Local Winds

Dana Desonie, Ph.D.

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Printed: August 31, 2014

AUTHORDana Desonie, Ph.D.

www.ck12.org Chapter 1. Local Winds

CHAPTER 1 Local Winds• Describe the different types of local winds and explain how they are created.• Explain how types of local winds influence climate.

How can they stand up?

When you try to walk against a 20 mile an hour wind it’s not easy. Just standing up is like walking really fast!

Local Winds

Local winds result from air moving between small low and high pressure systems. High and low pressure cells arecreated by a variety of conditions. Some local winds have very important effects on the weather and climate of someregions.

Land and Sea Breezes

Since water has a very high specific heat, it maintains its temperature well. So water heats and cools more slowlythan land. If there is a large temperature difference between the surface of the sea (or a large lake) and the land nextto it, high and low pressure regions form. This creates local winds.

• Sea breezes blow from the cooler ocean over the warmer land in summer. Where is the high pressure zoneand where is the low pressure zone ( Figure 1.1)? Sea breezes blow at about 10 to 20 km (6 to 12 miles) perhour and lower air temperature much as 5 to 10oC (9 to 18oF).

• Land breezes blow from the land to the sea in winter. Where is the high pressure zone and where is the lowpressure zone? Some warmer air from the ocean rises and then sinks on land, causing the temperature overthe land to become warmer.

Land and sea breezes create the pleasant climate for which Southern California is known. The effect of land and seabreezes are felt only about 50 to 100 km (30 to 60 miles) inland. This same cooling and warming effect occurs to asmaller degree during day and night, because land warms and cools faster than the ocean.

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FIGURE 1.1How do sea and land breezes moderate coastal climates?

Monsoon Winds

Monsoon winds are larger scale versions of land and sea breezes; they blow from the sea onto the land in summerand from the land onto the sea in winter. Monsoon winds occur where very hot summer lands are next to the sea.Thunderstorms are common during monsoons ( Figure 1.2).

FIGURE 1.2In the southwestern United States rela-tively cool moist air sucked in from theGulf of Mexico and the Gulf of Californiameets air that has been heated by scorch-ing desert temperatures.

The most important monsoon in the world occurs each year over the Indian subcontinent. More than two billionresidents of India and southeastern Asia depend on monsoon rains for their drinking and irrigation water. Backin the days of sailing ships, seasonal shifts in the monsoon winds carried goods back and forth between India andAfrica.

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Mountain and Valley Breezes

Temperature differences between mountains and valleys create mountain and valley breezes. During the day, air onmountain slopes is heated more than air at the same elevation over an adjacent valley. As the day progresses, warmair rises and draws the cool air up from the valley, creating a valley breeze. At night the mountain slopes cool morequickly than the nearby valley, which causes a mountain breeze to flow downhill.

Katabatic Winds

Katabatic winds move up and down slopes, but they are stronger mountain and valley breezes. Katabatic windsform over a high land area, like a high plateau. The plateau is usually surrounded on almost all sides by mountains.In winter, the plateau grows cold. The air above the plateau grows cold and sinks down from the plateau through gapsin the mountains. Wind speeds depend on the difference in air pressure over the plateau and over the surroundings.Katabatic winds form over many continental areas. Extremely cold katabatic winds blow over Antarctica andGreenland.

Chinook Winds (Foehn Winds)

Chinook winds (or Foehn winds) develop when air is forced up over a mountain range. This takes place, forexample, when the westerly winds bring air from the Pacific Ocean over the Sierra Nevada Mountains in California.As the relatively warm, moist air rises over the windward side of the mountains, it cools and contracts. If the air ishumid, it may form clouds and drop rain or snow. When the air sinks on the leeward side of the mountains, it formsa high pressure zone. The windward side of a mountain range is the side that receives the wind; the leeward side isthe side where air sinks.

The descending air warms and creates strong, dry winds. Chinook winds can raise temperatures more than 20oC(36oF) in an hour and they rapidly decrease humidity. Snow on the leeward side of the mountain melts quickly. Ifprecipitation falls as the air rises over the mountains, the air will be dry as it sinks on the leeward size. This dry,sinking air causes a rainshadow effect ( Figure 1.3), which creates many of the world’s deserts.

FIGURE 1.3As air rises over a mountain it cools andloses moisture, then warms by compres-sion on the leeward side. The resultingwarm and dry winds are Chinook winds.The leeward side of the mountain experi-ences rainshadow effect.

Santa Ana Winds

Santa Ana winds are created in the late fall and winter when the Great Basin east of the Sierra Nevada cools,creating a high pressure zone. The high pressure forces winds downhill and in a clockwise direction (because of

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Coriolis). The air pressure rises, so temperature rises and humidity falls. The winds blow across the Southwesterndeserts and then race downhill and westward toward the ocean. Air is forced through canyons cutting the San Gabrieland San Bernardino mountains. ( Figure 1.4).

FIGURE 1.4The winds are especially fast throughSanta Ana Canyon, for which they arenamed. Santa Ana winds blow dust andsmoke westward over the Pacific fromSouthern California.

The Santa Ana winds often arrive at the end of California’s long summer drought season. The hot, dry winds dryout the landscape even more. If a fire starts, it can spread quickly, causing large-scale devastation ( Figure 1.5).

FIGURE 1.5In October 2007, Santa Ana winds fueled many fires that together burned426,000 acres of wild land and more than 1,500 homes in SouthernCalifornia.

Desert Winds

High summer temperatures on the desert create high winds, which are often associated with monsoon storms. Desertwinds pick up dust because there is not as much vegetation to hold down the dirt and sand. ( Figure 1.6). A haboobforms in the downdrafts on the front of a thunderstorm.

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FIGURE 1.6A haboob in the Phoenix metropolitanarea, Arizona.

Dust devils, also called whirlwinds, form as the ground becomes so hot that the air above it heats and rises. Air flowsinto the low pressure and begins to spin. Dust devils are small and short-lived, but they may cause damage.

Summary

• Water has high specific heat, so its temperature changes very slowly relative to the temperature of the land.This is the reason for sea and land breezes and monsoon winds.

• The cause of all of these winds is the differential heating of Earth’s surface, whether it’s due to the differencein water and land, the difference with altitude, or something else.

• Winds blow up and down slope, on and off land and sea, through deserts or over mountain passes.

Explore More

Use these resources to answer the questions that follow.

https://www.youtube.com/watch?v=1Qtf7M1s__w

1. What is the cause of monsoon winds? How is this the same or different from the cause of land and sea winds?2. Why does wind blow from land to sea in winter?3. Why does wind blow from sea to land in summer?4. What causes the monsoon rain and winds?

https://www.youtube.com/watch?v=4pNQ4g4zOwY

1. Describe the movement of the Santa Ana winds through and outward from Southern California?2. In which positions do high and low pressure cells need to be to generate these winds?3. What causes the winds to blow from the east across Southern California?4. What is the adiabatic process?5. How does the adiabatic process work to create the Santa Ana Winds?6. Why do fires often accompany the Santa Ana winds?

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Review

1. How does the high specific heat of water result in the formation of sea and land breezes?2. Describe the conditions that lead to Santa Ana winds.3. How do Chinook winds lead to rainshadow effect?

References

1. Jodi So. Land and sea breezes blow in different directions if it is day or night. CC BY-NC 3.02. User:RyanW124/Wikipedia. Picture of a monsoon in the southwestern United States. Public Domain3. Courtesy of US Federal Aviation Administration. Diagram of Chinnok winds. Public Domain4. Courtesy of NASA. Satellite image of Santa Ana winds. Public Domain5. Image copyright mikeledray, 2014. Santa Ana winds fuel wildfires in Southern California. Used under license

from Shutterstock.com6. User:Junebug172/Wikipedia. Picture of a haboob in Phoenix, Arizona. Public Domain

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