WINDS

Wind:-Horizontal movement of air is called wind.

Air Current:-The vertical or nearly vertical movement of air is referred to as air current.

Factors controlling the direction and speed of wind:-

• Pressure gradient force
• Frictional Force
• The Coriolis Effect

Pressure Gradient Force and Wind Movement

• The rate of change of pressure with respect to distance is the pressure gradient.
• Pressure Gradient Force operates from the high pressure area to a low pressure area and causes wind movement.
• The pressure gradient is strong where the isobars are close to each other and is weak where the isobars are apart.
• Since a closely spaced gradient implies a steep pressure change, it also indicates a strong wind speed.
• The wind direction follows the direction of change of pressure, i.e. perpendicular to the isobars.

Coriolis Force and Wind Movement

• The rotation of the earth about its axis affects the direction of the wind. This force is called the Coriolis force. It has great impact on the direction of wind movement.
• Due to the earth’s rotation, winds do not cross the isobars at right angles as the pressure gradient force directs, but get deflected from their original path.
• This deviation is the result of the earth’s rotation and is called the Coriolis effect or Coriolis force.
• Due to Coriolis effect, winds in the northern hemisphere get deflected to the right of their path and those in the southern hemisphere to their left, following Farrell’s Law.
• This deflection force does not seem to exist until the air is set in motion and increases with wind velocity, air mass and an increase in latitude.
• The Coriolis force acts perpendicular to the pressure gradient force (pressure gradient force is perpendicular to an isobar)

Why are there no tropical cyclones at the equator?

The Coriolis force is directly proportional to the angle of latitude. It is maximum at the poles and is absent at the equator.

At the equator (Coriolis force is zero) wind blows perpendicular to the isobars. The low pressure gets filled instead of getting intensified i.e., there is no spiraling of air due to zero Coriolis effect.

The winds directly gets uplifted vertically to form thunderstorms.

Cyclone & anticyclone

• The wind movement at the earth’s surface around low and high on many occasions is closely related to the wind circulation at higher level.
•  Generally, over low pressure area the air will converge and rise.
• Over high pressure area the air will subside from above and diverge at the surface.

geostrophic wind

When isobars are straight and when there is no friction, the pressure gradient force is balanced by the Coriolis force and the resultant wind blows parallel to the isobar.

This wind is known as the geostrophic wind.

Frictional Force and Wind Movement

• The irregularities of the earth’s surface offer resistance to the wind movement in the form of friction.
• It affects the speed of the wind. It is greatest at the surface and its influence generally extends up to an elevation of 1 – 3 km. Over the sea surface the friction is minimal.
• Over uneven terrain, however, due to high friction, the wind direction makes high angles with, isobars and the speed gets retarded.

Causes of the Coriolis Effect

• The main cause of the Coriolis effect is the earth’s rotation. As the earth spins in a counter-clockwise direction on its axis anything flying or flowing over a long distance above its surface appears to be deflected.
• As latitude increases and the speed of the earth’s rotation decreases, Coriolis effect increases.
• A plane flying along the equator itself would be able to continue flying on the equator without any apparent deflection. A little to the north or south of the equator, the plane would be deflected.
• In addition to the speed of the earth’s rotation and latitude, the faster the object itself is moving, the more deflection there will be.

Impact of the Coriolis Effect

• Some of the most important impacts of the Coriolis effect in terms of geography are the deflection of winds and currents in the ocean.
• It also has a significant effect on man-made items like planes and missiles.

Types of Winds

1. Planetary / permanent  winds
2. Periodic Winds
3. Local Winds

Primary Winds or Prevailing Winds or Permanent Winds or Planetary Winds

• These are the planetary winds which blow extensively over continents and oceans.
• The two most well- understood and significant winds for climate and human activities are trade winds and westerly winds.

Trade Winds

• The trade winds are those blowing from the sub-tropical high pressure areas towards the equatorial low pressure belt.
• Therefore, these are confined to a region between 30°N and 30°S throughout the earth’s surface.
• They flow as the north-eastern trades in the northern hemisphere and the south-eastern trades in the southern hemisphere.
• This deflection in their ideally expected north-south direction is explained on the basis of Coriolis force and Farrel’s law.

Westerlies

• The winds which blow from the Horse latitudes towards the polar region throughout the year with varying intensity and cause rain near the Polar Regions.
•  Westerlies are stronger in Southern Hemisphere because of the vast expanse of ocean water.
• They are also described as Roaring Forties, furious Fifties and Shrieking Sixties.

Polar easterlies

• The Polar easterlies are dry, cold prevailing winds blowing from north-east to south-west direction in Northern Hemisphere and south-east to north-west in Southern Hemisphere.
• They blow from the polar high-pressure areas of the sub-polar lows.

Periodic Winds

• These winds change their direction with change in season.
• Monsoons are the best example of large-scale modification of the planetary wind system.
• Other examples of periodic winds include land and sea breeze, mountain and valley breeze, cyclones and anticyclones, and air masses.

Monsoon

Land & sea breeze

• The land and sea absorb and transfer heat differently.
• During the day the land heats up faster and becomes warmer than the sea.
• Therefore, over the land the air rises giving rise to a low pressure area, whereas the sea is relatively cool and the pressure over sea is relatively high.
• Thus, pressure gradient from sea to land is created and the wind blows from the sea to the land as the sea breeze.
• In the night the reversal of condition takes place.
• The land loses heat faster and is cooler than the sea.
• The pressure gradient is from the land to the sea and hence land breeze results.

Mountain and Valley Winds

• In mountainous regions, during the day the slopes get heated up and air moves upslope and to fill the resulting gap the air from the valley blows up the valley.
• This wind is known as the valley breeze. During the night the slopes get cooled and the dense air descends into the valley as the mountain wind.
• The cool air, of the high plateaus and ice fields draining into the valley is called katabatic wind.

Tertiary Winds or Local Winds

• Local differences of temperature and pressure produce local winds.
• Such winds are local in extent and are confined to the lowest levels of the troposphere.

Local Winds

Local Winds

Introduction:

Local winds occur on a small spatial scale, their horizontal dimensions typically several tens to a few hundreds of kilometres. They also tend to be short-lived lasting typically several hours to a day. There are many such winds around the world, some of them cold, some warm, some wet, some dry. There are many hazards associated with the winds.

Planetary winds or Primary Winds are those which blow extensively over continents and oceans. The winds that blow constantly throughout the year and blow constantly in a particular direction. There are types of permanent winds namely Trade winds, Easterlies and Westerlies.

Body:

Types and Impact of local winds on the weather:

Periodical winds: The winds originating from diurnal temperature and pressure variation are known as Periodical and they generally complete their cycle in a day/ 24 hour like Land & Sea Breeze and Mountain & Valley Breeze.

Land and Sea Breeze: Land and Sea Breeze is generated by the diurnal variation of pressure. Due to this reason, the Land and Sea Breeze are sometimes known as diurnal Monsoon.

Land Breeze:

• At night reversal of sea breeze may occur but with somewhat weaker characteristics as the temperature and pressure gradient are less steeper during the night.
• During night land breeze is established since land cools to a temperature lower than the adjacent water setting up a pressure gradient from land to sea
• The horizontal and vertical extent of the Land Breeze helps in moderation of temperature of a coastal area during night time as it maintains regular circulation
• Land Breeze usually attains its maximum intensity in the early morning hours and dies out soon after sunup.

Sea Breeze:

• The sea breeze develops along seacoasts or large inland water bodies when the land heats much faster than the water on a clear day and a pressure gradient is directed high over the water to low over the land.
• Impact of Sea breeze rapidly declines landward and impact is limited to 50km.
• Land- Sea Breeze system is very shallow as the average depth of the land and sea breeze, varies from 1000-2000M in tropical regions and over the lakes, the depth is even lesser.
• Sea Breeze brings cool marine air and thus help in moderation of coastal temperature and due to the sea breeze, coastal regions record a drop of 5-10 0C in their temperature
• It also frequently causes late afternoon rainfall in these coastal areas, particularly during summer.
• Due to the location nearer to the lakes, places experience the Lake Effect like Chicago, due to its location near a lake presents a typical example of lake effect- where lakeside areas are cooler than the much warmer outlying areas in the summer.

Mountain and Valley Breeze: These winds develop over areas with large differences in relief and majorly caused by the temperature gradient that exists between Mountain Slopes and valleys.

Valley Breeze:

• Due to the intense insolation during the daytime, the slopes of the mountain heat up rapidly but the free atmosphere above the lowlands is not heated to some extent.
• As the valleys receive comparatively lesser insolation so relatively high pressure sets up in the valleys while along the mountain slopes due to more heating the warm air is uplifted, and low pressure sets up.
• Thus, the air moves from the Valleys towards the slopes (High pressure to the low pressure) and this upslope movement of air is known as valley breeze.
• Valley breezes are also known as Anabatic Wind.
• Weather associated with the Valley Breeze
• This type of upslope winds in the Mountainous region may cause occasional and afternoon thundershowers on warm and humid days.
• Sometimes, the valley breezes are also accompanied by the formation of cumulus cloud near mountain peaks or over slopes and escarpments.

Mountain Breeze:

• On mountain-sides under the clear night sky, the higher land (upslope land) radiates heat and is cooled and in turn cools the air in contact with it. The cool denser air flows down the mountain slope due to the pressure difference since the valley is warmer and at relatively lower pressure.
• This flow of the air is termed as Mountain Breeze and they are also known as Katabatic wind.
• Weather associated with the Mountain Breeze
• By the morning the mountain breeze produces temperature inversions and valley bottom becomes colder than the Mountain Slopes.
• Thus, the valley floors are characterised by frost during the night while upper part/ hill-side are free from frost in cold areas.

Non-Periodical winds: Only present during a season and are classified as Hot and Cold Winds.

Hot Local Winds: Hot Local winds are produced generally by the mechanism of downslope compressional heating also known as adiabatic heating. The examples of the Hot Local Winds include Chinook, Harmattan, Foehn, Sirocco, Norwester, Brickfielder, Khamsin, Santa Ana, Loo etc.

• Chinook:
  • Chinook is the name of hot and dry local wind, which moves down the eastern slopes of the Rockies in U.S.A. and Canada.
  • The literal meaning of chinook is snow eater as they help in melting the snow earlier.
  • During winter Great Plain of North America are very cold and frozen, Chinook with its arrival increase the temperature and bring relief to the people and at the same time, the rise in temperature due to Chinook also helps in early sowing of spring wheat in the USA.
  • They keep the grasslands clear of snow. Hence they are very helpful to ranchers.
• Foehn:
  • Foehn is strong, dusty, dry and warm local wind, which develops on the leeward side of the Alps mountain ranges.
  • Regional pressure gradient forces the air to ascend and cross the barrier.
  • Ascending air sometimes causes precipitation on the windward side of the mountains.
  • After crossing the mountain crest, the Foehn winds start descending on the leeward side or northern slopes of the mountain as warm and dry wind.
  • The temperature of the winds varies from 15°C to 20°C, which help in melting snow.
  • Thus making pasture land ready for animal grazing and help the grapes to ripe early.
• Harmattan:
  • Harmattan, hot, dry wind that blows from the northeast or east in the western Sahara.
  • It usually carries large amounts of dust, which it transports hundreds of kilometres out over the Atlantic Ocean.
  • The dust often interferes with aircraft operations and settles on the decks of ships.
  • The interaction of the Harmattan with monsoon winds can cause tornadoes.
  • Humidity drops to as low as 15 per cent, which can result in spontaneous nosebleeds for some people.
  • The wind can also increase fire risk and cause severe crop damage
• Loo:
  • Loo is hot and dry winds, which blow very strongly over the northern plains of India and Pakistan in the months of May and June.
  • Their direction is from west to east and they are usually experienced in the afternoons.
  • Their temperature varies between 45°C to 50°C.
  • They have desiccating effects and are considered as environmental hazards.
  • It causes heat waves and can cause heat stroke.

Cold Local Winds: Cold local winds are dust-laden winds and as they have a temperature below freezing point, they create Cold Wave condition. The examples of Cold Local winds include-Mistral, Bora, Northers, Blizzard, Purga, Laventer, Pampero, Bise etc.

• Mistral:
  • Mistrals are most common local cold winds.
  • They originate on the Alps and move over France towards the Mediterranean Sea through the Rhone valley.
  • They are very cold, dry and high-velocity winds.
  • They bring down temperature below freezing point in areas of their influence.
  • People in these areas protect their orchards and gardens by growing thick hedges and build their houses facing the Mediterranean sea.
• Bora:
  • These are cold and dry north-easterly winds which blow from the mountains towards the eastern shore of Adriatic Sea.
  • Bora is more effective in North Italy since here it descends the southern slopes of the Alps, although due to descend it gets adiabatically heated still its temperature is very low in comparison to the coastal area and these are the typical example of fall winds.
  • Bora has often associated with the passage of a temperate Cyclone and at times the Bora winds themselves attain the hurricane force at the foot of the mountain and may cause disastrous impacts on properties.
• Blizzard:
  • Blizzard is cold, violent, powdery polar winds (pick dry snow from the ground)
  • They are prevalent in the north and south polar regions, Canada, USA, Siberia etc. Due to the absence of any east-west Mountain barrier, these winds reach to the southern states of USA.

Conclusion:

Local differences of temperature and pressure produce local winds. Such winds are local in extent and are confined to the lowest levels of the troposphere.