Atmospheric Circulation And Weather System

Air expands when heated and gets compressed when cooled. This creates differences in atmospheric pressure.

Because of pressure differences, air moves from high-pressure areas to low-pressure areas. This movement of air creates winds.

Winds redistribute heat and moisture across the planet. They help maintain a broad temperature balance at the planetary scale. When moist air rises vertically, it cools, forms clouds and may bring precipitation.

Atmospheric Pressure

Atmospheric pressure is the weight of a column of air contained in a unit area from mean sea level to the top of the atmosphere.

Atmospheric pressure is expressed in millibars, or mb.

At sea level, the average atmospheric pressure is 1,013.2 millibars.

Due to gravity, air is denser near the Earth’s surface and therefore pressure is higher near the surface.

Atmospheric pressure is measured by:

• Mercury barometer
• Aneroid barometer

Vertical Variation Of Pressure

Atmospheric pressure decreases with height.

In the lower atmosphere, pressure decreases rapidly with altitude.

The decrease is about 1 mb for every 10 m increase in elevation, though it does not always decrease at the same rate.

The vertical pressure gradient force is much larger than the horizontal pressure gradient. But it is generally balanced by an almost equal and opposite gravitational force.

Because of this balance, strong upward winds are not normally experienced.

Horizontal Distribution Of Pressure

Pressure also varies from place to place at the same elevation.

This horizontal variation in pressure is the main cause of air motion.

The horizontal distribution of pressure is studied by drawing isobars.

Isobar are lines joining places having equal pressure.

For comparison, pressure readings at stations are reduced to sea level to remove the effect of altitude.

Low-Pressure System

A low-pressure system is enclosed by one or more isobars with the lowest pressure at the centre.

High-Pressure System

A high-pressure system is enclosed by one or more isobars with the highest pressure at the centre.

World Distribution Of Sea Level Pressure

Pressure belts are found at different latitudes.

Near the equator, sea-level pressure is low. This is called the equatorial low.

Around 30° N and 30° S, high-pressure areas occur. These are called subtropical highs.

Around 60° N and 60° S, low-pressure belts occur. These are called subpolar lows.

Near the poles, pressure is high. This is called the polar high.

These pressure belts are not permanent in one fixed position. They shift with the apparent movement of the Sun.

In the Northern Hemisphere:

• In winter, they move southwards.
• In summer, they move northwards.

Forces Affecting Wind Direction And Velocity

Air moves because of differences in atmospheric pressure.

Air in motion is called wind.

Wind blows from high pressure to low pressure, but the rotation of the Earth also affects its direction.

Horizontal winds near the Earth’s surface are affected by:

• Pressure gradient force
• Frictional force
• Coriolis force

Gravitational force acts downward.

Pressure Gradient Force

The difference in atmospheric pressure between two places produces a force.

The rate of change of pressure with distance is called the pressure gradient.

Pressure gradient force is perpendicular to isobars.

Greater pressure gradient means greater wind velocity.

Frictional Force

Friction affects the speed of wind.

It is strongest at the Earth’s surface.

Its influence generally extends up to about 1–3 km above the surface.

Over the sea surface, friction is minimal.

Coriolis Force

The force caused by the rotation of the Earth is called the Coriolis force.

It was mathematically expressed by Gaspard-Gustave de Coriolis in 1835.

Coriolis force deflects winds:

• To the right in the Northern Hemisphere
• To the left in the Southern Hemisphere

Coriolis force is directly proportional to latitude.

It is:

• Maximum at the poles
• Absent at the equator

Deflection is greater when wind velocity is high.

Because Coriolis force is absent at the equator, tropical cyclones do not form near the equator.

Geostrophic Wind

At about 2–3 km above the Earth’s surface, frictional force becomes weak.

At this height, pressure gradient force may be balanced by Coriolis force.

When isobars are straight and winds blow parallel to them, such winds are called geostrophic winds.

Pressure And Wind

Wind circulation around low and high-pressure systems is connected with circulation at higher levels.

Over a low-pressure area:

• Air converges.
• Air rises.

Over a high-pressure area:

• Air descends from above.
• Air diverges at the surface.

Apart from convergence, rising of air may also occur due to:

• Eddies
• Convection currents
• Orographic uplift

Rising air is important for cloud formation and precipitation.

Wind circulation around a low-pressure system is called cyclonic circulation.

General Circulation Of The Atmosphere

The pattern of movement of planetary winds is called the general circulation of the atmosphere.

General circulation also sets ocean water in motion and influences the Earth’s climate.

The pattern of planetary winds depends on:

• Latitudinal variation of atmospheric heating
• Emergence of pressure belts
• Migration of pressure belts following the apparent path of the Sun
• Distribution of continents and oceans
• Rotation of the Earth

Inter Tropical Convergence Zone

At the Inter Tropical Convergence Zone, or ITCZ, air rises due to convection caused by high insolation.

This creates low pressure.

The converged air rises within a convective cell and reaches the upper troposphere, around 14 km.

Then it moves towards the poles.

This causes accumulation of air around 30° N and 30° S.

Near the surface, air flows back towards the equator as easterlies.

Such circulations from surface upward and back are called cells.

Wind Systems

Wind is the horizontal movement of air from high-pressure areas to low-pressure areas.

Due to Coriolis force, winds do not move in a straight path.

Wind direction is identified by a wind vane.

Wind speed is measured by an anemometer.

Winds are classified into three types:

• Primary winds
• Secondary winds
• Tertiary winds

Primary Winds

Primary winds are also called planetary winds.

They blow constantly throughout the year.

There are three types of primary winds:

• Trade winds
• Westerlies
• Polar easterlies

Trade Winds

Trade winds blow between 30° N and 30° S.

They flow from subtropical high-pressure belts towards the equatorial low-pressure belt.

They are associated with the Hadley cell.

Trade winds blow:

• From north-east in the Northern Hemisphere
• From south-east in the Southern Hemisphere

As these winds move towards the equator, they become warm and pick up moisture.

Near the equator, they rise and cause heavy rainfall.

Westerlies

Westerlies blow between 30° and 60° latitudes in both hemispheres.

They flow from subtropical highs towards subpolar lows.

They are associated with the Ferrel cell.

Westerlies blow:

• From south-west to north-east in the Northern Hemisphere
• From north-west to south-east in the Southern Hemisphere

Westerlies are stronger in the Southern Hemisphere because there are no large landmasses to interrupt them.

In the Southern Hemisphere, westerlies are known as:

• Roaring Forties near 40° S
• Furious Fifties near 50° S
• Shrieking Sixties near 60° S

Polar Easterlies

Polar easterlies blow from polar high-pressure belts towards subpolar low-pressure belts.

They blow between 90° and 60° latitudes in both hemispheres.

They are associated with the Polar cell.

Polar easterlies blow:

• From north-east to south-west in the Northern Hemisphere
• From south-east to north-west in the Southern Hemisphere

Secondary Winds Or Seasonal Winds

Wind circulation patterns are modified in different seasons because regions of maximum heating, pressure and wind belts shift.

The most pronounced effect of this shift is seen in monsoons, especially over Southeast Asia.

Local Winds

Differences in heating and cooling of the Earth’s surface create common local or regional winds.

These may develop daily or annually.

Land And Sea Breezes

Land and sea absorb and transfer heat differently.

Sea Breeze

During the day, land heats faster than the sea.

Air over land rises and creates low pressure.

The sea remains relatively cool and has higher pressure.

Therefore, wind blows from sea to land.

This is called sea breeze.

Land Breeze

At night, the situation reverses.

Land loses heat faster and becomes cooler than the sea.

Pressure becomes higher over land and lower over sea.

Wind blows from land to sea.

This is called land breeze.

Mountain And Valley Winds

In mountainous regions, local winds develop due to day and night temperature differences.

Valley Breeze

During the day, mountain slopes heat up.

Air moves upslope.

Air from the valley moves upward to fill the gap.

This is called:

• Valley breeze
• Anabatic wind
• Upslope wind

Mountain Wind

At night, mountain slopes cool.

Dense air descends downslope into valleys.

This is called:

• Mountain wind
• Katabatic wind
• Downslope wind

Warm winds may also occur on the leeward side of mountain ranges.

When moist air crosses mountains, moisture condenses and falls as precipitation.

The remaining dry air descends on the leeward side and warms by the adiabatic process.

Such winds may melt snow rapidly.

Tertiary Winds

Tertiary winds form due to local pressure gradients caused by differences in heating and cooling of the Earth’s surface.

They blow only during a particular period of the day or year in a small area.

They are confined to the lowest levels of the troposphere.

Hot Local Winds

Wind	Place	Characteristics
Chinook	Rocky Mountains, USA and Canada	Warm, dry wind; known as “Snow Eater” because it removes snow quickly
Foehn	Alps	Warm, dry wind on leeward side; causes rapid temperature rise, clear sky and reduced humidity
Khamsin	Egypt	Name comes from Arabic word “fifty”; linked with low-pressure depressions over Sahara or southern Mediterranean; raises temperature and reduces humidity
Sirocco	Mediterranean Sea	Hot, dry south wind; becomes sticky after crossing the sea
Harmattan	West Africa	Dry season wind from late November to mid-March; gives relief from humidity and is called “Doctor Wind”
Norwester	New Zealand	Warm, dry wind
Santa Ana	South California	Strong, extremely dry wind; responsible for frequent wildfires
Loo	India and Pakistan	Hot, dry and dusty summer wind, most intense in May–June afternoons
Shamal	Iraq, Persian Gulf, Arabian Peninsula	Strong north-westerly wind; causes large sandstorms in Iraq

Cold Local Winds

Wind	Place	Characteristics
Bora	North Italy	Cold katabatic wind descending from high elevations; very high speed
Mistral	Alps and France	Violent, dry, cold north-west wind along coasts of Spain and France
Blizzard	Tundra Region	Extremely cold wind carrying dry powdery snow; visibility may reduce sharply
Purga	Russia	Cold local wind from Siberia, similar to blizzard
Levanter	Spain	Pleasant moist east wind bringing mild weather to the Mediterranean

Jet Streams

Jet streams are strong and rapidly moving air currents found in narrow belts in the upper troposphere.

They are a few hundred kilometres wide.

Their presence became widely known during the Second World War through observations by high-altitude aircraft pilots.

Types Of Jet Streams

Subtropical Westerly Jet Stream

It occurs in the upper troposphere to the north of subtropical surface highs.

It lies near the poleward limit of the Hadley cell, between 30° and 35° latitudes.

Polar Front Jet Stream

It occurs above the convergence zone of polar cold air and tropical warm air.

It lies between 40° and 60° latitudes.

Tropical Easterly Jet Stream

It develops in the upper troposphere over India and Africa during summer.

It is linked with intense heating of the Tibetan Plateau.

It plays an important role in the Indian monsoon.

Somali Jet

The Somali Jet is a strong low-level south-westerly flow during summer near Somalia and the Arabian Sea region.

It is most intense from June to August.

Significance Of Jet Streams

Jet streams are important because they:

• Influence weather
• Redistribute heat
• Influence monsoon patterns
• Help aircraft save time and fuel when flying with them

Air Masses

An air mass is a huge body of air with little horizontal variation in temperature and moisture.

It may extend several kilometres in length, breadth and thickness.

When air remains over a homogeneous area for a long time, it acquires the characteristics of that area.

Such homogeneous areas are called source regions.

Source regions may be:

• Vast ocean surfaces
• Vast plains
• Plateaus

Major source regions include:

• Subtropical high-pressure belts
• Polar regions

Five Major Source Regions

The five major source regions are:

• Warm tropical and subtropical oceans
• Subtropical hot deserts
• Relatively cold high-latitude oceans
• Very cold snow-covered continents at high latitudes
• Permanently ice-covered continents in the Arctic and Antarctica

Types Of Air Masses

Air Mass	Nature
Maritime tropical (mT)	Moist and warm
Continental tropical (cT)	Dry and warm
Maritime polar (mP)	Moist and cold
Continental polar (cP)	Dry and cold
Continental arctic (cA)	Dry and cold

Tropical air masses are warm.

Polar air masses are cold.

Fronts

When two different air masses meet, the boundary zone between them is called a front.

Fronts mainly occur in the middle latitudes between 30° and 60° N and S.

They show steep gradients in temperature and pressure.

Fronts are uncommon in tropical and polar regions.

The formation of fronts is called frontogenesis.

The dissipation of fronts is called frontolysis.

Mid-latitude or temperate cyclones develop due to frontogenesis.

Types Of Fronts

There are four types of fronts:

• Stationary front
• Cold front
• Warm front
• Occluded front

Stationary Front

When a front remains stationary, it is called a stationary front.

It forms when warm and cold fronts stop moving and cannot push against each other.

It brings significant precipitation along the front.

Cold Front

A cold front forms when cold air moves towards a warm air mass.

Cold air replaces warm air by advancing into it.

Cold fronts form cumulonimbus clouds and bring heavy rainfall with lightning and thunder.

Warm Front

A warm front forms when warm air moves towards a cold air mass.

Warm air actively moves over cold air.

Warm fronts form stratus and nimbostratus clouds over large areas.

They bring moderate to gentle rainfall.

Occluded Front

An occluded front forms when a cold air mass overtakes a warm air mass and moves underneath it.

In a rotating low-pressure system, the cold front catches up with the warm front.

The warm air between them is forced upward.

Weather along an occluded front is complex and includes features of both warm front and cold front weather.

Such fronts are common in western Europe.

Cyclones

Cyclones are atmospheric disturbances involving closed circulation of air around a low-pressure centre.

Pressure is higher at the periphery and lower at the centre.

Due to the Coriolis force, cyclones rotate:

• Anti-clockwise in the Northern Hemisphere
• Clockwise in the Southern Hemisphere

Cyclones are classified into two types based on the latitudes of origin:

• Tropical cyclones
• Temperate or extra-tropical cyclones

Tropical Cyclones

Tropical cyclones develop mainly over oceans in tropical areas.

They may move towards coastal regions and cause destruction through:

• Strong winds
• Very heavy rainfall
• Storm surges

They are known by different names in different regions:

Region	Name
Atlantic Ocean	Hurricanes
Western Pacific and South China Sea	Typhoons
Western Australia	Willy-willies
Indian Ocean	Cyclones

Factors Favourable For Tropical Cyclones

Large Sea Surface With Temperature Above 27°C

A large warm sea surface provides warm and moist air to feed the storm.

Condensation releases latent heat, which drives the storm.

Presence Of Coriolis Force

Coriolis force creates a cyclonic vortex.

It is zero at the equator and increases with latitude.

At about 5° latitude, Coriolis force becomes significant enough for storm formation.

About 65% of cyclonic activity takes place between 10° and 20° N and S latitudes.

Small Vertical Wind Speed Variation

Low wind shear is required.

This allows storm clouds to rise vertically to high levels.

Pre-Existing Weak Low-Pressure Area

A weak low-pressure area or low-level cyclonic circulation acts as a seed for cyclone development.

Upper Divergence Above The Sea-Level System

Upper-level divergence helps rising air continue upward and supports cyclone intensification.

Structure And Movement Of Tropical Cyclones

The centre of a mature tropical cyclone is called the eye.

It is a calm centre.

Around the eye lies the eyewall, which is the most violent part of the cyclone.

In the eyewall, wind speed may reach up to about 250 km per hour, and torrential rain occurs.

A cyclone weakens after landfall because its moisture supply is cut off.

In the Bay of Bengal and Arabian Sea region, storm diameter can be large.

The system generally moves a few hundred kilometres per day.

Tropical cyclones usually move with trade winds, generally from east to west, and often make landfall on eastern coasts.

Temperate Cyclones

Temperate cyclones occur between 30° and 60° latitudes in both hemispheres.

They occur:

• Between Tropic of Cancer and Arctic Circle in the Northern Hemisphere
• Between Tropic of Capricorn and Antarctic Circle in the Southern Hemisphere

They are also called:

• Extra-tropical cyclones
• Mid-latitude cyclones
• Frontal cyclones
• Wave cyclones

Temperate cyclones move with the westerlies and are oriented from west to east.

Unlike tropical cyclones, temperate cyclones form over both land and water.

They cover a larger area than tropical cyclones and last for a longer time.

Jet streams play an important role in temperate cyclones and influence their west-to-east direction.

Thunderstorms And Tornadoes

Thunderstorms and tornadoes are severe local storms.

They are short-lived and affect small areas, but they can be violent.

Thunderstorms

Thunderstorms form due to intense convection on moist and hot days.

A thunderstorm is a well-developed cumulonimbus cloud producing thunder and lightning.

If clouds extend into sub-zero levels, hail forms and the storm becomes a hailstorm.

If moisture is insufficient, thunderstorms may generate dust storms.

Thunderstorms show:

• Strong updrafts of rising warm air
• Downdrafts of cool air and rain

Tornadoes

Sometimes, a spiralling wind descends like an elephant’s trunk with very low central pressure.

This causes massive destruction and is called a tornado.

Tornadoes generally occur in middle latitudes.

A tornado over the sea is called a waterspout.

Thunderstorms and tornadoes show the atmosphere’s adjustment to uneven energy distribution.

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