Meteorology Today: An Introduction to Weather, Climate, and the Environment (2022) PDF

9 Wind: Small-Scale and Local Chapter Systems L e a r ning O b j e c t i v e s At the end of this section, you should be able to: LO1 List the four main scales of atmospheric...

9 Wind: Small-Scale and Local Chapter Systems L e a r ning O b j e c t i v e s At the end of this section, you should be able to: LO1 List the four main scales of atmospheric motion and at least one weather phenomenon associated with each scale. LO2 Describe the conditions that lead to mechanical turbulence and thermal turbulence. LO3 Explain how strong winds blowing over an obstruction can produce a wind from the opposite direction near the surface downstream. LO4 Identify the effects of wind on sand, soil, snow, vegetation, and water. LO5 Illustrate the mechanisms of sea, land, valley, and mountain breezes. LO6 Describe three types of strong downslope wind and the hazards they can pose. LO7 Describe the instruments used to measure wind direction and/or wind speed at the ground and aloft. ON DECEMBER 28, 1997, A UNITED AIRLINES BOEING 747 carrying 374 passengers was over the Pacific Ocean en route to Hawaii from Japan. About two hours into the flight, the aircraft was at a cruising altitude of 31,000 feet when suddenly, east of Tokyo, this routine, uneventful flight turned harrowing. Seat-belt signs were turned on because of reports of severe air turbulence nearby. The plane hurtled upward, then quickly dropped by about 30 meters (100 feet) before stabilizing. Terrified, screaming passengers not fastened to their seats were flung against the walls of the aircraft, then dropped. Bags, serving trays, and luggage that slipped out from under the seats were tossed about inside the plane. Within seconds, the entire ordeal was over. A total of 160 people were injured. Tragically, there was one fatality: A 32-year-old woman, who had been hurled against the ceiling of the plane, died of severe head injuries. What sort of atmospheric phenomenon could cause such turbulence? 229 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. T he aircraft in our opening vignette on p. 229 encountered a the smoke, small chaotic motions—tiny eddies—cause it to turbulent eddy—an “air pocket”—in perfectly clear air. Such tumble and turn. These eddies constitute the smallest scale of violent eddies are not uncommon, especially in the vicinity of atmospheric motion—the microscale. At the microscale level, jet streams. In this chapter, we will examine a variety of eddy cir- eddies with diameters of a few meters or less not only disperse culations. First, we will look at the different scales of motion found smoke, but they also sway branches and swirl dust and papers within our atmosphere, then we will see how eddies form and how into the air. They form by convection or by the wind blowing eddies and other small-scale winds interact with our environment. past obstructions and are usually short-lived, lasting only a few Next, we will examine slightly larger circulations—local winds— minutes at best. such as the sea breeze and the chinook, describing how they form In Fig. 9.1b observe that, as the smoke rises, it drifts toward and the type of weather they generally bring. In Chapter 10, we will the center of town. Here the smoke rises even higher and is examine winds and circulations as they unfold on the global scale. carried many kilometers downwind. This circulation of city The air in motion—what we commonly call wind—is a air constitutes the next larger scale—the mesoscale (meaning powerful phenomenon. It is invisible, yet we see evidence of it middle scale). Typical mesoscale circulations range from a few nearly everywhere we look. It sculpts rocks, moves leaves, blows kilometers to about a hundred kilometers in diameter. Gen- smoke, and lifts water vapor upward to where it can condense erally, they last longer than microscale motions, often many into clouds. The wind is with us wherever we go. On a hot day, minutes, hours, or in some cases, as long as a day. Mesoscale it can cool us off; on a cold day, it can make us shiver. A breeze circulations include local winds (which form along shorelines can sharpen our appetite when it blows the aroma from the and mountains), as well as thunderstorms, tornadoes, and local bakery or a food truck in our direction. Wind is the work- small tropical cyclones. horse of weather, moving storms and large fair-weather systems When we look at the smokestack on a surface weather map around the globe. It transports heat, moisture, dust, insects, (see Fig. 9.1c), neither the smokestack nor the circulation of bacteria, and pollen from one area to another. city air shows up. All that we see are the circulations around Circulations of all sizes exist within the atmosphere. Little high- and low-pressure areas—the cyclone and anticyclones whirls form inside bigger whirls, which are encompassed by of the middle latitudes, as well as the large tropical cyclones of even larger whirls—one huge mass of turbulent, twisting eddies.* lower latitudes. We are now looking at the synoptic scale, or For clarity, meteorologists arrange circulations according to weather-map scale. Circulations of this magnitude dominate their size. This hierarchy of motion from tiny gusts to giant regions of hundreds to even thousands of square kilometers and, storms is called the scales of motion. although the life spans of these features vary, they typically last for days and sometimes weeks. These include large hurricanes and typhoons as well as the frequent mid-latitude storm systems 9.1 Scales of Atmospheric Motion that bring rain, snow, and wind. The largest wind patterns are seen at the global scale, or LO1 planetary scale. Here, we have wind patterns ranging over the Consider smoke rising from a chimney into the otherwise clean entire earth. Together, the synoptic and global scales are referred air in an industrial section of a large city (see Fig. 9.1a). Within to as the macroscale—the largest scale of atmospheric motion. Figure 9.2 summarizes the various scales of motion and their *Eddies are spinning globs of air that have a life history of their own. average life spans. Figure 9.1 Scales of atmospheric motion. The tiny microscale motions constitute a part of the larger mesoscale motions, which, in turn, are part of the much larger synoptic scale. Notice that as the scale becomes larger, motions observed at the smaller scale are no longer visible. 230 Chapter 9 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Figure 9.2 The scales of atmospheric motion with the phenomenon’s average size and life span. (Because the actual size of certain features can vary, some of the features fall into more than one category.) Having looked at the different scales of atmospheric motion, The friction of fluid flow is called viscosity. When the we turn our attention to see the effect that microscale winds can slowing of a fluid—such as air—is due to the random motion have on our environment. of the gas molecules, the viscosity is referred to as molecular viscosity. Consider a mass of air gliding horizontally and smoothly (laminar flow) over a stationary mass of air. Even though the 9.2 Small-Scale Winds Interacting molecules in the stationary air are not moving horizontally, they are darting about and colliding with each other. At the boundary with the Environment separating the air layers, there is a constant exchange of mol- LO2 LO3 LO4 ecules between the stationary air and flowing air. The overall effect of this molecular exchange is to slow down the moving We begin our discussion of microscale winds by examining the air. If molecular viscosity were the only type of friction acting on important topic of turbulent flow, called turbulence, which rep- moving air, the effect of friction would disappear in a thin layer resents any disturbed flow of air that produces wind gusts and just above the surface. There is, however, another frictional effect eddies. that is far more important in reducing wind speeds. When laminar flow gives way to irregular turbulent motion, there is an effect similar to molecular viscosity, but which occurs 9.2a Friction and Turbulence in the throughout a much larger portion of the moving air. The internal Boundary Layer friction produced by turbulent whirling eddies is called eddy vis- We are all familiar with friction. If we rub our hand over the cosity. Near the surface, it is related to the roughness of the ground. top of a table, friction tends to slow its movement because of As wind blows over a landscape dotted with trees and buildings, it irregularities in the table’s surface. On a microscopic level, fric- breaks into a series of irregular, twisting eddies that can influence tion arises as atoms and molecules of the two surfaces seem to the airflow for hundreds of meters above the surface. Within each adhere, then snap apart, as the hand slides over the table. Fric- eddy, the wind speed and direction fluctuate rapidly, producing the tion is not restricted to solid objects; it occurs in moving fluids irregular air motion known as wind gusts. Eddy motions created by as well. Consider, for example, a steady flow of water in a stream. obstructions are commonly referred to as mechanical turbulence. When a paddle is placed in the stream, turbulent whirls (eddies) Mechanical turbulence creates a drag on the flow of air, one far form behind it. These eddies create fluid friction by draining greater than that caused by molecular viscosity. energy from the main stream flow, slowing it down. Let’s exam- The frictional drag of the ground normally decreases as we ine the idea of fluid friction in more detail. move away from Earth’s surface. Because of the reduced friction, Wind: Small-Scale and Local Systems 231 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. wind speeds tend to increase with height above the ground. In of the eddy, producing a momentary gust of wind. Because of the fact, at a height of only 10 m (33 ft), the wind is often moving increased depth of circulating eddies in unstable air, strong, gusty twice as fast as at the surface. As we saw in Chapter 8, the atmo- surface winds are more likely to occur when the atmosphere is spheric layer near the surface that is influenced by friction (tur- unstable. Greater instability also leads to a greater exchange of bulence) is called the planetary (atmospheric) boundary layer, faster-moving air from upper levels with slower-moving air also referred to as the friction layer. The top of the boundary at lower levels. In general, this exchange increases the average layer is usually near 1000 m (3300 ft), but this height may vary wind speed near the surface and decreases it aloft, producing somewhat as both strong winds and rough terrain extend the the distribution of wind speed with height shown in Fig. 9.4. region of frictional influence. Above the boundary layer, winds At heights above the boundary layer (typically around 1000 m or are generally not subject to these friction and turbulence effects. 3300 ft), friction is no longer a major influence on wind speeds. Weather Watch 1200 On a blustery night, the howling of the wind can be caused by eddies. 1100 As the wind blows past chimneys and roof corners, small eddies form. Wind speed above 3500 These tiny swirls act like pulses of compressed air that ultimately atmospheric 1000 boundary layer reach your eardrum and produce the sound of howling winds. 900 3000 Surface heating and instability also cause turbulence to 800 2500 extend to greater altitudes. As Earth’s surface heats, thermals rise 700 Elevation (m) Elevation (ft) and convection cells form. The resulting vertical motion creates thermal turbulence, which increases with the intensity of sur- 600 2000 face heating and the degree of atmospheric instability. During the 500 (b) Unstable early morning, when the air is most stable, thermal turbulence is 1500 normally at a minimum. As surface heating increases, instabil- 400 ity is induced and thermal turbulence becomes more intense. If 300 1000 this heating produces convective clouds that rise to great heights, there may be turbulence from Earth’s surface to the base of the 200 500 stratosphere, more than 10 km (6.2 mi) above the ground. 100 (a) Stable Although we have treated thermal and mechanical turbu- 0 lence separately, they occur together in the atmosphere—each 0 magnifying the influence of the other. Let’s consider a simple Wind speed example: the eddy forming behind the barn in Fig. 9.3. In stable air with weak winds, the eddy is nonexistent or small. As wind Figure 9.4 When the air is stable and the terrain fairly smooth (a), vertical speed and surface heating increase, instability develops, and the mixing is at a minimum, and the effect of surface friction only extends upward a eddy becomes larger and extends through a greater depth. The relatively short distance above the surface. When the air is unstable and the terrain rough (b), vertical mixing is at a maximum, and the effect of surface friction extends rising side of the eddy carries slow-moving surface air upward, upward through a much greater depth of atmosphere. Within the region of frictional causing a frictional drag on the faster flow of air aloft. Some of influence, vertical mixing increases the wind speed near the ground and decreases it the faster-moving air is brought down with the descending part aloft. (Wind at the surface is normally measured at 10 m [33 ft] above ground level.) Figure 9.3 Winds flowing past an obstacle. (a) In stable air, light winds produce small eddies and little vertical mixing. (b) Greater winds in unstable air create deep, vertically mixing eddies that produce strong, gusty surface winds. 232 Chapter 9 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  We can now see why surface winds are usually stronger in the afternoon. Vertical mixing during the middle of the day links surface air with the faster-moving air aloft. The result is that the surface air is pulled along more quickly. At night, when convection is reduced, the interchange between the air at the surface and the air aloft is at a minimum. Hence, the wind near the ground is less affected by the faster wind flow above, and so it blows more slowly. In summary, the friction of airflow (viscosity) is a result of the exchange of air molecules moving at different speeds. The exchange brought about by random molecular motions (molecular viscosity) is quite small in comparison with the exchange brought about by turbulent motions (eddy viscosity). Therefore, the frictional effect of the surface on moving air depends largely on mechanical and thermal turbulent mixing. The depth of mixing and, hence, frictional influence (in the boundary layer) depend primarily on three factors: 1. surface heating—producing a steep lapse rate and strong thermal turbulence 2. strong wind speeds—producing strong mechanical turbulent motions 3. rough or hilly landscape—producing strong mechanical Jeff Schmaltz/MODIS/NASA-GSFC turbulence When these three factors occur simultaneously, the fric- tional effect of the ground is transferred upward to considerable heights, and the wind at the surface is typically strong and gusty. Figure 9.5 Satellite image of eddies forming on the leeward (downwind) side 9.2b Eddies—Big and Small of the Cape Verde Islands during April 2004. As trade winds blow from the northeast past When the wind encounters a solid object, a whirl of air—an the islands, the air breaks into a variety of swirls toward the southwest, as indicated by the eddy—forms on the object’s leeward side (see Fig. 9.5). The size cloud pattern. (The islands are situated in the Atlantic Ocean, off Africa’s western coast.) and shape of the eddy often depend on the size and shape of the obstacle and on the speed of the wind. Light winds produce small stationary eddies. Wind moving past trees, shrubs, and even your body produces small eddies. (You may have had the experience of dropping a piece of paper on a windy day only to have it car- ried away by a swirling eddy as you bend down to pick it up.) Air flowing over a building produces larger eddies that will, at best, be about the size of the building. Strong winds blowing past an open sports stadium can produce eddies that may rotate in such a way as to create surface winds on the playing field that move in a direction opposite to the wind flow above the stadium. Wind blowing over a fairly smooth surface produces few eddies, but when the surface is rough, many eddies form. The eddies that form downwind from obstacles can produce a variety of interesting effects. For instance, wind moving over a mountain range in a stable atmosphere with a speed greater than Figure 9.6 Under stable conditions, air flowing past a mountain range can create eddies many kilometers downwind of the mountain itself. 40 knots usually produces waves and eddies, such as those shown in Fig. 9.6. We can see that eddies form both close to the moun- tain and beneath each wave crest. These are called roll eddies, or greater than 1000 km (600 mi). Because it is these migrating sys- rotors, and have violent vertical motions that produce extreme tems that make our middle latitude weather so changeable, we turbulence and hazardous flying conditions. Strong winds blowing will examine the formation and movement of these systems in over a mountain in stable air sometimes provide a mountain wave Chapters 11 and 12. eddy on the downwind side, with a reverse flow near the ground. Turbulent eddies form aloft as well as near the s­ urface. The largest atmospheric eddies form as the flow of air ­Turbulence aloft can occur suddenly and unexpectedly, espe- becomes organized into huge spiraling whirls—the cyclones cially where the wind changes its speed or direction (or both) and anticyclones of middle latitudes—which can have diameters abruptly. Such a change is called wind shear. The shearing Wind: Small-Scale and Local Systems 233 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. creates forces that produce eddies along a mixing zone. If the around, up, and over it. When the barrier is long and low like a eddies form away from clouds or storms, this form of turbulence water wave, the slight updrafts created on the windward side sup- is called clear air turbulence (CAT). When an airplane is flying port the wings of birds, allowing them to skim the water in search through such turbulence, the bumpiness can range from small of food without having to flap their wings. Elongated hills and vibrations to violent up-and-down motions that force passen- cliffs that face into the wind create upward air motions that can gers against their seats and toss objects throughout the cabin. support a hang glider in the air for a long time. The cliffs in the (Additional information on CAT is given in Focus section 9.1.) Hawaiian Islands and along the California coast, with their steep escarpments, are especially fine areas for hang-gliding and para- Weather Watch gliding (see Fig. 9.8). Wind speeds greater than about 15 knots blowing over a smooth yet moderately sloping ridge may provide The British scientist Lewis Fry Richardson, who proposed the idea of excellent ridge-soaring for the sailplane enthusiast. computational weather prediction in the early 20th century, realized As stable air flows over a ridge, it increases in speed. the importance of turbulence and eddies in the atmosphere. He Thus, winds blowing over mountains tend to be stronger than composed this rhyme: “Big whirls have little whirls that feed on their winds blowing at the same level on either side. In fact, one velocity, and little whirls have lesser whirls and so on to viscosity.” of the greatest wind speeds ever recorded near the ground occurred at the summit of Mt. Washington, New Hampshire, 9.2c The Strong Force of the Wind elevation 1909 m (6262 ft), where the wind gusted to 201 knots The force of the wind on an object is proportional to the wind speed squared, which means that a small increase in wind speed can greatly increase the force of the wind acting on an object. So, strong winds may blow down trees, overturn mobile homes, and even move railroad cars. For example, in February 1965 the wind presented people in North Dakota with a “ghost train” as it pushed five railroad cars from Portal to Minot (about 125 km or 77 mi) without a locomotive. On May 2, 2009, while the Dallas Cowboys rookie football players were going through workouts at the indoor practice facilities near Dallas, Texas, a strong wind—estimated at over 60 knots—ripped the roof off the facility, injuring 12 people. And on March 8, 2017, more than a million people lost power when a vast swath of high wind uprooted trees and knocked down utility lines across the Great Lakes states, all under bright blue skies. Wind blowing with sufficient force to rip the roof off build- ings is uncommon. However, wind blowing with enough force to Figure 9.7 Strong winds flowing past an obstruction, such as these hills, can move an automobile is very common, especially when the auto- produce a reverse flow of air that strikes an object from the side opposite the general mobile is exposed to a strong crosswind. On a normal road, the wind direction. force of a crosswind is usually insufficient to move a car sideways because of the reduced wind flow near the ground. However, when the car crosses a high bridge, where the frictional influence of the ground is reduced, the increased wind speed can be felt by the driver. Near the top of a high bridge, where the wind flow is typically strongest, complex eddies pound against the car’s side as the air moves past obstructions, such as guard railings and posts. In a strong wind, these eddies may even break into extremely turbulent whirls that buffet the car, causing difficult handling as it moves from side to side. If there is a wall on the bridge, the wind may swirl around and strike the car from the side opposite the wind direction, producing hazardous driving conditions. A similar effect occurs where the wind moves over low hills paralleling a highway (see Fig. 9.7). When the vehicle moves by the obstruction, a wind gust from the opposite direction can iStock.com/Daniel Cardiff suddenly and without warning push it to the opposite side. This wind hazard is a special problem for trucks, campers, and trail- ers. Consequently, highway signs warning of gusty wind areas are often posted. Up to now we’ve seen that, when the wind meets a barrier, it Figure 9.8 With the prevailing wind blowing from off the ocean, the steep cliffs exerts a force upon it. If the barrier doesn’t move, the wind moves along the coast of Southern California promote rising air and good hang-gliding conditions. 234 Chapter 9 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Focus on an Observation 9.1 Eddies and “Air Pockets” To better understand how eddies form along As we learned earlier, when these huge eddies encountered a region of severe clear air a zone of wind shear, imagine that, high in the develop in clear air, this form of turbulence is turbulence and reportedly plunged about atmosphere, there is a stable layer of air having referred to as clear air turbulence (CAT). 600 m (2000 ft) toward Earth before stabilizing. vertical wind speed shear (changing wind speed The eddies that form in clear air may have Twenty-one of the 154 people aboard were with height) as depicted in Fig. 1a. The top diameters ranging from a couple of meters injured; one person sustained a fractured hip half of the layer slowly slides over the bottom to several hundred meters. An unsuspecting and another person, after hitting the ceiling, half, and the relative speed of both halves is aircraft entering such a region may be in for jabbed himself in the nose with a fork, then low. As long as the wind shear between the more than just a bumpy ride. If the aircraft landed in the seat in front of him.* Clear air top and bottom of the layer is small, few if any flies into a zone of descending air, it may drop turbulence has occasionally caused structural eddies form. However, if the shear and the suddenly, producing the sensation that there is damage to aircraft by breaking off vertical corresponding relative speed of these layers no air to support the wings. Consequently, these stabilizers and tail structures. Fortunately, the increases (Figs. 1b and 1c), wavelike undulations regions have come to be known as air pockets. effects are usually not this dramatic. may form. When the shearing exceeds a Commercial aircraft entering air pockets The potential adverse effects of clear certain value, the waves break into large swirls, have dropped hundreds of meters, injuring air turbulence is one important reason why with significant vertical movement (Fig. 1d). passengers and flight attendants not strapped passengers are frequently told to “fasten your Eddies such as these often form in the upper into their seats. Five passengers and crew seat belts” while flying, even when there are no troposphere near jet streams, where large wind members had to be hospitalized in February thunderstorms or obvious hazards in sight. speed shears exist. 2014 after severe turbulence struck a Boeing Turbulent eddies also occur in conjunction 737 jetliner as it approached Billings, Montana. *Another example of an aircraft that experienced with mountain waves, which may extend In April 1981, a DC-10 jetliner flying at severe turbulence as it flew into an air pocket is given upward into the stratosphere (see Fig. 2). 11,300 m (37,000 ft) over central Illinois in the opening vignette on p. 229. Figure 1 The formation of clear air turbulence (CAT) along a boundary of increasing wind speed shear. The wind in the top layer increases in speed from (a) through (d) as it flows over the bottom layer. Figure 2 Turbulent eddies forming downwind of a mountain chain in a wind shear zone produce these waves called Kelvin-Helmholtz waves. The visible clouds that form are called billow clouds. Brooks Martner—www.cloudphotos.net Wind: Small-Scale and Local Systems 235 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. (231 mi/hr) on April 12, 1934. A similar increase in wind speed occurs where air accelerates as it funnels through a narrow con- striction, such as a low pass or saddle in a mountain crest. Weather Watch A ski resort in California’s Sierra mountain range was blasted with winds that gusted as high as 199 mi/hr on February 20, 2017. The fierce winds occurred atop Ward Mountain (elevation 2634 m or 8643 ft) as a powerful jet stream plowed into the Sierra at a nearly perpendicular angle. The peak gust set a new wind-speed record for California. Lillac/Shutterstock.com 9.2d Wind and Soil Where the wind blows over exposed soil, it takes an active role in shaping the landscape. This is especially noticeable in deserts. Figure 9.10 The shape of this sand dune reveals that the wind was blowing Tiny, loose particles of sand, silt, and dust are lifted from the from left to right when it formed. Note also the shape of the sand ripples on the dune. surface and carried away by the wind, leaving the surface lower than it once was. These same winds may also help to move des- high enough, acts as an obstacle itself. If the wind speed is strong ert rocks across wet ground, as shown in Fig. 9.9. and continues to blow in the same direction for a sufficient time, Blowing sand eventually comes to rest behind obstacles, the sand piles up higher and eventually becomes a sand dune. which can be anything from a rock to a clump of vegetation. As On the dune’s surface, the sand rolls, slides, and gradually creeps the sand grains accumulate, they pile into a larger heap that, when along, producing wavelike patterns called sand ripples. Each ripple forms perpendicular to the wind direction, with a gentle slope on the upwind side and a steeper slope on the downwind side. (If the wind direction frequently changes, the ripple becomes more symmetric.) On a larger scale, the dune itself may take on a more symmetric shape. Sand is carried forward and up the dune until it reaches the top. Here, the airflow is strongest, and the sand con- tinues its forward movement and cascades down the backside of the dune into quieter air. The effect of this migration is to create a dune whose windward slope is more gentle than its leeward slope. Therefore, the shape of a sand dune reveals the prevailing wind direction during its formation (see Fig. 9.10). 9.2e Wind and Snow Wind blowing over a snow-covered landscape may also create wavelike patterns several centimeters high and oriented at right angles to the wind. These snow ripples are similar to sand ripples. On a larger scale, winds may create snow drifts and even snow dunes, which are quite similar to sand dunes. Irregularities at the surface can cause a strong wind (40 knots) to break into turbu- lent eddies. If the snow on the ground is moist and sticky, some of it may be picked up by the wind and sent rolling. As it rolls along, it collects more snow and grows bigger. If the wind is suf- ficiently strong, the moving clump of snow becomes cylindrical, Michael Melford/National Geographic Creative often with a hole extending through it lengthwise. These snow rollers range from the size of eggs to that of small barrels. The tracks they make in the snow are typically less than 1 centimeter deep and several meters long. Snow rollers are rare, but, when they occur, they create a striking winter scene (see Fig. 9.11). In populated areas, they may escape notice as they are often mis- taken as having been made by children rather than by nature. Strong winds blowing over a vast region of open plains can Figure 9.9 Winds may have helped to push a rock across a wet surface alter the landscape in a different way. Consider, for example, a at Death Valley National Park, California. light snowfall several centimeters deep covering a large portion 236 Chapter 9 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  © C. Donald Ahrens Figure 9.12 Snow drifts accumulating behind snow fences in Wyoming. 9.2f Wind and Vegetation Strong winds can have an effect on vegetation, too. Armed with sand, winds can damage or destroy tender new vegetation, decreasing crop productivity. Most plants increase their rate of transpiration as wind speed increases.* This leads to rapid water Tom Uhlman/Alamy Stock Photo loss, especially in warmer areas having low humidities, and may actually dry out plants. If sustained, this drying-out effect may stunt plant growth, and, in some windy, dry regions, mature trees that might otherwise be many meters tall grow only to the height of a small shrub. Wind-dried vegetation can result in an area of high fire danger. If a fire should begin here, any additional wind helps it Figure 9.11 Snow rollers—natural cylindrical rolls of snow—grow along, directing its movement, adding oxygen for combustion, larger as the wind blows them down a hillside. and carrying burning embers elsewhere to start new fires. On the open plains, where the wind blows practically unimpeded, of central South Dakota. After the snow stops falling, strong wind-whipped prairie fires can imperil homes and livestock as winds may whip it into the air, leaving fields barren of snow. The the fires burn out of control over large areas. cold, dry wind also robs the soil of any remaining moisture and Wind erosion is greatly reduced by a continuous cover of freezes it solid. Meanwhile, the snow settles out of the air when vegetation. The vegetation screens the surface from the direct the wind encounters obstacles. Because the greatest density of force of the wind and anchors the soil. Soil moisture also helps to such obstructions is normally in towns, municipal snowfall resist wind erosion by holding particles together. From this fact, measurements may show an accumulation of many centimeters, we can see that land where natural vegetation has been removed while the surrounding countryside, which may desperately need for farming purposes—followed by several years of drought— the snow, has practically none. is ripe for wind erosion. This situation happened in parts of To help remedy this situation, snow fences are constructed in the Great Plains in the 1930s, when winds carried millions of open spaces (see Fig. 9.12). Behind the snow fence, the wind tons of dust into the air, creating vast dust storms that buried speed is reduced because the air is broken into small eddies, whole farmhouses, reduced millions of acres to unproductive allowing the snow to settle to the ground. Added snow cover is wasteland, and financially ruined thousands of families. Because important for open areas because it acts like an insulating blanket of these disastrous effects of the wind, portions of the western that protects the ground from the bitter cold air, which often fol- plains became known as the Dust Bowl. lows in the wake of a major snowstorm. In regions of low rainfall, To protect crops and soil, windbreaks—commonly called moisture from the spring snowmelt can be a critical factor during shelterbelts—are planted. Shelterbelts usually consist of a series long, dry summers. Snow fences are also built to protect major of mixed conifer and deciduous trees or shrubs planted in rows highways in these areas. Hopefully, the snow will accumulate perpendicular to the prevailing wind flow. They greatly reduce the behind the fence rather than in huge drifts on the road. wind speed behind them (see Fig. 9.13). As air filters through Strong winds can whip the snow about, reducing visibility to the belt, the flow is broken into small eddies, which have little practically zero, often closing side streets and even interstate high- ways. When sustained winds or frequent gusts reach 35 mi/hr, *This effect actually drops above a certain wind speed and varies greatly among the blowing or falling snow can produce blizzard conditions. plant species. Wind: Small-Scale and Local Systems 237 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Figure 9.13 A properly designed shelterbelt can reduce the airflow downwind Figure 9.14 Wind blowing over a wave creates a small eddy of air that helps for a distance of 25 times the height of the belt. The minimum wind flow behind the to reinforce the up-and-down motion of the water. belt is typically measured downwind at a distance of about four times the belt’s height. mixing effect on the air near the surface. However, if trees are wind upward, producing an undulation in the airflow just above planted too close together, several unwanted effects may result. the water. This looping air motion establishes a small eddy of For one thing, the air moving past the belt may be broken into air between the two crests. The upward and downward motions larger, more turbulent eddies, which swirl soil about. Further- of the eddy reinforce the upward and downward motions of the more, in high winds, strong downdrafts may damage the crops. water. Consequently, the eddy helps the wave to build in height. The use of properly designed shelterbelts has benefited Traveling in the open ocean, waves represent a form of energy. agriculture. In some parts of the Central Plains, these belts As they move into a region of weaker winds, they gradually have stabilized the soil and increased wheat yield. Despite their change: Their crests become lower and more rounded, forming advantages, many of the shelterbelts planted during the drought what are commonly called swells. When waves reach a shoreline years of the mid-1930s have been removed. Some are economi- they transfer their energy—sometimes catastrophically—to the cally unfeasible because they occupy valuable crop land. Others coast and structures along it. High, storm-induced waves can hurl interfere with the large center pivot sprinkler systems now in thousands of tons of water against the shore. If this happens dur- use. At any rate, one wonders how the absence of these shel- ing an unusually high tide, resort homes overlooking the ocean terbelts would affect this region if it were struck by a drought can be pounded into a twisted mass of boards and nails by the similar to that experienced in the 1930s. surf. Bear in mind that the storms creating these waves may be thousands of kilometers away and, in fact, may never reach the shore. Some of the largest, most damaging waves ever to strike 9.2g Wind and Water the beach communities of Southern California arrived on what The impact of the wind on Earth’s surface is not limited to land; was described as “one of the clearest days imaginable.” On the wind also influences water—it makes waves. Waves forming by more positive side, the remote location of the Hawaiian Islands— wind blowing over the surface of the water are known as wind thousands of kilometers from any continent—allows for a large waves. Just as air blowing over the top of a water-filled pan creates fetch, and high waves excellent for surfing are common. tiny ripples, so waves are created as the frictional drag of the wind Frequently, as winds cross a large body of water, they will transfers energy to the water. In general, the greater the wind speed, change their speed and direction. Figure 9.15 shows the wind the greater the amount of energy added, and the higher will be the speed and direction as air flows over a large lake. At position A, waves. The amount of energy transferred to the water (and thus on the upwind side, the wind is blowing at 10 knots from the the height to which a wave can build) depends on three factors: northwest; at position B, the wind speed is 15 knots and has 1. the wind speed shifted to a more northerly direction; at position C, the wind 2. the length of time that the wind blows over the water speed is again blowing at 10 knots from the northwest. Why does the wind blow faster and from a slightly different direction in the 3. the fetch, or distance, of deep water over which the wind blows center of the lake? As the air moves from the rough land over A sustained 50-knot wind blowing steadily for nearly three the relatively smooth lake, friction with the surface lessens, and days over a minimum distance of 2600 km (1600 mi) can gen- the wind speed increases. The increase in wind speed, however, erate waves with an average height of 15 m (49 ft). Thus, a sta- increases the Coriolis force, which turns the wind flow slightly tionary storm system centered somewhere over the open sea is to the right of its intended path as shown by the wind report at capable of creating large waves with wave heights occasionally position B. When the air reaches the opposite side of the lake, it measuring over 31 m (100 ft). again encounters rough land, and its speed slows. This process Microscale winds actually help waves grow taller. Consider, reduces the Coriolis force, and the wind responds by shifting to for example, the wind blowing over the small wave depicted a more westerly direction, as shown by the report at position C. in Fig. 9.14. Observe that both the wind and the wave are mov- Changes in wind speed along the shore of a large lake can ing in the same direction, and that the wave crest deflects the inhibit cloud formation on one side and enhance it on the other. 238 Chapter 9 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Strong winds blowing over an open body of water, such as a lake, can cause the water to slosh back and forth rhythmically. This sloshing causes the water level to periodically rise and fall, much like water does at both ends of a bathtub when the water is disturbed. Such water waves that oscillate back and forth are called seiches (pronounced “sayshes”). In addition to strong winds, seiches may also be generated by sudden changes in atmospheric pressure or by earthquakes.* Around the Great Lakes, seiche applies to any sudden rise in water level whether or not it oscillates. In November 2003, strong westerly winds gusting to more than 50 knots created a seiche on Lake Erie that caused a 4 m (12 ft) difference in lake level between Toledo, Ohio (on its western shore) and Buffalo, New York (on its east- ern shore). Severe thunderstorms along Lake Erie’s northern shore in May 2012 generated a seiche that pushed a 2 m (7 ft) wave over a seawall northeast of Cleveland, Ohio, on the lake’s south shore. Several children were rescued after having been washed into the lake by the unexpected wave. Figure 9.15 Wind can change in both speed and direction when crossing a In summary, we’ve discussed how the wind blowing over large lake. Earth can produce a variety of features, from snow rollers to ocean waves. We also saw how the wind can influence a mov- Suppose warm, moist air flows over a lake, as illustrated in ing auto. To see how the wind can influence someone riding a Fig. 9.16. Observe that clouds are forming on the downwind bicycle, read Focus section 9.2. side, but not on the upwind side. The lake is slightly cooler than the air. Consequently, by the time the air reaches the downwind Brief Review side of the lake, it will be cooler, denser, and less likely to rise. Why, then, are clouds forming on this side of the lake? As air Up to this point we’ve been examining microscale winds and how they affect moves from the land over the water, it travels from a region of our environment. Before we turn our attention to winds on a larger scale, greater friction into a region of less friction, so it increases in here is a brief review of some of the main points presented so far: speed, which causes the surface air to diverge—to spread apart. Viscosity is the friction of fluid flow. The small-scale fluid friction due to the Such spreading of air forces air from above to slowly sink, which, random motion of the molecules is called molecular viscosity. The larger- of course, inhibits the formation of clouds. Hence, there are no scale internal friction produced by turbulent flow is called eddy viscosity. clouds on the upwind side of the lake. Out over the lake, the separation between air temperature and dew point lessens. Mechanical turbulence is created by twisting eddies that form as the wind As this nearly saturated air moves onshore, friction with the blows past obstructions. Thermal turbulence results as rising and sinking rougher ground slows it down, causing it to “bunch up” or con- air forms when Earth’s surface is heated unevenly by the sun. verge (which forces the air upward). This slight upward motion The planetary boundary layer (or friction layer) is usually considered to be coupled with surface heating is often sufficient to initiate the the first 1000 m (3300 ft) above the surface. formation of clouds along the downwind side of the lake. When Wind shear is a sudden change in wind speed or wind direction (or both). temperature and moisture contrasts are particularly strong over a large lake, heavy rain or snow squalls can develop in a narrow The wind can shape a landscape, influence crop production, transport band along the downwind shore. Lake-effect precipitation often material from one area to another, and generate waves. occurs along the south and east shores of the Great Lakes. Snow totals of up to several feet in a single day have been observed. 9.3 Local Wind Systems LO5 LO6 Every summer, millions of people flock to the New Jersey shore, hoping to escape the oppressive heat and humidity of the inland region. On hot, humid afternoons, these travelers often encoun- ter thunderstorms about 30 km or so from the ocean, thunder- storms that invariably last for only a few minutes. In fact, by the *Earthquakes and other disturbances on a lake floor can cause the water to slosh back and forth, producing a seiche. Earthquakes on the ocean basin floor can cause Figure 9.16 Sinking air develops where surface winds move offshore, speed up, and a tsunami, a Japanese word meaning “harbor waves” because these waves build in diverge. Rising air develops as surface winds move onshore, slow down, and converge. height as they enter a bay or harbor. Wind: Small-Scale and Local Systems 239 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Focus on a Special Topic 9.2 Pedaling into the Wind Anyone who rides a bicycle knows that it is much easier to pedal with the wind than against it (see Fig. 3). The reason is obvious: As we saw earlier in this chapter, when wind blows against an object, it exerts a force upon it. The amount of force exerted by the wind over an area increases as the square of the wind velocity. This relationship is shown by F , V2 where F is the wind force andV is the wind velocity. From this we can see that, if the wind velocity doubles, the force goes up by a factor iStock.com/technotr of 22 , or 4, which means that pedaling into a 40-knot wind requires four times as much effort as pedaling into a 20-knot wind. Wind striking an object exerts a pressure Figure 3 Pedaling into a 15-knot wind requires nine times as much effort as pedaling into on it. The amount of pressure depends upon a 5-knot wind. the object’s shape and size, as well as on the amount of reduced pressure that exists on the 10 mi/hr into a head wind of 40 mi/hr. With the This force is enough to make pedaling into object’s downwind side. Without concern for total velocity of the wind against the rider (wind the wind extremely difficult. To remedy this all the complications, we can approximate speed plus bicycle speed) being 50 mi/hr, the adverse effect, cyclists—especially racers— the wind pressure on an object with a simple pressure of the wind is bend forward as low as possible to expose a formula. For example, if the wind velocity (V ) P 5 0.004 V 2 minimum surface area to the wind. is in miles per hour, and the wind force (F ) is Runners also experience wind impacts. At P 5 0.004(502 ) in pounds, and the object’s surface area ( A ) is competitions, wind affects records set during measured in square feet, the wind pressure (P ), P 5 10 lb / ft 2 track events to the extent that when runners race in pounds per square feet, is If the rider has a surface body area of 5 ft 2, for 200 meters or less (i.e., in one direction) with F the total force exerted by the wind becomes a tail wind of more than 2 meters per second 5 P 5 0.004 V 2 (about 4.5 mi/hr), their results are asterisked A F 5P 3A We can look at a practical example of this F 5 10 lb / ft 2 3 5 ft 2 with the qualifier “wind-aided” and cannot be expression if we consider a bicycle rider going F 5 50 lb accepted for any world or national records. time the vacationers arrive at the beach, skies are generally clear temperature), and there is no pressure gradient and no wind. and air temperatures are much lower, as cool ocean breezes greet Suppose in Fig. 9.17b the atmosphere is cooled to the north them. If the travelers return home in the afternoon a few days (left) and warmed to the south (right). In the cold, dense air later, these “mysterious” showers often occur at just about the above the surface, the isobars bunch closer together vertically, same location as before. while in the warm, less-dense air, they spread farther apart. This The showers are not really mysterious, of course. They are shift of the isobars produces a horizontal pressure gradient force caused by a local wind system, the sea breeze. As cooler ocean air aloft that causes the air to move from higher pressure (warm air pours inland, it forces the warmer, conditionally unstable humid aloft) toward lower pressure (cold air aloft). air to rise and condense, producing majestic clouds and rain At the surface, the air pressure changes as the air aloft showers along a line that separates the contrasting temperatures. begins to move. As the air aloft moves from south to north, air The sea breeze forms as part of a thermally driven circula- leaves the southern area and “piles up” above the northern area. tion. Consequently, we will begin our study of local winds by This redistribution of air reduces the surface air pressure to the examining the formation of thermal circulations. south and raises it to the north. Consequently, a pressure gradi- ent force is established at Earth’s surface from north to south 9.3a Thermal Circulations and, hence, surface winds begin to blow from north to south. Consider the vertical distribution of pressure shown in We now have a distribution of pressure and temperature Fig. 9.17a. The isobaric surfaces all lie parallel to Earth’s and a circulation of air, as shown in Fig. 9.17c. As the cool sur- surface; thus, there is no horizontal variation in pressure (or face air flows southward, it warms and becomes less dense. 240 Chapter 9 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Figure 9.18 The vertical distribution of pressure with thermal highs and thermal lows. 9.3b Sea and Land Breezes The sea breeze is a type of thermal circulation. The uneven heat- ing rates of land and water (described in Chapter 3) cause these mesoscale coastal winds. During the day, the land heats more quickly than the adjacent water, and the intensive heating of the air above produces a shallow thermal low. The air over the water remains cooler than the air over the land; hence, a shallow ther- mal high exists above the water. The overall effect of this pressure distribution is a sea breeze that blows at the surface from the sea toward the land (see Fig. 9.19a). Because the strongest gradients of temperature and pressure occur near the land-water bound-