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).
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