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Chapter 6 GENERAL WINDS

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Chapter 6 GENERAL WINDS Chapter 6 GENERAL WINDS The two most important weather, or weather related, elements affecting wildland fire behavior are wind and fuel moisture. Of the two, wind is the most variable and the least predictable. Winds, particularly near the earth's surface, are strongly affected by the shape of the topography and by local heating and cooling. This accounts for much of their variability and is the reason why there is no substitute for an adequate understanding of local wind behavior. Wind affects wildfire in many ways. It carries away moisture-laden air and hastens the drying of forest fuels. Light winds aid certain firebrands in igniting a fire. Once a fire is started, wind aids combustion by increasing the oxygen supply. 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. The direction of fire spread is determined mostly by the wind direction. 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. GENERAL WINDS The atmosphere is in continuous motion. In weather, however, we should remember that winds the previous chapter we considered the large scale can also have an appreciable vertical motions-the primary circulation resulting from the component which will influence fire behavior, unequal heating of the equatorial and polar regions particularly in mountainous topography. of the earth, and the secondary circulations around high- and low-pressure areas produced by unequal At weather stations making regular weather heating and cooling of land and water masses. observations, surface wind direction is determined by a wind vane mounted on a mast and pointing In this chapter and the next we will investigate into the wind. The direction can be determined the local wind-the wind that the man on the ground visually or, with more elaborate instruments, it can can measure or feel. Why does it persist or change be indicated on a dial or recorded on a chart. as it does? Is it related to the general circulation patterns, or is it produced or modified by local Wind direction is ordinarily expressed as the influences? We find that local winds may be direction from which the wind blows. Thus, a related to both, and we will discuss them north wind blows from the north toward the south, a separately. northeast wind from the northeast, and so on around the points of the compass. Direction is also In this chapter we will consider local winds that described in degrees of azimuth from north-a are produced by the broadscale pressure gradients northeast wind is 45°, a south wind 180°, and a which are shown on synoptic weather maps, but northwest wind 315°. may be modified considerably by friction or other topographic effects. We will call these general winds. They vary in speed and direction as the synoptic-scale Highs and Lows develop, move, and decay. In the next chapter, under the heading of convective winds, we will consider local winds produced by local temperature differences. Cer- tainly 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. Wind is air in motion relative to the earth's surface. Its principal characteristics are its direction, speed, and gustiness or turbulence. Wind direction and speed are usually measured and expressed quantitatively, while in field practice turbulence is ordinarily expressed in qualitative or relative terms. Ordinarily only the horizontal components of direction and speed are measured and reported, and this is adequate for most A wind vane indicates wind direction by pointing into the wind-the direction from which the wind blows. purposes. In fire 86 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. 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. Here it is common to express the wind direction as the direction toward which the wind is headed. Thus, an upslope or upcanyon wind is actually headed up the slope or up the canyon. Wind is described as blowing along the slopes, through the passes, or across the ridges. Similarly, "offshore" or "onshore" are used to describe the directions toward which land and sea breezes are blowing. Surface wind speeds are measured with anemometers. Many types of anemometers are in use, but the most common is the cup anemometer. It indicates either the air speed at any given instant Horizontal wind speed is measured by the rate of rotation of a cup or the miles of air that pass the instrument in a anemoter. given time period. The latter gives an average wind for the selected time period. Normally, a 2-minute ascent rate is not constant because of vertical air average is used. The standard height at which currents. If a radiosonde unit (which transmits wind speed is measured is 20 feet above open temperature, moisture, and pressure data during ground. ascent) is added to the balloon, the height of the balloon at the time of each reading can be In the United States, wind speed is usually calculated fairly accurately, and the computed measured in miles per hour or knots (nautical miles winds are more accurate. per hour). One knot is 1.15 miles per hour. Weather Bureau and military weather agencies use The most refined of present systems has the knots for both surface and upper winds, while miles further addition of a self-tracking, radio per hour is still in common use in many other direction-finding unit that measures elevation and agencies and operations, including fire weather. azimuth angles, and slant range from the observing station to the balloon. This unit, known as a The direction and speed of winds aloft are rawinsonde, yields quite accurate upper-air determined most commonly by tracking an information. All of these methods furnish wind ascending, gas-filled balloon from the surface up soundings for meteorological use and through the atmosphere. interpretation. The simplest system employs a pilot balloon The speed and direction of upper winds are followed visually with a theodolite. If a constant sampled at regular intervals each day at selected rate of rise of the balloon is assumed, periodic weather stations across the continent. These readings of elevation and azimuth angles with the stations are often more than 100 miles apart. theodolite allow computation of average wind Although winds aloft tend to be more uniform than direction and speed between balloon positions. surface winds, there are exceptions. The wind Errors are introduced when the structure over an area some distance from a sampling station may differ considerably from that indicated by the nearest sounding. 87 MECHANICAL AND THERMAL TURBULENCE either mechanical or thermal in We learned in the previous chapter that friction with the earth's surface slows down the wind and results in changes of direction so that the surface wind blows at an angle across the isobars from high to low pressure. The amount of reduction in speed and change of direction depends upon the roughness of the earth's surface. It follows then that the effect of friction is least over smooth water and greatest over mountainous topography. The depth of the air layer through which the frictional force is effective also varies with the roughness of the surface; it is shallower over smooth surfaces and deeper over rough topography. The depth may also vary with the stability of the lower atmosphere. A low inversion will confine the frictional effect to a shallow surface layer, but a deep layer can be affected if the air is relatively unstable. These effects vary widely both with time and between localities. Usually the friction layer is considered to be The wind direction at surface stations may differ widely from the about 2,000 feet deep. The top of the friction layer windflow above the friction layer, as shown by this weather map. is the gradient wind level above which the windflow Surface wind direction is indicated on weather maps by a wind arrow flying with the wind. The number of barbs on the tail tends to parallel the isobars or Pressure-surface represent the wind speed. At the top of the friction layer the wind contours. blows parallel to the isobars, as shown by the large arrow. Surface winds often vary considerably in both speed and direction over short intervals of time. nature. At the surface, turbulence is commonly They tend to blow in a series of gusts and lulls with identified in terms of eddies, whirls, and gusts; aloft the direction fluctuating rapidly. This irregular air it is associated with "bumpy" flying. Surf ace friction produces mechanical turbu- motion is known as turbulence, which may be lence in the airflow. The flow of stable air Roughness creates mechanical turbulence, while surface heating causes thermal turbulence in the airflow. 88 near the surface is similar to the flow of water in a shows diurnal changes, and is most pronounced creekbed. At low speeds the currents of air tend to In the early afternoon when surface heating is at a follow the general contours of the landscape. But maximum and the air is unstable in the lower when the speed increases-as when a creek layers.
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