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Lesson 7-06 Regional Wind Systems

Page history last edited by debra.krohn@gmail.com 14 years, 1 month ago

 

Lesson 7.06 – Regional Wind Systems

Standard:  ES5.e Students know rain forests and deserts on Earth are distributed in bands at specific latitudes.

ES6.b Students know the effects on climate of latitude, elevation, topography, and proximity to large bodies of water and cold or warm ocean currents.

ES5. f.* Students know the interaction of wind patterns, ocean currents, and mountain ranges results in the global pattern of latitudinal bands of rain forests and deserts.

ES5. g.* Students know features of the ENSO (El Niño southern oscillation) cycle in terms of sea-surface and air temperature variations across the Pacific and some climatic results of this cycle.

 

 

INTRODUCTION

Circulation in the middle latitudes is complex and does not fit the convection system described for the tropics. Between about 30 and 60 degrees latitude, the general west-to-east flow, known as the westerlies, is interrupted by migrating cyclones and anticyclones. In the Northern Hemisphere, these pressure cells move from west to east around the globe.

 

 

INSTRUCTION

Local Winds

Small-scale winds produced by a locally generated pressure gradient are known as local winds. The local winds are caused either by topographic effects or by variations in surface composition—land and water—in the immediate area.

 

 

Land and Sea Breezes

In coastal areas during the warm summer months, the land surface is heated more intensely during the daylight hours than an adjacent body of water is heated. As a result, the air above the land surface heats, expands, and rises, creating an area of lower pressure. As shown in Figure 12, a sea breeze then develops because cooler air over the water at higher pressure moves toward the warmer land and low pressure air. The breeze starts developing shortly before noon and generally reaches its greatest intensity during the mid- to late afternoon. These relatively cool winds can be a moderating influence on afternoon temperatures in coastal areas.

 

 

Figure 12 Sea Breeze During daylight hours, the air above land heats and rises, creating a local zone of lower air pressure.

 

 

At night, the reverse may take place. The land cools more rapidly than the sea, and a land breeze develops, as shown in Figure 13. The cooler air at higher pressures over the land moves to the sea, where the air is warmer and at lower pressures. Small-scale sea breezes also can develop along the shores of large lakes. People who live in a city near the Great Lakes, such as Chicago, recognize this lake effect, especially in the summer. They are reminded daily by weather reports of the cool temperatures near the lake as compared to warmer outlying areas.

 

 

Figure 13 Land Breeze At night, the land cools more rapidly than the sea, generating an offshore flow called a land breeze. Inferring How would the isobar lines be oriented if there was no air pressure change across the land-water boundary?

 

 

Valley and Mountain Breezes

A daily wind similar to land and sea breezes occurs in many mountainous regions. During daylight hours, the air along the slopes of the mountains is heated more intensely than the air at the same elevation over the valley floor. Because this warmer air on the mountain slopes is less dense, it glides up along the slope and generates a valley breeze, as shown in Figure 14A. The occurrence of these daytime upslope breezes can often be identified by the cumulus clouds that develop on adjacent mountain peaks.

 

 

Figure 14 A Valley Breeze Heating during the day generates warm air that rises from the valley floor. B Mountain Breeze After sunset, cooling of the air near mountain slopes can result in cool air moving into the valley.

 

 

After sunset, the pattern may reverse. The rapid cooling of the air along the mountain slopes produces a layer of cooler air next to the ground. Because cool air is denser than warm air, it moves downslope into the valley. Such a movement of air, illustrated in Figure 14B, is called a mountain breeze. In the Grand Canyon at night, the sound of cold air rushing down the sides of the canyon can be louder than the sound of the Colorado River below.

 

 

The same type of cool air drainage can occur in places that have very modest slopes. The result is that the coldest pockets of air are usually found in the lowest spots. Like many other winds, mountain and valley breezes have seasonal preferences. Although valley breezes are most common during the warm season when solar heating is most intense, mountain breezes tend to be more dominant in the cold season.

 

 

How Wind Is Measured

Two basic wind measurements—direction and speed—are particularly important to the weather observer. Winds are always labeled by the direction from which they blow. A north wind blows from the north toward the south. An east wind blows from the east toward the west. The instrument most commonly used to determine wind direction is the wind vane, shown in the upper right of Figure 15. Wind vanes commonly are located on buildings, and they always point into the wind. The wind direction is often shown on a dial connected to the wind vane. The dial indicates wind direction, either by points of the compass—N, NE, E, SE, etc.—or by a scale of 0° to 360°. On the degree scale, 0° or 360° are north, 90° is east, 180° is south, and 270° is west.

 

 

Wind Direction

When the wind consistently blows more often from one direction than from any other, it is called a prevailing wind. Recall the prevailing westerlies that dominate circulation in the middle latitudes. In the United States, the westerlies consistently move weather from west to east across the continent. Along within this general eastward flow are cells of high and low pressure with the characteristic clockwise and counterclockwise flows. As a result, the winds associated with the westerlies, as measured at the surface, often vary considerably from day to day and from place to place. In contrast, the direction of airflow associated with the trade winds is much more consistent.

 

 

Wind Speed

Shown in the upper left of Figure 15, a cup anemometer (anemo = wind,metron = measuring instrument) is commonly used to measure wind speed. The wind speed is read from a dial much like the speedometer of an automobile. Places where winds are steady and speeds are relatively high are potential sites for tapping wind energy.

 

 

Figure 15 Wind Vane and Cup Anemometer Interpreting Photographs How does the position of a wind vane tell you which direction the wind is blowing?

 

 

El Niño and La Niña

Look at Figure 16. The cold Peruvian current flows toward the equator along the coasts of Ecuador and Peru. This flow encourages upwelling of cold nutrient-filled waters that are the primary food source for millions of fish, particularly anchovies. Near the end of the year, however, a warm current that flows southward along the coasts of Ecuador and Peru replaces the cold Peruvian current. During the nineteenth century, the local residents named this warm current El Niño (“the child”) after the Christ child because it usually appeared during the Christmas season. Normally, these warm countercurrents last for a few weeks and then give way to the cold Peruvian flow again.

 

 

Figure 16 Normal Conditions Trade winds and strong equatorial ocean currents flow toward the west.

 

 

El Niño

At irregular intervals of three to seven years, these warm countercurrents become unusually strong and replace normally cold offshore waters with warm equatorial waters. Scientists use the term El Niño for these episodes of ocean warming that affect the eastern tropical Pacific.

 

 

The onset of El Niño is marked by abnormal weather patterns that drastically affect the economies of Ecuador and Peru. As shown in Figure 17, these unusually strong undercurrents accumulate large quantities of warm water that block the upwelling of colder, nutrient-filled water. As a result, the anchovies starve, devastating the local fishing industry. At the same time, some inland areas that are normally arid receive an abnormal amount of rain. Here, pastures and cotton fields have yields far above the average. These climatic fluctuations have been known for years, but they were originally considered local phenomena. It now is understood that El Niño is part of the global circulation and that it affects the weather at great distances from Peru and Ecuador.

 

 

Figure 17 El Niño Warm countercurrents cause reversal of pressure patterns in the western and eastern Pacific.

 

 

When an El Niño began in the summer of 1997, forecasters predicted that the pool of warm water over the Pacific would displace the paths of both the subtropical and midlatitude jet streams, as shown in Figure 17. The jet streams steer weather systems across North America. As predicted, the subtropical jet brought rain to the Gulf Coast. Tampa, Florida, received more than three times its normal winter precipitation. The mid-latitude jet pumped warm air far north into the continent. As a result, winter

 

 

La Niña

The opposite of El Niño is an atmospheric phenomenon known as La Niña. Once thought to be the normal conditions that occur between two El Niño events, meteorologists now consider La Niña an important atmospheric phenomenon in its own right. Researchers have come to recognize that when surface temperatures in the eastern Pacific are colder than average, a La Niña event is triggered that has a distinctive set of weather patterns. A typical La Niña winter blows colder than normal air over the Pacific Northwest and the northern Great Plains. At the same time, it warms much of the rest of the United States. The Northwest also experiences greater precipitation during this time. During the La Niña winter of 1998–99, a world-record snowfall for one season occurred in Washington State. La Niña impact can also increase hurricane activity. A recent study concluded that the cost of hurricane damages in the United States is 20 times greater in La Niña years as compared to El Niño years.

 

 

The effects of both El Niño and La Niña on world climate are widespread and vary greatly. These phenomena remind us that the air and ocean conditions of the tropical Pacific influence the state of weather almost everywhere.

 

 

Global Distribution of Precipitation

Figure 18 (Click here) shows that the tropical region dominated by the equatorial low is the rainiest region on Earth. It includes the rain forests of the Amazon basin in South America and the Congo basin in Africa. In these areas, the warm, humid trade winds converge to yield abundant rainfall throughout the year. In contrast, areas dominated by the subtropical high-pressure cells are regions of extensive deserts. Variables other than pressure and wind complicate the pattern. For example, the interiors of large land masses commonly experience decreased precipitation. However, you can explain a lot about global precipitation if you apply your knowledge of global winds and pressure systems.

 

 

PRACTICE

  1. Take notes on the information above.
  2. Click here (http://videos.howstuffworks.com/hsw/16796-hands-on-weather-ii-a-look-at-wind-video.htm) to watch a video about wind direction.
  3. Click here (http://videos.howstuffworks.com/hsw/15817-natural-disasters-el-nino-video.htm) to watch a video about El Nino and La Nina.

 

 

ASSESSMENT

  1. Turn in your notes.
  2. Take the 7.06 Quiz


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