19 Winds and Global Circulation of Winds

M K Nanda

1. Learning outcomes
2. Introduction
3. Atmospheric pressure
3.1. Measurement of atmospheric pressure
3.2. Mean sea level pressure
3.3. Constant pressure surface
3.4. Isobar and pressure gradient
3.5. Temporal variation of atmospheric pressure
4. Pressure gradient and wind
4.1. – Measurement of wind
– 4.1.1. Anemometers
– 4.1.2. Wind vane
– 4.1.3. Beaufort’s scale
4.2. – Forces acting on wind : driving force vs motion dependent force
4.2.1. Pressure gradient force
4.2.2. Coriolis force
4.2.3. Frictional force
4.2.4. Centrifugal force
5. Global circulation system
5.1. Single cell vs three cell model
5.1.1. Hardley cell
5.1.2. Ferrel cell
5.1.3. Polar cell
5.2. Surface features of global atmospheric circulation system
5.2.1. Global pressure belts
5.2.2. Global wind system
6. High altitude winds (wind aloft)
6.1. Geostrophic and gradient winds
6.2. Jet streams and Rossby waves
7. Local wind
7.1. Land and sea breezes
7.2. Mountain and valley breezes
8. Summary

1. Learning outcomes

• know about the different forces acting on wind understand the principles of wind measurement
• know about the global pressure belts and tri cellular wind system understand about the surface and upper air circulation system
• know about the land and sea breeze, mountain and valley breeze and other local wind systems

2. Introduction

The horizontal motion of air in attempts to equalize lateral differences in atmospheric pressure (and consequently other atmospheric properties such as temperature and humidity as well) is called as wind. In this tendency to reach equilibrium or stability in one part of the atmosphere invariably creates new differences elsewhere; the result is blowing of wind is a never ending process considering the entire atmosphere over the surface of the earth as a whole continuum. Wind is a vector quantity that is it has both speed and direction. Some wind movement is confined only to very small localities while others involve the entire globe. Simply speaking, temperature inequalities among adjacent air parcels is the root cause and consequently the formation of convective cells within the atmosphere is the principal mechanism of wind flow/ air circulation at any scale of movement. The above fact is more apparent at smaller scales. Large scale wind movements, say planetary scale wind, are relatively complicated due to connection with certain other factors apart from temperature such as rotational effects of earth and frictional resistance offered by the earth surface/atmosphere.

3. Atmospheric pressure

The pressure of air (Atmospheric pressure) at a given place is defined as force exerted by air on a unit surface by continuous collision of air molecules. The amount of pressure is determined by two factors: a) Temperature and b) Density of air

3.1.       Measurement of atmospheric pressure

Atmospheric pressure, P = h.ρ.g, [Here, h = Height of mercury column, ρ = Density of mercury and g =Acceleration due to gravity]

• = 76 x 13.5951 x 980.665 dynes cm-2 = 1013250.144 dynes cm-2
• = 01325 x 106 dynes cm-2 = 1.01325 x 105 newtons m-2

1 newton m-2 = 10 dynes cm-2 = 1 Pascal

1 hecto pascal (hPa) = 100 Pascal = 1 millibar

Atmospheric pressure is measured by the instrument called Barometer. Mercury barometers are used worldwide to measure atmospheric pressure. Aneroid barometer is another instrument used to measure atmospheric pressure. It has partially evacuated chamber with spring pointer arrangement on its wall so that the pressure exerted on it by the air outside, is indicated by the pointer. Aneroid  barometer is also used as altimeter to measure the elevation with assumption of a nonlinear relation between atmospheric pressure and altitude.

where, z = altitude T = absolute temperature, P = atmospheric pressure (0 subscript for sea level and c = constant

3.2. Mean sea level pressure

The atmospheric pressure decreases with elevation. The pressure at surface level is always higher than the pressure at higher altitude. For all practical purposes of weather reports (synoptic study and weather analysis on radio, television, and newspapers) the mean sea level pressure (MSLP) is used for reference. The mean sea level pressure is the atmospheric pressure reduced to sea level. Average sea-level pressure (or standard atmospheric pressure) is 1013.25 mbar (101.325 kPa; 760 mm Hg).

3.3. Constant pressure surface

A constant pressure (or isobaric) surface is a surface in the atmosphere where the pressure is equal everywhere along that surface. For example, the 100 millibar (mb) surface is the surface in the atmosphere where the pressure at every point along that surface is 100 mb. Since pressure decreases with height, the altitude of the 100 mb surface is higher than the 500 mb surface. Meteorologists use pressure as a vertical coordinate to simplify thermodynamic computations on a routine basis

Table 1: Pressure surface and corresponding altitude in troposphere

Pressure	Approximate height
Sea level	0 m
1000 mb	100 m
700 mb	3000 m
500 mb	5000 m
200 mb	12000 m
100 mb	16000 m

3.4. Isobar and pressure gradient

Isobars are the imaginary lines joining the places of equal mean sea level pressure at a given time. Pressure gradient is the change in pressure between the two points along a line perpendicular to the isobars divided by the distance between the two points. Close isobars imply steep pressure gradient.

3.5. Temporal variation of atmospheric pressure

The atmospheric pressure is related to the temperature. When the temperature increases, the air expands and the density decreases. This lowers atmospheric pressure near that portion of the Earth’s surface. The average hourly pressure of a place over a long period of time shows a definite rhythm. The atmospheric pressure shows a bimodal trend with two maxima at 10 am and 10 pm and minima at 4 am and 4 pm at the time of equinoxes. Similarly, in a year the high pressure is recorded during cool season and low pressure in the hot season.

4. Pressure gradient and wind

Air in horizontal motion is generally referred to as wind. Wind is caused by a pressure gradient that is developed due to unequal heating. When an air column is heated it expands and the height of pressure surface in the air column increases leading to divergent flow of air at upper level. This results in fall of air pressure at bottom of the air column (surface level). At surface level wind blows from adjacent high pressure areas.

Wind is a vector term expressed as speed and direction (from which the wind is blowing). The wind speed is expressed as m s-1, Knot (Nautical miles per hour) or km h-1 etc.

1 mile = 1.61 km,	1 nautical mile = 1.852 km,	1 m s-1 =1.944 Knot = 2.237 miles h-1
1 Knot = 1.151 miles h-1 = 0.515 m s-1

4.1. Measurement of wind

4.1.1. Anemometers

The instrument that measures wind speed is called anemometer. There are different types of anemometers used for different purposes e.g., Cup-counter anemometer, propeller type anemometer, hot wire anemometer, sonic anemometer etc.

Cup-counter anemometer: Cup-counter anemometer (Robinson’s Cup counter anemometer) is the simple type of anemometer. It consisted of three or four hemispherical cups mounted symmetrically on horizontal arms, which are fitted to a vertical shaft. The air flow while passing the cups in any horizontal direction produces torque at the axis at a rate that was proportional to the wind speed. It is the most common type of anemometer used for routine observations in surface observatories.

Propeller type anemometer: It is a windmill type anemometer that often comes as a combination of a propeller and a tail on the same axis to obtain accurate and precise wind speed and direction measurements from the same instrument (vane anemometer).

Hot wire anemometer: Hot wire anemometers use a very fine wire (in the order of several micrometres) electrically heated to some temperature above the ambient. Air flowing past the wire cools the wire. The change in the electrical resistance of the metal indicates wind speed.

Sonic anemometer: Sonic (or ultrasonic) anemometer uses ultrasonic sound waves to measure wind velocity. Sonic anemometers have very fine temporal resolution, 20 Hz or better, which makes them suitable for turbulence measurements in Eddy Covariance System.

Plate anemometer: Plate anemometer consists of a flat plate, kept normal to the wind by a wind vane. The plate is fitted to a spring. The compression of the spring determines the actual force which the wind is exerting on the plate. This instrument is less sensitive to light winds and is generally used to trigger high wind alarms on bridges.

There are also several other devices used for measurement of wind speed 4.1.2. Wind vane Wind direction is measured by the simple instrument called wind vane. It has a horizontal an arrow like structure rotating on a vertical axis. The pointed end that offers less resistance to wind always shows the direction of wind. The wind direction is expressed in terms of the direction from which the wind is blowing. For example, the wind blowing from east direction is called easterly wind (E) and that from south west direction is called south-westerly or SW wind. The principal axis (North-South) comes first in the expression. Wind direction can also be expressed as the degree angle clockwise from north. For example, East is expressed as 900, SW as 2250 and so on. The north is represented by 360 (not 0).

4.1.3. Beaufort’s Scale

Beaufort’s scale is one such system originally developed by Admiral Beaufort (1805) and subsequently revised by other workers gives an approximate estimate of wind speed in absence of anemometer.

Table 2: Beaufort’s scale and corresponding wind speed

B.	Speed	Indicators	Description
No.	(Knot)
0	0	Smoke raises vertically	Calm
1	1-3	Drift of smoke	Light air
2	4-6	Wind felt on face, leaves rustle	Light breeze
3	7-10	Leaves and small twigs in motion	Gentle breeze
4	11-16	Dust and loose papers raise, small branches move	Moderate breeze
5	17-21	Small trees in sway	Fresh breeze
6	22-27	Leaves and branches move, telegraph wire whistle	Strong breeze

B. No.	Speed (Knot)	Indicators	Description
7	28-33	Whole tree sway, walking becomes difficult	Moderate gale
8	34-40	Twigs break off	Fresh gale
9	41-47	Slight structural damage	Strong gale
10	48-55	Trees uprooted , structural damage	Whole gale
11	56-63	Wide spread damage	Storm
12	64-71	Wide spread sever damage	Hurricane

4.2.  Forces acting on wind

Two different categories of forces act upon the wind to determine its path (Speed and direction) : a) Fundamental / Driving force, b) Motion dependent force

4.2.1.  Pressure gradient force

The pressure gradient force is the ratio of difference in atmospheric pressure between the two points (locations or isobars) and the distance between them. This is the driving force which causes the wind movement. Wind moves from higher atmospheric pressure to lower pressure. Greater this difference greater will be the wind speed. The direction of pressure gradient force is perpendicular to the isobars.

4.2.2. Coriolis force

Coriolis force arises due to rotation of earth which forms an accelerating coordinate system in which the body moves. It is proportional to the speed of the moving body and acts at right angle to it. The direction of Coriolis force is to the right of the direction of motion in north hemisphere and to the left of the direction of motion in south hemisphere. It is expressed as :

f = 2 Ω sin Φ v

where, Ω = Angular velocity of earth, V = velocity of moving body

Φ = Latitude and

4.2.3.   Frictional force

The frictional force is the drag near the surface and acts opposite to the direction of motion. The magnitude of frictional force decreases with height. Frictional force affects the wind upto 1 km above the surface.

4.2.4. Centrifugal force

When the movement of wind is in a curved path the centrifugal force acts away from the center of curvature of the path.

5. Global circulation system

Atmospheric circulation is the movement of air at all levels of the atmosphere over all parts of the planet. The driving force behind atmospheric circulation is solar energy, which heats the surface and the atmosphere with different intensities at the equator, the middle latitudes, and the poles. This latitudinal heat imbalance drives the circulation of the atmosphere and oceans. Around 60% of the heat energy is redistributed around the planet by the atmospheric circulation and around 40% is redistributed by the ocean currents. The rotation of Earth on its axis and the unequal arrangement of land and water masses on the planet also contribute to various features of atmospheric circulation.

5.1. Single cell model vs Three cell model

One way to transfer heat from the equator to the poles would be to have a Single cell model that has a one large meridional cell driven by a giant low pressure in the equatorial region and high pressure at polar region in both the hemispheres. This single cell circulation model was first proposed by Hadley in the 1700’s. The single cell model has oversimplified assumption that – a) Earth surface is homogenous, b) Sun is always directly over the equator and c) Coriolis force is ignored. High insolation at the equatorial region creates giant low pressure belt there and similarly, the cold polar regions have high pressure in both the hemispheres. This creates a pressure gradient force that causes surface air to flow from the poles towards the equator. Air rises at equator and flows towards poles at upper levels.

Since the Earth rotates, its axis is tilted and there is more land in the Northern Hemisphere than in the Southern Hemisphere, the actual global air circulation pattern thus is much more complicated. Instead of a single cell, the global circulation is explained by a three cell model that consists of three circulation cells in each hemisphere known as Hadley cells, Ferrell cells and polar cells.

5.1.1. Hadley Cell

At low latitude air gets heated up due to high insolation and rises vertically. In the wake of the warm rising air, low pressure develops at the equator known as equatorial low. When the air reaches the top of the troposphere (at about 16 kilometers) it can rise no farther and begins to move toward the poles, cooling and descending in the process leading to high pressure at subtropical regions of both the hemispheres. The surface flow of air from subtropical high to equatorial low completes a convection cell known as Hadley cell that dominates tropical and subtropical climates. Hadley cells are named after the English scientist George Hadley who first described them in 1753.

5.1.2. Ferrel Cell

While most of the trade wind air that sinks at 30 degree latitude and returns to the equator, some of it flows poleward. At about 60 degrees latitude, this air mass meets much colder polar air (the areas where this occurs are known as polar fronts). The warmer air is forced upward by the colder air to the tropopause, where most of it moves back toward the equator, sinking at about 30 degrees latitude to continue the cycle again. These second circulation belts over the middle latitudes between 30 degrees and 60 degrees are known as Ferrell cells, named after the American meteorologist William Ferrell who discovered them in 1856.

5.1.3.  Polar Cell

The air at the top of polar fronts that does not return toward the equator moves, instead, poleward. At the poles, this air cools, sinks, and flows back to 60 degree latitude north and south. These third circulation belts over the poles are known as polar cells (or polar Hadley cells) because they flow in the same direction as the Hadley cells near the equator. However, they are not as powerful since they lack the solar energy present at the equator.

Both Hadley cell and polar cell have their rising branches over warm temperature zones and sinking braches over the cold temperature zone. Both the cells directly convert thermal energy to kinetic energy. They are thermally direct cells being formed due to latitudinal difference in insolation. The Ferrel cell on the other hand, rises over cold temperature zone and sinks over warm temperature zone. This cell is not driven by thermal forcing but driven by eddy (weather systems) forcing. Thus the Ferrel cell is thermally indirect cell.

5.2. Surface features of global atmospheric circulation system

The surface feature of global circulation is described by six major wind belts, three in each hemisphere. From pole to equator, they are the polar easterlies, the westerlies, and the trade winds. The pressure belts are – a) Equatorial Low, b) Sub-tropical high, c) Sub-polar low, d) Polar high. The distribution of pressure belts and wind system over earth surface is described in this section.

5.2.1. Global pressure belts

Inter-tropical Convergence Zone (ITCZ):

The equatorial region is associated with a low pressure belt called equatorial low. This is the region where the north east and south east trade winds convergences. Hence, this is popularly known as Intertropical Convergence Zone (ITCZ). This region is characterized by light and irregular wind broken by occasional thunderstorms and squalls. The width and exact location of the doldrums is hard to predict. Sailing ships are sometimes becalmed here for many days waiting for a proper wind. Thus the ITCZ is also called Doldrums, that represents the depressed mental state of the sailors while passing this region.

Because of the converging moist air and high potential for rainfall in the doldrums, this region coincides with the world’s latitudinal belt of heaviest precipitation and most persistent cloud cover.

Subtropical Highs:

This is the areas of high pressure, generally located between latitudes 25° and 35° latitudes in North and South hemispheres. The subtropical highs are areas of sinking and air from higher altitudes, which settle to build up the atmospheric pressure. From this region the surface wind blow poleward to become the westerlies and equatorward as the trade winds.

The subtropical highs are areas, like the doldrums, in which there are no strong prevailing winds. However, unlike the doldrums, the weather conditions here are typically clear, sunny, and rainless, especially over the eastern portions of the oceans where the high pressure cells are strongest. The subtropical highs are often called the “horse latitudes.” This name comes from the occasional need by the Spanish conquistadors to eat their horses or throw them overboard in order to conserve drinking water and lighten the weight when their ships were becalmed in these latitudes. The Horse latitudes are called ‘Variables of Cancer’ and ‘Variables of Capricorn’ in north and south hemispheres respectively.

Sub-polar Lows:

When the cold air flowing out of the polar regions and the warmer air moving in the path of the westerlies meet, the heavier cold air pushes the warm air upward, forcing it to rise rapidly forming a transition zone of low pressure called sub-polar low or polar front.. The weather of this frontal region can be very stormy.

Polar Highs:

The polar highs are areas of high atmospheric pressure around the north and south poles; the south polar high being the stronger one because land gains and losses heat more effectively than sea. The cold temperatures in the polar regions cause air to descend to create the high pressure (subsidence). The surface temperatures of the polar highs are the coldest on Earth. The polar high regions get very low amount of precipitation, and are thus known as “polar deserts”.

5.2.2. Global wind system

Trade winds:

The trade winds blow out of the subtropical highs toward the equatorial trough in both the Northern and Southern Hemispheres between latitudes 5° and 25°. Because of the Coriolis effect, the northern trades move away from the subtropical high in a clockwise direction out of the northeast. In the Southern Hemisphere, the trades diverge out of the subtropical high toward the equatorial trough from the southeast, as their movement is counterclockwise. Because the trades tend to blow out of the east, they are also known as the tropical easterlies.

The trade winds tend to be constant, steady winds, consistent in their direction. This is most true when they cross the eastern sides of the oceans (near the eastern portion of the subtropical high). The area of the trades varies somewhat during the solar year, moving north and south a few degrees of latitude with the sun. Near their source in the subtropical highs, the weather of the trades is clear and dry, but after crossing large expanses of ocean, the trades have a high potential for stormy weather. Early Spanish sea captains depended on the northeast trade winds to drive their galleons to destinations in Central and South America in search of gold, spices, and new lands.

Westerlies:

The westerlies flow poleward out of the subtropical high pressure cells in both the hemispheres of earth surface. The path of these become westerly due to Coriolis impact which is more noticeable at higher latitude and thus these winds have been labeled the westerlies. They tend to be less consistent in direction than the trades, but they are usually stronger winds and may be associated with stormy weather. The westerlies occur between about 35° and 65°N and S latitudes. In the Southern Hemisphere, where there is less land than in the Northern Hemisphere to affect the development of winds, the westerlies attain their greatest consistency and strength.

Polar easterlies:

The polar easterlies are the dry, cold prevailing winds that blow from the high-pressure areas of the polar highs at the North and South Poles towards the subpolar low (Polar front) in between the 600 to 900 latitude of each hemisphere. The direction of polar wind is being deflected westward by the Coriolis effect. The polar easterlies are often weak and irregular.

The pressure belts are discontinuous semi permanent and quasi stationary in nature. The non-homogeneous distribution of land and water makes the pressure belt discontinuous since the land mass and ocean have different heat storage capacity. The seasonal change insolation pattern of north and south hemisphere causes seasonal shift of pressure belts. All of the belts tend to move northward during the northern summer and southward during the northern winter. Because global heating and cooling lags behind the position of the sun, they reach their northernmost latitude at or after the end of the northern summer. This brought the trade winds within reach of the Spain and Portugal and determined the sailing time of the Spanish treasure fleet. The northernmost position of the wind belts corresponds to the Atlantic hurricane season.

6. High altitude winds (Winds aloft)

When the wind blows at high altitude it is not affected by surface friction. Air aloft will move in response to pressure gradients and will be influenced primarily by the Coriolis effect.

6.1. Geostrophic and gradient winds

At upper levels in the atmosphere, a parcel of air is subjected to a pressure gradient force and a Coriolis force. As a parcel of air moves in response to a pressure gradient, it is turned progressively sideways until the gradient force and Coriolis force balance, producing the geostrophic wind. The geostrophic winds blow parallel to the isobars.

When the wind blows in a curved path the centrifugal force acts on wind, outward from the centre of curvature and the resultant wind is a balance among the pressure gradient force, centrifugal force and Coriolis force. The resultant wind is called gradient wind. The gradient wind occurs during the cyclonic/anticyclonic movement of air when there is a low pressure zone surrounded by high pressure or the reverse.

6.2.  Jet stream and Rossby waves

The jet stream forms high in the upper troposphere under thermal effect between two air masses of very different temperature and the impact of Coriolis force. The greater the temperature difference between the air masses, the faster the wind blows in the jet stream. If the warm and cold air masses are quite deep, higher altitudes in the atmosphere experience progressively larger air pressure differences that leads to very strong winds. The jet stream is like narrow zones of very high wind speed winds, the wind speed often exceeds 100 miles per hour, and sometimes peak over 200 mph. Jet streams are strong in the winter, when there is a greater contrast in temperature between cold continental air masses and warm oceanic air masses. There are three kinds of jet streams: the Polar jet stream (westerly) that forms near the interface of the Polar and Ferrel circulation cells, the subtropical jet (westerly) that forms near the boundary of the Ferrel and Hadley circulation cells and tropical easterly jets that occurs during the Northern Hemisphere summer in tropical regions, typically where dry air encounters more humid air at high altitudes.

The smooth westward flow of the upper-air westerlies frequently forms undulations called Rossby waves. The waves arise in a zone of contact between cold polar air and warm tropical air, called the polar front. The number of Rossby waves ranges from three to seven.

7. Local wind

This occurs due to local situation, generally diurnal in character. Ex. Land breeze/Sea breeze, Mountain breeze / Valley breeze etc.

7.1. Land and sea breezes

A sea breeze or onshore breeze is a gentle wind blowing from sea toward land that develops over bodies of water (sea) near land due to differences in air pressure created by their different heat capacity. It is a common occurrence along coasts during the morning as solar radiation heats the land more quickly than the water. A land breeze or offshore breeze, blowing from land to sea, is the reverse effect, caused by land cooling more quickly than water in the evening. The land breeze or sea breeze do not occur when the measure weather systems like depression, cyclone etc. forms.

7.2. Mountain and valley breezes

The mountain and valley breeze are the part of wind system of a mountain valley region that blows downhill (mountain breeze) at night and uphill (valley breeze) during the day. The mountain and valley breezes form through a process similar to sea and land breezes.

During the day, the mountainside is directly heated by the sun, the temperature is higher, air expands, air pressure reduces, and therefore air will rise up the mountainside from the valley and generate a valley breeze. At around 10 a.m., wind flows from the valley and up along the mountainside, the higher the temperature the stronger the breeze. The valley breeze reaches its maximum force at around 2 pm that slows down gradually and comes to a complete stop by sunset. By nightfall, the mountainside region is able to dissipate heat more quickly, due to its higher altitude and therefore temperature drops rapidly. Cold air will then travel down the mountainside from the top and flow into the valley, forming a mountain breeze. Mountain breeze is also called gravity wind or drainage wind.

The valley breezes normally carry water vapor to the peak, which will often condense into clouds. When the mountain breeze travels down and gathers in the valleys, water vapor will condense and forms clouds and fog that are usually occur before sunrise. During late spring or early autumn, the cold air trapped in the valley and basin will often generate frost. The mountain breeze is usually stronger than the valley breeze.

The valley breeze and valley breeze are the types of anabatic (Greek term for “climbing”) referring to the wind that blows uphill and katabatic wind (Greek term for “going down”) that blows downhill respectively. However, the katabatic and anabatic winds are conventionally referred to the winds of a larger and stronger scale.

8. Summary

• Atmospheric pressure is the force exerted by air on a unit surface which is determined by temperature and density of air. Atmospheric pressure is measured barometers.
• The standard atmospheric pressure is the pressure at mean sea level (1013.25 milibar). For synoptic study and weather reports the mean sea level pressure (MSLP) is used for reference.
• The atmospheric pressure shows a bimodal trend with two maxima at 10 am and 10 pm and minima at 4 am and 4 pm at the time of equinoxes. Similarly, in a year the high pressure is recorded during cool season and low pressure in the hot season.
• Isobars are the imaginary lines joining the places of equal mean sea level pressure at a given time. Close isobars imply steep pressure gradient.
• The surface wind generated by pressure gradient force is being affected by Coriolis force resulted from rotation of earth, frictional force offered by surface roughness. The centrifugal force acts upon the wind when it moves on a curved path. The Geostrophic wind at high altitude blows parallel to the isobars as the pressure gradient force is balanced by the Coriolis force.
• The wind speed is measured by anemometer and is expressed as m s-1, Knot (nautical miles per hour) or other equivalent units. Beaufort scale gives an approximate estimate of wind speed.
• The wind direction is measured by wind vane and expressed as acronym of standard direction (like SW, N, NE etc) or as degree angle, clockwise from north.
• The latitudinal distribution of pressure shows Equatorial low at 50  N to 50  S latitude (also known as Inter-tropical Convergence Zone, ITCZ or Doldrums), Sub-tropical high (STH) or
• Horse latitude at 250 to 350 latitude, Sub-polar low (SPL) or Polar front at 600 to 700 and Polar high (PH) near 900 latitude in North and South hemispheres
• The surface wind system includes northeast and southeast trade winds from STH to ITCZ, westerlies from STH to SPL and polar easterlies from PH to SPL in both north and south hemispheres. The entire system moves southward in January and northward in June.
• The three dimensional system of global circulation expressed in terms of Hadley, cell Ferrel cell and polar cell with high speed jet streams and Rossby waves at high altitude.
• The local wind system that includes land-sea breeze, mountain-valley breeze show diurnal pattern of variation in intensity and direction.