be modified considerably by friction or other topographic effects. Why does it persist or change as it does? Is it related to the general circulation patterns, or is it produced or modified by local influences? We find that local winds may be related to both, and we will discuss them separately. In this chapter and the next we will investigate the local wind-the wind that the man on the ground can measure or feel. In the previous chapter we considered the large scale motions-the primary circulation resulting from the unequal heating of the equatorial and polar regions of the earth, and the secondary circulations around high- and low-pressure areas produced by unequal heating and cooling of land and water masses. Thus the fire control plan, in the case of wildfire, and the burning plan, in the case of prescribed fire, must be based largely on the expected winds. The direction of fire spread is determined mostly by the wind direction. It aids fire spread by carrying heat and burning embers to new fuels, and by bending the flames closer to the unburned fuels ahead of the fire. Once a fire is started, wind aids combustion by increasing the oxygen supply. Light winds aid certain firebrands in igniting a fire. It carries away moisture-laden air and hastens the drying of forest fuels. This accounts for much of their variability and is the reason why there is no substitute for an adequate understanding of local wind behavior. Winds, particularly near the earth's surface, are strongly affected by the shape of the topography and by local heating and cooling. Of the two, wind is the most variable and the least predictable. The two most important weather, or weather related, elements affecting wildland fire behavior are wind and fuel moisture.
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