4.4 Wind Systems: Sea Breezes to Gravity Winds

Identify local and regional wind systems and explain how they form.

The underlying theme that has been with us since the beginning of Chapter 1 is that the atmosphere is set in motion by uneven heating of Earth’s surface by the Sun. The release of latent heat through condensation and Earth’s rotation also generate winds and determine the direction of airflow. In this section, we examine major local and regional wind systems in the atmosphere. These wind systems can be arranged by their geographic and temporal scales, as summarized in Figure 4.23.

Figure 4.23

Geographic scale of wind systems. Microscale systems, such as local breezes, last hours and cover up to 2 kilometers. Mesoscale systems, such as Santa Ana winds, are up to several hundred kilometers in extent and last for days. Synoptic-scale systems, such as the Asian monsoon, last months and span a thousand kilometers or more.

All the wind systems described in this section can locally override global wind patterns. The first two wind systems we explore, sea and land breezes and the Asian monsoon, are created by unequal heating of land and ocean surfaces.

Sea and Land Breezes

Sea breezes and land breezes are microscale breezes created by heating and cooling differences between water and land (Figure 4.24). Sea and land breezes are most pronounced in the tropics because of strong daytime heating of the land. They also form at midlatitudes, where cold ocean water contrasts with warm temperatures just inland. Given their localized nature, sea and land breezes can easily be disrupted by synoptic-scale storm systems.

Figure 4.24

GEO-GRAPHIC: Sea and land breezes.

sea breeze

A local onshore breeze created by heating and cooling differences between water and land.

land breeze

A local offshore breeze created by heating and cooling differences between water and land.

The Asian Monsoon

Like sea and land breezes, the Asian monsoon is created by heating differences between land and sea. The word monsoon is derived from the Arabic mausim, which means “season.” A monsoon is a seasonal reversal of winds, characterized by summer onshore airflow and winter offshore airflow. About half the world’s population experiences the influence of a monsoon system and knows only two seasons: a warm and rainy summer and a slightly cooler but dry winter. The Asian monsoon is the largest monsoon system in the world, and in summer it brings heavy rains, and sometimes flooding, to South Asia.

monsoon

A seasonal reversal of winds, characterized by summer onshore airflow and winter offshore airflow.

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The Asian monsoon is separated into the South Asian monsoon, which affects India, and the East Asian monsoon, which affects Indonesia, northern Australia, southern China, Korea, and Japan. All monsoons consist of a summer monsoon, a warm and moist onshore airflow, and a relatively cool (but still warm) winter monsoon, an offshore flow of dry air originating in the continental interior.

In summer, Asian monsoon rains do not necessarily fall every day. Dry break periods can last a few weeks to more than a month. When these break periods are prolonged, drought and crop failures can result. Too little rain brings food shortages and potential famine for millions. When the break periods are short, too much rain results, and flooding, soil erosion, and waterborne disease outbreaks sometimes occur.

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Animation

Asian monsoon

http://qrs.ly/3b434ek

The Asian monsoon system is complex, and climatologists still cannot predict with accuracy how it will behave from one year to the next. There are three synoptic-scale controls on the South Asian monsoon: (1) onshore airflow created by summer landmass heating, (2) orographic lifting by the Himalayas, and (3) movement of the ITCZ over the Himalayas and Tibetan Plateau (Figure 4.25).

Figure 4.25

Asian monsoon system. (A) In summer, the ITCZ draws air north (shown with red arrows) from the Indian Ocean. This humid onshore airflow brings summer rains. (B) In winter, air flows away from the Siberian high over central Eurasia. As a result, dry offshore winds develop. The timing and strength of the summer monsoon rains depend on three factors: onshore airflow, orographic uplift, and the ITCZ.

Onshore airflow, orographic uplift, and the ITCZ affect one another and overlap in their influence on the strength, duration, and timing of the South Asian monsoon, creating an unpredictable system. For example, the high Tibetan Plateau acts like a chimney when the ITCZ forms over it and causes heated air to be injected high into the troposphere, further strengthening onshore airflow and precipitation. Extensive snow cover on the plateau can reduce the strength of this chimney effect by cooling the air and making it more stable. This diminishes the strength of the summer monsoon. As Picture This shows, the Asian monsoon brings significant amounts of rainfall to many regions in South Asia.

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During the winter monsoon in Asia, the winds reverse and flow from land to sea. The cold interior of the Eurasian landmass develops the semipermanent Siberian high, from which dry air flows outward across Asia and out to sea.

The southwestern United States and northern Mexico also have a monsoon system. During summer, heating over the mountains and the resulting thermal low pressure create a pressure gradient that draws air inland from the warm Gulf of California and the Gulf of Mexico, resulting in summer thunderstorms.

The remaining four wind systems we will describe are all caused by sloped terrain. We will explore them in order of their spatial extent, starting with the most localized winds in mountains.

Picture This

(© DINODIA/Age Fotostock Inc.)

The Wettest Places on Earth

Mawsynram, India, has the distinction of being recognized by Guinness World Records as the wettest place on Earth. Each year it receives an average of 1,187 cm (467.4 in or 38.9 ft) of rainfall (shown with climate diagram at right). It is located in the Khasi Hills in Meghalaya State at about 1,400 m (4,560 ft) elevation. The nearby Nohakali Kai Falls, in Cherrapunji, are shown in this photo.

There are probably wetter locations, but they are not officially recognized. Lloró, Colombia, for example, claims an average yearly rainfall amount of 1,329 cm (523 in or 43.6 ft). Just 11 km (7 mi) to the east of Mawsynram, the town of Cherrapunji also claims to have the world’s highest average annual rainfall. Cherrapunji does hold the official record for the greatest 12-month precipitation total. Between August 1, 1860, and July 31, 1861, Cherrapunji recorded 2,646.1 cm (1,041.77 in or 86.8 ft) of rainfall. Interestingly, Mawsynram and Cherrapunji (25° north latitude) are located at nearly the same latitude as the Empty Quarter (see Section 4.3), one of the most arid regions on Earth.

Consider This

  1. Question 4.7

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  2. Question 4.8

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Valley and Mountain Breezes

Valley and mountain breezes are local breezes produced by heating and cooling differences in mountainous areas. In summer, mountain slopes that face toward the afternoon Sun are heated and form warm, buoyant parcels of air. As this warmed air rises, it creates a pressure gradient that draws in air from adjacent valley floors, resulting in a valley breeze. As the air parcels rise, they expand and cool adiabatically. If there is sufficient vapor pressure in the rising air parcels, their temperature may drop to the dew point, and condensation will follow. Clouds such as cumulus and cumulonimbus formed from this condensation often produce afternoon summer thunderstorms in mountainous regions as valley breezes develop.

valley breeze

A local upslope breeze produced by heating and cooling differences in mountainous areas.

This situation is reversed after the Sun sets, as upper elevations cool faster than lower elevations. Cold, dense, and heavy mountain breezes flow downslope through canyons, finding the lowest valleys. Mountain breezes are strongest in winter when air is coldest (Figure 4.26).

Figure 4.26

Valley and mountain breezes.

mountain breeze

A local downslope breeze produced by heating and cooling differences in mountainous areas.

Chinook and Foehn Winds

Downslope winds on the leeward side of the Rocky Mountains are called chinook winds. Foehn winds are the same phenomenon in the European Alps, and there are other local names for this type of wind. As we saw in Section 3.3, the leeward side of a mountain range is typically warmer and drier than the windward side. This difference is due to the release of latent heat and precipitation on the windward side and adiabatic heating as air flows down the leeward side (Figure 4.27).

Figure 4.27

Chinook and foehn winds. Warm chinook and foehn winds form as air flows up, over, and back down the leeward side of a mountain range.

chinook wind

A local downslope wind that forms on the leeward side of the Rocky Mountains.

foehn wind

(pronounced FEH-rn) A downslope wind that forms on the leeward size of the European Alps.

Chinook winds are warm, dry winds that often come in sharp contrast to cold winter conditions. With the arrival of chinook winds, temperatures can rise 20°C (36°F) or more within a matter of minutes. These winds quickly melt and sublimate snow.

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Santa Ana Winds

Santa Ana winds sometimes create a major fire hazard for parts of southern California and northern Baja California, Mexico. Santa Ana winds originate in the high desert of the Great Basin and flow to coastal California (Figure 4.28).

Figure 4.28

Santa Ana winds. Santa Ana winds occur when high pressure occupies the Great Basin desert and relatively low pressure occurs off the Southern California coast.

Santa Ana winds

Winds that originate in the Great Basin and are heated adiabatically as they descend to sea level on the southern California coast and northern Baja California; often associated with major wildfires.

Santa Ana winds form as high pressure develops in the Great Basin and cool air flows downslope toward coastal southern California. This flow is the result not only of the pressure gradient, but also of gravity, because the cool, relatively dense and heavy air sinks and flows downslope. As the air descends in elevation, it is compressed and warmed adiabatically. Because it originates in the desert, it is dry. Thus the temperature of Santa Ana winds may approach 32°C (90°F), and the relative humidity of these winds is often in the single digits.

Santa Ana winds occur from fall through spring and usually peak in December.Santa Ana wind speeds can exceed 100 km/h (62 mph) in constricted valleys. Wildfires sometimes burn out of control in these hot, dry winds. As Figure 4.29 shows, rural residences can be vulnerable to Santa Ana wildfires.

Figure 4.29

October 2003 Santa Ana fires. (A) Outlined in red, fires burning in many areas from southern California to northern Baja California can be seen in this satellite image taken October 26, 2003. The strong offshore airflow is apparent as fire smoke blows over the Pacific Ocean. About 3,000 km2 (1,150 mi2) burned, 3,000 homes were destroyed, and 26 people were killed in the 2003 fires. The cost totaled $2.5 billion. (B) The Cedar fire, shown here, burned in San Diego County in 2003 and was a result of Santa Ana winds. The fire burned 1,134 km2 (438 mi2) and was the largest fire in California history.
(A. NASA; B. David Hume Kennerly/Getty Images)

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Katabatic Winds

Question 4.9

What are gravity winds?

Gravity winds are also called katabatic winds. They are winds that flow rapidly downslope because they are cold and heavy.

Katabatic winds, or gravity winds (katabatikos is Greek for “downhill movement”), form mainly over ice sheets or glaciers when intensely cold, dense, and heavy air spills downslope by the force of gravity. Katabatic winds are similar to mountain breezes and Santa Ana winds, but they are stronger, cover greater geographic distances, and are far colder. Where airflow is constricted and focused in valleys, the wind speeds can exceed those of hurricanes. Greenland and Antarctica experience katabatic winds commonly, but they can occur wherever high, cold plateaus are found. The effects of katabatic winds can be seen in the satellite image in Figure 4.30.

Figure 4.30

Katabatic winds. The winds flowing into Terra Nova Bay, Antarctica, are pushing sea ice into long lines. Arrows show the direction of the wind.
(NASA)

katabatic wind

(or gravity wind) Wind that forms mainly over ice sheets or glaciers when intensely cold, dense, and heavy air spills downslope by the force of gravity.