• There are two mechanisms of energy transfer: circulation patterns in the atmosphere and in the oceans
• These mechanisms shift warmth from low latitudes towards high latitudes, moderating both high and low latitudes
• The atmosphere and oceans are enormous thermal engines, driven by latitudinal imbalance of energy
• The energy drives currents of air and water, which in turn transfer energy and modify the imbalance
• The general circulation of the atmosphere is the more important of the two mechanisms
• There is a broad planetary circulation pattern that moves warm air poleward and cool air equatorward
• Some 75 to 80 percent of all horizontal energy transfer is accomplished by atmospheric circulation

Atmospheric Circulation

• The movement of air in the atmosphere is called atmospheric circulation.
• Earth’s atmospheric circulation balances the energy surplus of tropics and the energy deficit of poles.
• Atmospheric circulation involves the movement of air in both horizontal and vertical directions.
• Horizontal movement of air is called wind, and winds are named based on the direction from which they come.
• Vertical movement of air is called updraft if the air parcel is moving up and downdraft if the air parcel is moving down.
• 

Oceanic Circulation

• Winds disturb the surface of the ocean with swells and waves.
• Wind can also propel the surface of the water to move forward in the form of a current.
• Surface ocean currents can flow at about 1 to 2 percent of wind speed.
• The wind blowing over the surface of the water is the principal force driving the major surface ocean currents.
• Energy stored in the oceans has important effects on patterns of atmospheric circulation.
• A close relationship exists between the general circulation patterns of the atmosphere and oceans.
• 

Impact of pressure and Wind

• Wind responds to pressure changes caused by atmospheric pressure
• Wind has the energy to transport solid particles in the air, causing visible effects on the landscape
• Vegetation may bend in the wind and loose material such as dust or sand may be shifted from one place to another
• Effects of pressure and wind on the landscape are usually short-run and temporary
• Pressure and wind are major elements of weather and climate
• Their interaction with other atmospheric components and processes cannot be overestimated
• Atmospheric pressure is the force exerted by the weight of gas molecules on a unit of area on Earth’s surface or on any other body
• The atmosphere has weight because the mass of its gases is pulled toward Earth by gravity

Coriolis effect and Wind

• The atmosphere is constantly in motion.
• Air can move in any direction, shaped by various factors.
• Some airflow is weak and brief, while others are strong and persistent.
• Atmospheric motions involve both horizontal and vertical movement.
• Wind refers to horizontal air movement.
• The term “wind” only applies to horizontal movement, even though both vertical and horizontal motions are important in the atmosphere.
• Wind direction is determined by three factors: pressure gradient, Coriolis effect, and friction.
• Pressure gradient force causes air to move from high to low pressure.
• Coriolis effect is a deflecting force caused by Earth’s rotation.
• In the Northern Hemisphere, air appears to turn right due to the Coriolis effect, and in the Southern Hemisphere, it appears to turn left.
• The Coriolis effect always acts in the perpendicular direction of the motion of air.
• The Coriolis effect is zero at the equator and increases towards the poles.
• 
• Gravity influences all vertical motion near Earth’s surface.
• The Coriolis effect is a pervasive influence on the direction of objects moving horizontally around Earth.
• Due to the Coriolis effect, all things moving over the surface of Earth or in Earth’s atmosphere appear to drift sideways as a result of Earth’s rotation beneath them.
• The path of any free-moving object appears to deflect to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere as a result of Earth’s rotation.
• The following are four basic points to remember about the Coriolis effect:
  • Freely moving objects appear to deflect right in Northern Hemisphere and left in Southern Hemisphere due to Coriolis effect.
  • Deflection is strongest at the poles and decreases toward the equator where it is zero.
  • Coriolis effect is proportional to the speed of an object, so faster objects are deflected more.
  • Coriolis effect influences only the direction of movement, not the speed of an object.
• Friction force causes resistance to the motion of air on the surface and decreases with height.
• The effect of surface friction is felt up to the height of 500m above the surface.
• Surface friction varies with surface texture, wind speed, time of day and year, and atmospheric conditions.

Cyclones and Anticyclones

• High-Pressure Wind Patterns: A high-pressure center is known as an anticyclone, and the flow of air associated with it is described as being anticyclonic.
• The four patterns of anticyclonic circulation are shown in Figure –
• 
• Upper atmosphere of Northern Hemisphere: Winds move clockwise in a geostrophic manner parallel to isobars.
• Friction layer of Northern Hemisphere: Divergent clockwise flow with air spiraling out away from center of anticyclone.
• Upper atmosphere of Southern Hemisphere: Counterclockwise, geostrophic flow parallel to isobars.
• Friction layer of Southern Hemisphere: Diverging in counterclockwise pattern, mirror image of Northern Hemisphere.

• Low-Pressure Wind Patterns: Low-pressure centers are called cyclones, and the associated wind movement is said to be cyclonic.
• Northern Hemisphere cyclonic circulations and Southern Hemisphere cyclonic circulations are mirror images of each other.
• In the upper atmosphere of the Northern Hemisphere, air moves counterclockwise in a geostrophic pattern parallel to the isobars.
• In the friction layer of the Northern Hemisphere, there is a converging counterclockwise flow.
• In the upper atmosphere of the Southern Hemisphere, a clockwise geostrophic flow occurs parallel to the isobars.
• In the friction layer of the Southern Hemisphere, the winds converge in a clockwise spiral.
• Cyclones and anticyclones have a prominent vertical component of air movement.
• Air descends in anticyclones and rises in cyclones.
• These motions are especially notable in the lower troposphere.
• In anticyclones, upper air sinks down into the center of the high and then diverges near the ground surface.
• In cyclones, the air converges horizontally into the cyclone and then rises.
• Cyclones and rising air are associated with clouds.
• Anticyclones and descending air are associated with clear conditions.

Air Mass

• An air mass is a homogenous body of air in terms of temperature, humidity, and lapse rate.
• Air masses take on the characteristics of their source regions.
• Air masses are classified based on source region, latitudinal position, temperature, and moisture properties.
• There are two main categories of air masses: tropical/subtropical and polar/subpolar.
• The subcategories of air masses are based on whether the source region is oceanic or continental and the modifications they experience as they move from their source regions.
• To identify different types of air masses, letter symbols are placed first in the designation: T for tropical, P for polar, E for equatorial, A for Arctic, and AA for Antarctic.
• ‘k’ is used for air colder than the underlying surface, and ‘w’ is used for air warmer than the surface.
• On the basis of origin it can be maritime and continental.

Air mass	Symbol	Source Region	Properties
Maritime Equatorial	mE	warm ocean in equatorial zone	Unstable, warm, very moist
Maritime Tropical	mT	warm ocean in the tropical zone	warm, moist
Continentel Tropical	cT	Subtropical deserts	warm, dry
Maritime Polar	mP	Midlatitude oceans	cool, moist(winter)
Continental Polar	cP	Nothern continental interiors	cool, dry(winter)
Continental Arctic and Continental Antarctic	cAA	Regions near north and south poles	Very cold, very dry, very stable

• The border between the two air masses with contrasting physical properties is known as fronts. A warm front marks the leading edge of a sector of warm air. Cold front denotes the influx of cold air.

Fronts

• Frontogenesis is the development of fronts and frontal waveforms in well-defined areas.
• When unlike air masses meet, a boundary zone called a front develops between them.
• A front is a narrow three-dimensional transition zone with rapidly changing air properties.
• The frontal concept was developed by Norwegian meteorologists during World War I.
• Types of fronts are determined by the temperature difference between air masses.
• A cold front forms where a cold air mass meets and displaces warmer air, creating cloudiness and precipitation.
• A warm front forms where a warm air mass meets colder air, causing widespread cloudiness and precipitation.
• Stationary Fronts:
  • Stationary fronts occur when two air masses meet but do not displace each other or when a cold front or warm front stalls.
  • Weather along a stationary front is difficult to generalize but often produces limited precipitation similar to a warm front.
  • Stationary fronts are portrayed on a weather map by alternating warm and cold front symbols.
  • 
• Occluded Fronts:
  • An occluded front is formed when a cold front overtakes a warm front.
  • Occluded fronts are shown on a weather map by alternating warm and cold front symbols on the same side of the line.
  • 

Temperate Cyclones

• Temperate cyclones occur in the mid-latitude regions of both hemispheres
• Cyclones are born along the polar front, particularly in the region of the Icelandic and Aleutian sub-polar low pressure areas in the northern hemisphere
• The development and strengthening of mid-latitude wave cyclone is known as cyclogenesis
• On average, a temperate cyclone takes 3-10 days to progress through the stages of development
• The life cycle of a cyclone is completed through six successive stages:
  • Stage A: the convergence of two air masses of contrasting physical properties and direction
  • Stage B: the incipient stage, during which the warm and cold air masses penetrate into the territories of each other
  • Stage C: the mature stage when the cyclone is fully developed and isobars become almost circular
  • Stage D: warm sector is narrowed in extent due to the advancement of cold front at a faster rate than a warm front, and cold front comes nearer to the warm front
  • Stage E: this stage starts with the occlusion of the cyclone when the advancing cold front finally overtakes the warm front and an occluded front is formed
  • Stage F: in the final stage, warm sector completely disappears, occluded front is eliminated, and ultimately the cyclone dies out.
• 
• Characteristics of Temperate Cyclone:
  • Moves counter clockwise in northern hemisphere and clockwise in southern hemisphere.
  • May be 1600km wide, covering a large area.
  • Isobars are elliptical in shape.
  • Cold air mass moves faster than warm air mass.
  • Move at a gentle pace of 5-25km per hour.
  • Give light showers which are beneficial for crops and human health.
  • Thunder and lightning occur in the ending part of the cyclone.
  • Each cyclone is followed by clear weather.

Tropical Cyclones

• Tropical cyclone is a weather system of low pressure originating in the tropics.
• It can move into temperate waters if the water temperature is high enough to sustain it.
• The energy in an average hurricane may be equivalent to more than 10,000 atomic bombs the size of Nagasaki bomb.
• Tropical cyclones range in size from a few kilometers to several hundred kilometers in diameter.
• The eye of the cyclone can be as large as 65km across, and the total area involved may be as much as 52000 sq km.
• The tropical cyclones originate between 10° and 25 degree latitudes in both the hemispheres.

• Conditions conducive for tropical cyclone:
  • Continuous supply of abundant warm and moist air
  • Sea temperature in lower latitude should be around 26-27 degree C
  • Existence of weak tropical depression
  • Presence of coriolis force
• Characteristics of tropical cyclones:
  • Isobars are generally circular and close to each other, resulting in a steep pressure gradient
  • They may be a thousand kilometers in diameter and about 15km in height
  • The central area is designated as an ‘eye’ of cyclone, which is surrounded by clouds so high and dense that the day time sky above looks dark
  • They do not have fronts
  • They derive their energy from the latent heat
  • The clouds in the cyclone are cumulonimbus having vertical extension up to about 12-15km
  • They give torrential rainfall
  • Majority of tropical cyclones decay when they come over the land or when they recurve northward over oceans
• 
• Origin of Tropical Cyclone:
  • Tropical cyclones generally develop from a small tropical depression.
  • Tropical depressions form easterly waves, areas of lower pressure within the easterly trade winds.
  • Circular winds begin to blow in the vicinity of the wave when air containing the disturbance is heated by the proximity of tropical waters with a temperature of about 26 degree C or more.
  • Some of the warm humid air is forced upward, and condensation begins, shaping the storm.
  • Under ideal conditions, the embryo storm reaches hurricane status (i.e. with wind speed in excess of 118 km per hr) in two to three days.
• Places of occurrence:
  • The Caribbean Sea and the Gulf of Mexico.
  • The northwest pacific from the Philippines to the China Sea.
  • The Pacific Ocean west of Mexico.
  • The South Indian Ocean east of Madagascar.
  • The North Indian Ocean in the Bay of Bengal.
  • The Arabian Sea.
• Large tropical cyclones are called Hurricanes in the North Atlantic and Eastern Pacific, Typhoons in China, Taiphoo in Japan, cyclone or chakravaat in the Bay of Bengal, Baguio in the Philippines, and Willi Willies in Australia.
• 

How are Cyclones named?

• Cyclones are named by regional specialized meteorological centers (RSMCs) and Tropical Cyclone Warning Centres (TCWCs) in every ocean basin across the world.
• There are six RSMCs and five TCWCs.
• The World Meteorological Organisation (WMO) maintains lists and names of cyclones and guidelines for countries in every region to name cyclones.
• The India Meteorological Department (IMD) is one of the six RSMCs mandated to issue advisories and name tropical cyclones in the north Indian Ocean region.
• Advisories are issued to 13 member countries under WMO/ESCAP Panel.
• Benefits of naming include identifying individual cyclones, creating awareness, avoiding confusion, remembering cyclones easily, and disseminating warnings effectively.
• The WMO/ESCAP Panel on Tropical Cyclones assigns names to tropical cyclones in the Bay of Bengal and the Arabian Sea.
• The naming of tropical cyclones over the north Indian Ocean commenced from September 2004.
• The Panel on Tropical Cyclones is an intergovernmental body established by the WMO and the Economic and Social Commission for Asia and the Pacific (ESCAP) in 1972.
• Its membership comprises countries affected by tropical cyclones in the Bay of Bengal and the Arabian Sea.
• The main objective of the WMO/ESCAP Panel on Tropical Cyclones is to promote measures to improve tropical cyclone warning systems in the Bay of Bengal and the Arabian Sea.

Differences between Temperate Cyclone and Tropical Cyclone

Tropical Cyclone	Temperate Cyclone
These move from east to west.	These move from west to east
A tropical cyclone has an effect on a comparatively smaller area than a Temperate cyclone.	Temperate cyclone affect a much larger area.
The velocity of wind in a tropical cyclone is much higher and it is more damaging.	The velocity of air is comparatively lower
Tropical Cyclone forms only on seas with temperature more than 26-27degree C and dissipate on reaching the land.	Temperate cyclones can be formed on both land and sea
A tropical cyclone doesn’t last for more than 7 days	Temperate cyclone can last for a duration of 15 to 20 days