Winds – Global and Local Winds – UPSC World Geography Notes

In basic terms, Wind is the sideways motion of air, driven by variations in air pressure within the Earth’s atmosphere. Air flows from regions of high pressure to those of low pressure. This discussion will delve into the various categories of wind.

Types of Wind

• Persistent Winds (Primary/Prevailing/Planetary Winds):
  • Trade winds
  • Westerlies
  • Easterlies
• Intermittent or Periodic Winds:
  • Seasonal Winds: Exhibit directional changes with the seasons (e.g., Indian monsoons)
  • Periodic Winds: Include land and sea breezes, mountain, and valley breezes
• Localized Winds:
  • Occur within specific timeframes during a day or season in limited geographical areas
  • Examples: Loo, Mistral, Foehn, Bora

Primary Winds or Prevailing Winds or Permanent Winds

• These are the planetary winds that sweep extensively across continents and oceans.
• Among them, the two winds with the most comprehensive understanding and significant impact on climate and human activities are the trade winds and westerly winds.

Trade Winds

• Trade winds blow from the sub-tropical high-pressure areas towards the equatorial low-pressure belt.
• Confined to the region between 30°N and 30°S across the Earth’s surface.
• In the northern hemisphere, they flow as north-eastern trades, and in the southern hemisphere, as south-eastern trades.
• Named trade winds because of their consistent and regular direction, aiding sea merchants in navigation.
• Coriolis force and Ferrel’s law explain the deflection of trade winds to the right in the northern hemisphere and to the left in the southern hemisphere.
• Originating in stable sub-tropical high-pressure belts, trade winds become humid and warmer as they pick up moisture on their way to the equator.
• Convergence of trade winds from both hemispheres at the equator leads to rising air and heavy rainfall.
• The eastern parts of trade winds, associated with cool ocean currents, are drier and more stable compared to the western parts of the ocean.

Westerlies

• The westerlies originate from the subtropical high-pressure belts (30°-35°) and move towards the sub-polar low-pressure belts (60°-65°) in both hemispheres.
• In the northern hemisphere, they blow from southwest to northeast, and in the southern hemisphere, they blow from northwest to southeast.
• The southern hemisphere westerlies are stronger and more consistent due to the expansive oceans, while the northern hemisphere westerlies are irregular due to the uneven land relief.
• In the northern hemisphere, the westerlies are more complex and less effective during summer, becoming more vigorous in winter.
• These winds, especially in the southern hemisphere, gain strength and become stormy due to the lack of land and dominance of oceans.
• The westerlies pick up moisture over oceans, resulting in significant precipitation in western continental areas (e.g., north-west European coasts).
• The westerlies are most pronounced between 40° and 65°S latitudes, known as Roaring Forties, Furious Fifties, and Shrieking Sixties, which are challenging for sailors.
• The poleward boundary of the westerlies is highly variable with seasonal and short-term fluctuations, influencing weather patterns and producing wet spells.

Polar easterlies

• Dry and cold prevailing winds known as the Polar easterlies blow from the northeast to southwest in the Northern Hemisphere and from southeast to northwest in the Southern Hemisphere.
• These winds originate from the polar high-pressure areas towards the sub-polar lows.

Secondary Winds or Periodic Winds

• Their direction varies with the seasons.
• Monsoons represent a significant alteration of the planetary wind system on a large scale.
• Additional instances of periodic winds encompass phenomena like land and sea breezes, mountain and valley breezes, cyclones and anticyclones, as well as air masses.

Monsoons

• Monsoons were traditionally interpreted as large-scale land and sea breezes, constituting a massive convectional circulation.
• These atmospheric phenomena exhibit a seasonal reversal of wind direction.
• In summer, the northward movement of the sun and a powerful low-pressure core in the north-west of the Indian subcontinent draw the trade winds of the southern hemisphere towards the north. Crossing the equator, they deflect to the right due to the Coriolis force, arriving at the Asian landmass as south-west monsoons. Traveling over vast water expanses, they become moisture-laden and lead to substantial rainfall in India and neighboring regions.
• Conversely, in winter, a high-pressure core forms to the north of the Indian subcontinent, generating divergent winds that move southwards toward the equator. The apparent southward movement of the sun reinforces this anticyclonic movement, giving rise to north-east or winter monsoons responsible for precipitation along the east coast of India.
• The monsoon winds impact various regions, including India, Pakistan, Bangladesh, Myanmar, Sri Lanka, the Arabian Sea, Bay of Bengal, southeastern Asia, northern Australia, and China.
• Beyond India, in eastern Asiatic countries like China and Japan, the winter monsoon is more potent than the summer monsoon. (Detailed study of monsoons will be undertaken while examining the Indian climate.)

Land Breeze and Sea Breeze

• Land and sea exhibit distinct heat absorption and transfer characteristics. In daylight, land heats up rapidly, surpassing the warmth of the sea. Consequently, a low-pressure area forms over the land, while the relatively cool sea maintains high pressure. This establishes a pressure gradient from sea to land, initiating the sea breeze as the wind blows from the sea towards the land. During the night, conditions reverse. Land loses heat more swiftly and becomes cooler than the sea, leading to a pressure gradient from land to sea. This reversal results in the land breeze, with the wind blowing from the land towards the sea.

Valley Breeze and Mountain Breeze

• In mountainous terrains, daytime heating causes the slopes to warm up, leading to upslope air movement. To fill the resulting gap, air from the valley ascends, resulting in the valley breeze. Conversely, at night, as the slopes cool, dense air descends into the valley, creating the mountain wind. The katabatic wind, deriving from the cool air of high plateaus and ice fields draining into the valley, is a distinct form of this phenomenon.
• On the leeward side of mountain ranges, another warm wind, known as the katabatic wind, emerges. As these winds cross the mountains, moisture condenses and precipitates. Upon descending the leeward slope, the dry air undergoes warming through the adiabatic process. This dry, warmed air has the potential to rapidly melt snow.

Tertiary Winds or Local Winds

• Differences in temperature and pressure at the local level give rise to localized winds.
• These winds are limited to specific areas and remain within the lowest layers of the troposphere. The following are examples of such local winds.

Loo

• Destructive Breeze
• Across the plains of northern India and Pakistan, an occasionally scorching and arid wind sweeps in from the west during the months of May and June, typically in the afternoons. Referred to as, its temperature consistently hovers between 45°C and 50°C, posing a potential threat of sunstroke to individuals.

Foehn or Fohn

• Favorable Breeze
• The Foehn, a warm wind regionally significant in the Alps, is a robust, gusty, dry, and warm wind that arises on the downwind side of a mountain range. With the windward side extracting moisture from the incoming air through orographic precipitation, the descending air on the leeward side becomes dry and warm (known as Katabatic Wind).
• The wind’s temperature ranges from 15°C to 20°C, contributing to animal grazing by melting snow and promoting the ripening of grapes.

Chinook

• Helpful Breeze
• Winds resembling the Foehn in the USA and Canada descend along the western slopes of the Rockies.
• This wind proves advantageous for ranchers situated to the east of the Rockies, as it prevents snow accumulation on the grasslands for a significant portion of the winter.

Mistral

• Destructive Breeze
• Mistral is a regional term for winds that sweep from the Alps across France towards the Mediterranean Sea, funneled through the Rhone River valley.
• Characterized by high speed, these winds are extremely cold and dry, often ushering blizzards into southern France.

Sirocco

• Detrimental Gale
• Sirocco, a Mediterranean wind originating from the Sahara, attains hurricane speeds in North Africa and Southern Europe.
• It emerges from a warm, dry, tropical air mass drawn northward by eastward-moving low-pressure cells over the Mediterranean Sea, originating in the Arabian or Sahara deserts.
• The hotter, drier continental air combines with the cooler, wetter air of the maritime cyclone, and the counterclockwise circulation of the low propels the amalgamated air across the southern coasts of Europe.
• The Sirocco induces dry and dusty conditions along the northern coast of Africa, prompts storms in the Mediterranean Sea, and brings about cool and wet weather in Europe.

Table of Major Local Wind Systems

• Brick Fielder: A scorching north-eastern summer wind in Australia that carries dust and sand across the region.
• Chinook: A warm, dry wind in the Rocky Mountains, USA, valued by cattlemen for its ability to rapidly clear snow cover; named after a local Indian tribe.
• Foehn: A warm, dry European wind descending from mountainsides.
• Haboob: The Arabic term for a violent wind that raises sandstorms, particularly in North Africa.
• Levanter: A pleasant, moist east wind bringing mild weather to the Mediterranean.
• Mistral: A violent, dry, cold north-west wind along the coasts of Spain and France.
• Sirocco: The hot, dry south wind crossing North Africa from the Sahara, becoming intensely hot and humid upon reaching the sea.
• Elephanta: On the Malabar coast, a south-easterly wind marking the end of the southwest monsoon.
• Nor’easter: In the northeast USA, powerful storm winds from the northeast.
• Nor’wester: Along the east coast of New Zealand, warm, dry winds.
• Santa Ana Winds: In Southern California, strong, extremely dry winds frequently contributing to wildfires.
• Shamal: In the Persian Gulf, a robust northwesterly wind causing significant sandstorms in Iraq.
• Calima: From the Sahara to the Canary Islands (west African coast), carrying dust from the Sahara.

How is Wind Measured?

Wind possesses both speed and direction, and two distinct devices are employed to measure these parameters:

1. Anemometers: Utilized for measuring the speed of the wind.
2. Wind vanes: Employed for determining the direction of the wind.

Causes of Wind

The primary factor leading to the generation of wind is the disparate heating of two regions.

Examples include:

1. Uneven heating between land and sea.
2. Uneven heating between the equator and the poles.

Significance of Winds

• Wind holds significance in forecasting current weather conditions and future predictions.
• It plays a crucial role in transporting moisture and temperature between different locations. Numerous daily weather patterns are impacted by the movement of the wind.
• The velocity of surface wind typically dictates the direction of flight operations at an airport, and airfield runways are no exception to this rule.

Conclusion

Wind stands as the primary catalyst for erosion in arid regions. The overarching wind circulation carries minuscule particles such as dust across vast distances, spanning thousands of kilometers downwind from their origin over expansive oceans. The west-to-east movement of ocean currents worldwide is propelled by westerly winds in the mid-latitudes of the planet. Wind assumes a vital role in the dissemination of seeds, spores, pollen, and other stationary organisms by plants and various immobile species. Although wind may not be the principal method of seed dispersal in plants, it significantly contributes to the biomass of terrestrial plants.

FAQs on Winds

Q1: What is the primary difference between global wind and local wind?

A1: Global winds are large-scale air movements that occur over vast regions, influenced by the Earth’s rotation and the sun’s uneven heating. Local winds, on the other hand, are smaller-scale winds that are influenced by local geographical features.

Q2: How are global winds generated?

A2: Global winds are generated due to the uneven heating of the Earth’s surface by the sun, creating pressure differences and causing air to move from high to low-pressure areas.

Q3: What are some examples of global winds?

A3: Examples of global winds include the trade winds, westerlies, and polar easterlies.

Q4: What factors influence local winds?

A4: Local winds are influenced by factors such as temperature variations between land and water, the presence of mountains, and the specific characteristics of local geography.

Q5: Can local winds change direction frequently?

A5: Yes, local winds can change direction more frequently than global winds due to their dependence on local geographical features and temperature variations.

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