World Geography

WIND CIRCULATION

Overview

The global circulation patterns of wind on Earth

• Winds that blow predominantly from a single direction over a particular point on the Earth’s surface are called Prevailing Winds
• The general trends in wind direction are called dominant winds
• In general regional winds can be divided into two groups
  • Global winds like easterlies, westerlies
  • Local winds like land breeze, sea breeze
• Prevailing winds greatly influence climate patterns such as rainfall gradients, where the windward side of mountains have high rainfall while leeward side experience desert conditions
• Wind rose is a graphic plotting tool that is used to describe the speed and direction of wind at a particular location
• Insects drift along prevailing winds, while birds are able to fly independent of them

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Distribution of prevailing winds

• In general, easterly winds flow at low and high altitudes i.e. near the tropics and the poles
• Westerly winds flow at the mid-latitudes
• Directly under the subtropical ridge i.e. close to the equator, winds are lighter in intensity. These subtropical regions are called the doldrums, or horse latitudes
• The strongest winds are usually in the mid-latitudes, where cold air from the Artic meets warm air from the tropics
• Most of the earth’s deserts are found near the subtropical ridge, with high pressure leading to low humidity

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GLOBAL WINDS

• Trade Winds
  • Trade winds are the prevailing easterly winds that blow across the tropics
  • They blow from the northeast in the northern hemisphere and from the southeast in the southern hemisphere
  • Trade winds act as the steering for tropical storms that form in the Atlantic, Pacific and south Indian Oceans. These storms make landfall in North America, Southeast Asia and India, respectively
    
  • Trade winds steer African desert dust across the Atlantic Ocean towards North America (esp. the Caribbean and Florida)
    
  • The weaker a trade wind becomes, the more rainfall it brings
  • Trade winds are stronger in winter than summer
  • The one region of the Earth where trade winds are absent is the north Indian Ocean
• Westerlies
  • Westerlies are the prevailing winds in the mid-latitudes i.e. between 35 and 65 degrees latitude
  • They blow from high pressure areas in the horse latitudes towards the poles
  • Westerlies blow from the southwest in the northern hemisphere and from the northwest in the southern hemisphere
  • Westerlies are instrumental in carrying warm equatorial winds towards the western coasts of continents
  • They are responsible for carrying desert dust from the Gobi Desert into North America
  • They are stronger in winter than in summer, and over regions that have less land to interrupt their flow. They are stronger in the Southern Hemisphere because of the vast ocean expanses uninterrupted by land mass
  • Westerlies are strongest in the Roaring Forties i.e. between 40 and 50 degrees latitude
• Polar Easterlies
  • Polar easterlies are the prevailing winds that blow at the north and south poles
  • They are cold and dry winds
  • They blow from high pressure areas near the poles towards low pressure areas within the mid-latitudes
  • They blow from the east to the west
  • Polar easterlies are often weak and irregular
  • They are also called Polar Hadley Cells, named after George Hadley who discovered them in 1753

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LOCAL WINDS

Notable local winds

1. Sea and land Breeze
   1. Sea and land breezes are caused by the temperature differential between the sea and coastal areas
   2. Sea breeze occurs when the land gets heated during the day creating a low pressure, and cool air from the sea rushes in
   3. Land breeze occurs when the land cools off rapidly at night causing low pressure over the sea, and warm air flows from the land to the sea
   4. Sea breeze occurs during the day while land breeze occurs at night
2. Mountain winds
   1. In elevated surfaces heating of the ground exceeds heating of surrounding air, thereby changing wind circulation
   2. Hills and valleys significantly distort airflow by acting as physical barriers. This is known as barrier jet
   3. Jagged terrain results in unpredictable flow patterns and turbulence
   4. Passes in the mountain range experience lower pressure resulting in high wind speeds and erratic and turbulent air currents
   5. These conditions are dangerous to ascending and descending airplanes

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MONSOON WINDS

Overview

• Monsoons are defined as seasonal reversing winds accompanied by seasonal changes in precipitation
• The major monsoon systems of the world are the West African and Asia-Australian monsoon systems
• The origin of monsoons about 15-20 million years ago has been linked to the uplift of the Tibetan Plateau after the collision of India and Asia 50 million years ago

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Cause of monsoons

• Monsoons are caused by the larger amplitude of seasonal land temperature cycle compared to that of nearby oceans. This temperature differential arises because air over land warms faster and reaches higher temperature than the air over nearby ocean
• The hot air over land tends to rise creating a low pressure
• This creates a steady wind blowing from the ocean towards land, bringing moist air from the oceans
• In winter the land cools off quickly creating a high pressure that blows wind from land to sea
• In essence, monsoons are similar to sea and land breezes, except that they occur on a much larger scale

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The Southwest Monsoon

The Southwest monsoon in India. It brings about 80% of India's annual rainfall

• The southwest monsoon occurs from June to September
• The southwest monsoon is caused by rapid heating of the Thar desert and north-central India in summer, creating a low pressure that is filled by moisture laden winds from the Indian Ocean
• The Himalayas prevent the wind from blowing towards Central Asia and redirect them inwards to cause rainfall
• The Arabian Sea branch of the southwest monsoon brings rainfall to the Malabar coast and central India
• The Bay of Bengal branch of the southwest monsoon picks up additional moisture in the Bay of Bengal and arrives at the eastern Himalayas, and then turns west towards the Indo-Gangetic plains
• Mawsynram in Shillong is the wettest place on Earth with about 12,000 mm of rainfall annually
• The traditional start date of the southwest monsoon is June 01
• The southwest monsoon accounts for 80% of rainfall in India

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The Northeast Monsoon

• The northeast monsoon occurs October to December in India
• Around September, northern India begins to cool rapidly creating a high pressure zone
• This brings dry cold winds from the Himalayas towards the Deccan and into the Indian Ocean
• While travelling towards the Indian Ocean, the wind picks up moisture in the Bay of Bengal and pours it over southern peninsular India
• The northeast monsoon accounts for 50-60% of rainfall in Tamil Nadu

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JET STREAMS

Overview

The polar and subtropical jet streams

• Jet streams are fast, narrow air currents in the atmosphere
• Jet streams are usually located near the tropopause (transition between troposphere and stratosphere)
• The main jet streams are westerly winds, flowing from the west to the east
• Jet streams are used for weather forecasting and aviation. It is hypothesised that they could be used as an energy source as well
  
• Jet streams have been observed in the atmosphere of Jupiter as well

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Cause of jet streams

• Jet streams are caused by a combination of atmospheric heating and the rotation of the earth
• They form near boundaries of adjacent air masses with significant differences in temperature

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Occurrence of jet streams

• The strongest jet streams are the polar jets (23,000-39,000 ft) and the somewhat weaker subtropical jets (33,000-52,000 ft)
• Other weaker jets also exist, especially over central USA
• There is one polar jet stream and one subtropical jet stream each in the northern and southern hemispheres
• The northern hemisphere polar jet is situated over the northern latitudes of North America, Europe and Asia, while the southern polar jet always circles Antarctica
• The northern and southern hemisphere jet streams have been found to be drifting towards the poles at a rate of 2.1 km per year
• Jet streams are typically a few hundred miles wide and about 3 miles thick vertically
• Wind speeds usually exceed 92 km/h, although speeds of over 398 km/h have been observed

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Jet streams and aviation

• Jet streams are often as the preferred flight plans for commercial airliners
• Flying with the jet streams decreases travel time and reduced fuel consumption
• Conversely, flying against jet streams can add to travel time and increase fuel consumption. For this reason, flight plans use circuitous routes to avoid flying against jet streams
• Commercial use of jet streams began in 1952 on the Tokyo-Honolulu route cutting travel time from 18 hours to 11.5 hours

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A FEW NOTEWORTHY LOCAL WINDS

Wind	Location	Description
Calima	Sahara to Canary Islands (west African coast)	Carries dust from the Sahara
Chinook	Rocky mountains	Warm, dry westerly winds
Elephanta	Malabar coast	South easterly wind Marks end of southwest monsoon
Nor’easter	North east USA	Strong storm winds from the northeast
Nor’wester	East coast of New Zealand	Warm dry winds
Santa Ana winds	Southern California	Strong, extremely dry winds Responsible for frequent wildfires
Sirocco	North Africa, Europe	Strong winds from the Sahara that cause dusty dry conditions in north Africa and cold wet conditions in Europe Reaches hurricane speeds, can last hours to days
Shamal	Persian Gulf	Strong Northwesterly wind Causes large sandstorms in Iraq