PRESSURE SYSTEMS AND WIND SYSTEMS

A) Atmospheric pressure

  • The weight of a column of air contained in a unit area from the mean sea level to the place of measurement of pressure is called the atmospheric pressure.
  • The atmospheric pressure at sea level is 1034 gm per square centimetre.
  • Atmospheric pressure varies from place to place due to differences in topography, sun’s insolation and related weather and climatic factors.

B) Atmospheric pressure cells

  • When heated, the volume of a parcel of air increases (air expands) and hence the pressure within the air parcel falls creating a low-pressure cell.
  • When cooled, the volume of the air parcel decreases (air is compressed) and hence the pressure within the air parcel increases creating a high-pressure cell.
  • A combination of atmospheric pressure cells give rise to distinct pressure systems in atmospheric system
  • Distribution of continents and oceans have a marked influence over the distribution of pressure.
  • In winter, the continents are cooler than the oceans and tend to develop high-pressure centres, whereas, in summer, they are relatively warmer and develop low pressure. It is just the reverse with the oceans.

C) Vertical Variation of Pressure

  • In the lower atmosphere, the pressure decreases rapidly with height. however, this decrease in pressure is not directly proportional to increasing height but varies with temperature and water vapour density of air.
  • Since air pressure is proportional to density as well as temperature, it follows that a change in either temperature or density will cause a corresponding change in the pressure.
  • the atmospheric pressure decreases at rate of about 34 millibars every 300 metres of height.
  • The vertical pressure gradient force is much larger than that of the horizontal pressure gradient. However, it is generally balanced by a nearly equal and opposite gravitational force. Hence, we do not experience strong upward winds.
  • At the height of Mt. Everest, the air pressure is about two-thirds less than what it is at the sea level.

 

FACTORS AFFECTING WIND MOVEMENT

Wind: horizontal movement of air

Currents: vertical movement of air.

  • The factors that affect wind movement are pressure gradient force, buoyant force, friction, Coriolis force, gravitational force and centripetal acceleration.
  1. Pressure Gradient Force
    • The pressure gradient (difference in pressure) between atmospheric pressure cells and the surroundings causes the movement of air from relatively high-pressure centres to relatively low-pressure centres and it is called as wind. Greater the pressure difference, greater is the wind speed.
  1. Buoyant force.
    • The atmospheric pressure cells also determine whether the air sinks or rises at a place.
    • The surrounding atmosphere exerts buoyant force on low-pressure cells and hence the air within a low-pressure cell rises.
    • On the other hand, the air within a high-pressure cell sinks as it is denser than the surrounding atmosphere.
    • Rising air is associated with convergence and unstable weather (cyclonic conditions) whereas the sinking (subsiding) air is associated with divergence and stable conditions (anticyclonic conditions).
    • A rising pressure indicates increasing stability, while a falling pressure indicates the weather becoming more unstable.
    • The converging wind movement around a low is called cyclonic circulation.
    • Around a high, the wind diverges, and the movement is called anti-cyclonic circulation.
    • The wind circulation at the earth’s surface is associated with an exactly opposite wind circulation above in the upper troposphere.
    • Apart from convergence, convection currents, orographic uplift and uplift along fronts cause the rising of air, which is essential for the formation of clouds and precipitation.

 

Pressure System Pressure Condition Pattern of Wind Direction
Northern Hemisphere Southern Hemisphere
Cyclone Low Anticlockwise Clockwise
Anticyclone High Clockwise Anticlockwise

 

  1. Frictional Force
    • The irregularities of the earth’s surface resist the wind movement through friction.
    • The influence of friction generally extends up to an elevation of 2-3 km.
    • The friction is minimal over the oceans and maximum when wind flows close to the continental surface.
    • Friction causes change in wind speed and also its direction when Coriolis force, friction and pressure gradient (wind speed) acts together.
  1. Coriolis force
    • Due to rotation of the earth, 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

 

 

  • Coriolis force causes winds in the northern hemisphere to get deflected to right of their path and those in the southern hemisphere to left of their path (Farrell’s Law).
  • This deflection force does not act until the air is set in motion and increases with wind velocity and an increase in latitude. (frictional force acts opposite to Coriolis force)

Coriolis effect

  • The Coriolis effect is the apparent deflection of objects (such as aeroplanes, wind, missiles, sniper bullets and ocean currents) moving in a straight path relative to the earth’s surface.
  • Coriolis force is zero at the equator but increases with latitude, reaching a maximum at the poles.

 

 

5.Geostrophic Wind

  • The Coriolis force acting on a body increases with increase in its velocity.
  • The winds in the upper atmosphere, 2-3 km above the surface, are free from frictional effect of the surface and are controlled by the pressure gradient and the Coriolis
  • 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 (deflection of the wind is maximum).
  • This wind is known as the geostrophic wind.

 

 

No tropical cyclones at the equator.

  • The low pressure close to the equator gets filled instead of getting intensified, e., there is no spiralling of air due to zero Coriolis effect. The winds directly get uplifted vertically to form thunderstorms.

 

 

 

HORIZONTAL DISTRIBUTION OF PRESSURE

  • Horizontal distribution of pressure is studied by drawing isobars at constant levels. There are seven distinctly identifiable zones of horizontal pressure systems or pressure belts.
    1. Equatorial low,
    2. The sub-tropical highs (along 30° N and 30° S),
    3. The sub-polar lows (along 60° N and 60° S), and
    4. The polar highs.

Except the equatorial low, all others form matching pairs in the northern and southern hemispheres

 

 

  • The pressure belts are not permanently fixed but keep oscillating with the apparent movement of the sun. In the northern hemisphere in winter they move southwards and in the summer northwards.

A) Equatorial Low-Pressure Belt or ‘Doldrums’

  • Th equatorial low-pressure belt lies between 10°N and 10°S latitudes with position keeps changing with apparent movement of sun.Its width may vary seasonally between 5°N and 5°S and 20°N and 20°S.
  • This belt is thezone of convergence of trade winds (Intertropical Convergence Zone or ITCZ) from two hemispheres from sub-tropical high-pressure belts.
  • This belt is also called the doldrums, because of the extremely calm air movements.

 

 

How it is formed and what climatic conditions exists in equatorial lows.?

  • This region receives maximum insolation being nearest to equator. Due to intense heating, the air gets heated up creating a low-pressure region.
  • The air at the margins of the low-pressure region rises (convection) giving rise to clouds and turbulent weather along the margins.
  • Only vertical currents are found, and the surface windsare almost absent since winds rise near the margin
  • The region within the belt is characterised by extremely low pressure yet calm weather conditions.
  • As the larger part of the low-pressure belt passes along the oceans, the winds obtain huge amount of moisture.
  • Vertical winds carrying moisture from cumulonimbus thunderstorm clouds (convectional rainfall).The rising air loses all its moisture by the time it reaches the upper parts of the troposphere.
  • In spite of high temperatures and moisture, cyclones are not formed 5°N and 5°S of the equator because of negligible Coriolis.

B) Sub-Tropical High-Pressure Belt or Horse Latitudes

  • The sub-tropical highs extend from near the tropics (27-28 degree N or S) to about 35°N and S.

How it is formed and what climatic conditions exists in Subtropical highs?

  •  After shedding its moisture while reaching near the troposphere the wind moves away from the equatorial low-pressure belt and the subtropical low-pressure belt in the upper troposphere is dry and cold.
  • The blocking effect of air at upper levels caused by the Coriolis force forces the cold, dry air to subside at 30°N and S.
  • So, the high pressure (dynamically formed) along this belt is due to subsidence of air coming from the equatorial region and the subpolar region.
  • The subsiding air is warm and dry, therefore, most of the deserts are present along this belt, in both hemispheres.

 

 

  • A calm condition anticyclonic condition is created in this high-pressure belt.
  • The descending air currents feed the winds blowing towards adjoining low-pressure
  • This belt is frequently invaded by tropical and extra-tropical disturbances.

 Horse Latitudes

  • The corresponding latitudes of sub-tropical high-pressure belt are called horse latitudes.
  • In early days, the sailing vessels with cargo of horses found it difficult to sail under calm conditions of this high-pressure belt.
  • They had to throw horses into the sea when ships were struck in the middle of these latitudes.

 C) Sub-Polar Low-Pressure Belt

  • The subpolar low-pressure belts are located between 45°N and the Arctic circle (66.5° N) and 45°S and the Antarctic circles (66.5° S) respectively.

 Formation

  • These are dynamically produced due to
    1. Coriolis Forceand.
    2. Ascent of air as a result of convergence of westerlies (coming from the subtropical high-pressure regions) and polar easterlies (coming from the polar regions).
  • Subpolar low-pressure belts are mainly encountered above oceans.

 Seasonal behaviour

  • During winter, because of a high contrast between land and sea, this belt is broken into two distinct low centres – one in the vicinity of the Aleutian Islands and the other between Iceland and Greenland.
  • During summer, a lesser contrast results in a more developed and regular belt.
  • The belt in the southern hemisphere is not as well differentiated.

 Climate

  • The area of contrast between cold and warm air masses produces polar jet streams which encircles the earth at 60 degrees latitudes and is focused in these low-pressure areas.

D) Polar High-Pressure Belt

  • The polar highs are small in area and extend around the poles.
  • They lie around poles between 80 – 90° N and S latitudes.

Formation

  • The air from sub-polar low-pressure belts after saturation becomes dry. This dry air becomes cold while moving towards poles through upper troposphere.
  • The cold air (heavy) on reaching poles subsides creating a high-pressure belt at the surface of earth.

 

FACTORS CONTROLLING PRESSURE SYSTEMS

A) Thermal Factors

  • When air is heated, it leads to low pressure, and when it is cooled, it leads to high pressure.
  • Formation of equatorial low and polar highs are examples of thermal lows and thermal highs.

B) Dynamic Factors

  • Apart from variations of temperature, the formation of pressure belts may be explained by dynamic factors arising out of pressure gradient forces, apparent movement of sun and rotation of the earth (Coriolis force).

 

PRESSURE SYSTEMS AND GENERAL CIRCULATION

  • The pattern of planetary winds depends on:
    • latitudinal variation of atmospheric heating;

 

 

    • emergence of pressure belts;
    • the migration of belts following apparent movement of the sun;
    • the distribution of continents and oceans;
    • the rotation of earth.
  • The pattern of the movement of the planetary winds (permanent winds) is called the general circulation of the atmosphere.
  • The general circulation of the atmosphere also sets in motion the ocean water circulation which influences the earth’s climate.

 

 

A) Hadley Cell

  • The air at the equatorial low-pressure belt rises because of the convection currents.
  • The air after reaching the top of the troposphere (altitude of 14 km) starts to move towards the poles. The Coriolis force act strongly as there is no friction at this altitude.
  • This causes accumulation of air at about 30° N and S causing it to sink to the ground and forms a subtropical high.
  • At the surface a component of the diverging wind from the subtropical high flows towards the equator as the easterlies (northeast to southwest).
  • The easterlies from either side of the equator converge at the equatorial low pressure and the cycle repeats.
  • Such circulations of wind in the tropics is called Hadley Cell.

B) Ferrel Cell

  • In the middle latitudes, the circulation is that of sinking cold air that comes from the poles and the rising warm air that blows from the subtropical high.
  • At the surface, these winds are called westerlies, and the cell is known as the Ferrel cell

C) Polar Cell

  • At polar latitudes, the cold dense air subsides near the poles and blows towards middle latitudes as the polar easterlies. This cell is called the polar cell.

These three cells set the pattern for the general circulation of the atmosphere. The transfer of heat energy from lower latitudes to higher latitudes maintains the general circulation.

 

 

 

CLASSIFICATION OF WINDS

A) Permanent winds /Planetary Winds

  • The trade winds, westerlies and polar easterlies.

B) Secondary or Periodic Winds

  • Seasonal winds: These winds change their direction in different seasons. E.g. Monsoons in South Asia.
  • Periodic winds: Land and sea breeze, mountain and valley breeze etc.

C) Local winds

  • These are blow during a particular period of the day or year in a specific countrywhere they are named differently.
  • Winds like Loo, Mistral, Foehn, Bora etc.

 

PRIMARY WINDS OR PREVAILING WINDS OR PLANETARY WINDS

  • The winds blowing almost in the same direction throughout the year are called prevailing or permanent winds.
  • The two most significant winds for climate and human activities are trade winds and westerly winds.

 A) The Trade Winds

  • The trade winds are those blowing from the sub-tropical high-pressure(30 degree N or S) areas towards the equatorial low-pressure (equator)
  • They flow as the north-eastern trades in the northern hemisphere and the south-eastern trades in the southern hemisphere.
  • Trade winds are descending and stable in areas of their origin (sub-tropical high-pressure belt), and as they reach the equator, they become humid and warmer after picking up moisture on their way.
  • The trade winds from two hemispheres meet near the equator, and due to convergence, they rise and cause heavy rainfall.
  • The eastern parts of the trade winds associated with the cool ocean currents are drier and more stable than the western parts of the ocean.

 B) The Westerlies

  • The westerlies are the winds blowing from the sub-tropical high-pressure belts towards the sub-polarlow-pressure
  • They blow from south­west to northeast in the northern hemisphere and northwest to southeast in the southern hemisphere.
  • The westerlies of the southern hemisphere are stronger and persistent due to the vast expanse of water, while those of the northern hemisphere are irregularbecause of uneven relief of vast land-masses.
  • The westerlies are best developed between 40° and 65°S latitudes. These latitudes are often called Roaring Forties, Furious Fifties, and Shrieking Sixties.
  • The poleward boundary of the westerlies is highly fluctuating.
  • These winds produce wet spells and variability in weather.

 C) The 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 high-pressure polar areas of the sub-polar lows.

 

SEASONAL WINDS

A) Monsoons

  • The monsoons are characterized by seasonal reversal of wind direction.
  • During summer, the trade winds of southern hemisphere are pulled northwards by an apparent northward movement of the sun (ITCZ) and by an intense low-pressure core in the north-west of the Indian sub­continent.
  • While crossing the equator, these winds get deflected to their right under the effect of Coriolis
  • These winds now approach the Asian landmass as south-west monsoons.
  • During winter, these conditions are reversed, and a high-pressure core is created to the north of the Indian subcontinent.
  • Divergent windsare produced by this anticyclonic movement which travels southwards towards the equator. This movement is enhanced by the apparent southward movement of the sun.
  • These are north-east or winter monsoons which are responsible for some precipitation along the east coast of India.

 

 

  • The monsoon winds flow over India, Pakistan, Bangladesh, Myanmar (Burma), Sri Lanka, the Arabian Sea, Bay of Bengal, south-eastern Asia, northern Australia,China and Japan.
  • Outside India, in the eastern Asiatic countries, such as China and Japan, the winter monsoon is stronger than the summer monsoon.

 B) Land Breeze and Sea Breeze

  • 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.

 

 

 

TERTIARY OR LOCAL WINDS

A) Loo

  • In the plains of northern India and Pakistan becomes hot during summers and dry wind blows from the west in May and June, usually in the afternoons. It is known as loo.
  • Its temperature invariably ranges between 45 °C and 50 °C. It may cause sunstroke to people.

 B) Foehn or Fohn

  • Foehnis a hot wind of local importance in the Alps.
  • As the windward forces wind to shed moisture as orographic precipitation, the air that descends on the leeward side is dry and warm (katabatic wind).
  • The temperature of the wind varies between 15°C and 20°C.
  • The wind helps animal grazing by melting snow and aids the ripening of grapes.

 C) Chinook

  • Chinooks are foehn like winds in USA and Canada move down the leeward slopes of the Rockies.
  • It is beneficial to ranchers east of the Rockies as it keeps the grasslands clear of snow during much of the winter.

 D) Mistral

  • Mistral is one of the local names given to winds that blow from the Alps over France towards the Mediterranean Sea.
  • It is channelled through the Rhone valley. It is very cold and dry with high speed.
  • It brings blizzards into southern France.

 E) Sirocco

  • Sirocco is a Mediterranean wind that comes from the Sahara and reaches hurricane speeds in North Africa and Southern Europe.
  • It arises from a warm, dry, tropical air mass that is pulled northward by low-pressure cells moving eastward across the Mediterranean Sea, with the wind originating in the Sahara deserts.
  • The hotter, drier continental air mixes with the cooler, wetter air of the maritime cyclone, and the counter-clockwise circulation of the low propels the mixed air across the southern coasts of Europe.
  • The Sirocco causes dusty dry conditions along the northern coast of Africa, storms in the Mediterranean Sea, and cool, wet weather in Europe.

 

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