Maarg Manthan · Topic 5.9

Cyclonic and Anticyclonic Circulation

Geography › Climatology · Topic 5.9

Cyclonic circulation is the movement of air around a centre of low pressure: air converges, rises and spirals inward, anticlockwise in the northern hemisphere and clockwise in the southern, bringing cloud and rain. Anticyclonic circulation is the movement of air around a centre of high pressure: air sinks and spirals outward, clockwise in the northern hemisphere and anticlockwise in the southern, bringing clear, dry weather. The directions are set by the Coriolis force and the pressure gradient, and are shown by closed isobars.

6 min read · UPSC Prelims · Geography · Climatology

At a Glance

Cyclone (low) Anticlockwise in the north; air rises
Anticyclone (high) Clockwise in the north; air sinks
Cyclone weather Cloud and rain
Anticyclone weather Clear and dry
Law Buys Ballot
Exam link UPSC Prelims, Geography: Climatology

Where Does It Fit?

This topic applies the forces on wind to the spirals of air around pressure centres. It follows the topic on factors affecting wind and leads to the general circulation of the whole atmosphere. Previous topic: Factors Affecting Wind: Pressure Gradient Force, Frictional Force, Coriolis Force. Next topic: General Circulation of the Atmosphere: Hadley Cell, Ferrel Cell, Polar Cell.

What Is Geostrophic Wind and How Does Circulation Form?

Above the friction layer, the pressure gradient force and the Coriolis force are in balance, and the wind blows parallel to the isobars. This is the geostrophic wind. Upper-level winds, including jet streams, are close to geostrophic. Near the surface, friction reduces the Coriolis force, and the wind crosses the isobars at an angle.

System	Circulation in the northern hemisphere	Circulation in the southern hemisphere	Surface air movement
Low pressure (cyclone)	Anticlockwise	Clockwise	Converges and rises
High pressure (anticyclone)	Clockwise	Anticlockwise	Diverges and sinks

Buys Ballot’s law, formulated by the Dutch meteorologist Buys Ballot in 1857, says that if one stands with one’s back to the wind in the northern hemisphere, the low pressure is on the left, and in the southern hemisphere it is on the right. The wind pattern of the globe is built from these forces, as in General Circulation of the Atmosphere: Hadley Cell, Ferrel Cell, Polar Cell. As an analytical point, the Coriolis force is not a force in the strict physical sense but a consequence of observing motion from a rotating Earth, which is why it can deflect but never speed up or slow down air.

What Are Pressure Cells and Semi-Permanent Pressure Centres?

In the real world the belts are not continuous. Land and sea heat unequally, so the belts are broken into cells of high and low pressure. These semi-permanent centres of action control the climate of large regions.

Centre	Type	Season and location
Siberian (Asiatic) High	High	Winter; over cold Siberia; the source of cold dry winter winds towards India and China
Aleutian Low and Icelandic Low	Low	Winter; over the North Pacific and North Atlantic; subpolar lows
Azores (Bermuda) High and Hawaiian High	High	Year-round, stronger in summer; over the oceans at about 30 degrees north
Thermal lows over Asia and southwestern USA	Low	Summer; over hot land, including the Tibetan Plateau and northwest India, which draw in the monsoon
Mascarene High and Australian Low	High and low	Southern hemisphere: the Mascarene High over the southern Indian Ocean in the southern winter, which feeds the southwest monsoon

As an analytical point, the seven-belt model is an average, and the actual pressure on a day may differ greatly; the transitions of the pressure cells between seasons are what drive the seasonal reversal of winds, the monsoon, discussed in Seasonal and Local Winds: Land and Sea Breezes, Mountain and Valley Winds.

How Do Cyclones and Anticyclones Differ?

Feature	Cyclonic circulation	Anticyclonic circulation
Pressure	Low pressure at the centre	High pressure at the centre
Isobars	Closed, pressure falling inward	Closed, pressure rising inward
Wind direction, northern hemisphere	Anticlockwise, spiralling inward	Clockwise, spiralling outward
Wind direction, southern hemisphere	Clockwise	Anticlockwise
Air movement	Converges at the surface and rises	Sinks from above and diverges at the surface
Weather	Cloud, rain, strong winds	Clear sky, dry, light winds
Size and speed	Smaller, moves faster	Larger, moves slowly or stays

The direction of rotation follows from the Coriolis force. Air rushing towards a low is deflected to the right in the north and ends up circling it anticlockwise. Air pushed away from a high is deflected the same way and circles clockwise. The Buys Ballot law helps in finding the centre: in the northern hemisphere, a person standing with the back to the wind has low pressure to the left.

What Weather Do They Bring and Where Are They Found?

Cyclones: extra-tropical cyclones of the middle latitudes and tropical cyclones near the equator belong here. Tropical cyclones, tornadoes and other systems are in Tropical Cyclones: Formation Conditions, Paths, Naming Conventions, Early Warning Systems and Cumulonimbus Tornado Rotating funnel reaching the ground Scale Fujita scale for tornadoes Tornado Alley Great Plains of…">Thunderstorms and Tornadoes.
Anticyclones: include the subtropical highs over the oceans, such as the Azores and Pacific highs, and cold highs over continents in winter, such as the Siberian High. Anticyclones bring fine weather, but they also bring fog, frost and smog in winter by trapping cool air and pollution near the surface.
Blocking highs: persistent anticyclones that stop the usual passage of weather systems, causing heat waves or long dry spells.
India: the winter high over Siberia and the northwest of India sends cold, dry winds towards the sea. Western disturbances, which are cyclonic systems, bring winter rain to the north.

As an analytical point, the circulation around highs and lows is the basic building block of weather maps. The planetary pattern that these systems fit into is in General Circulation of the Atmosphere: Hadley Cell, Ferrel Cell, Polar Cell.

Why Is This Topic Important?

Concepts Coriolis and geostrophic wind asked
Cyclones Explains circulation
Navigation Winds and currents

What Came Next?

The next topic describes the general circulation of the atmosphere: the Hadley, Ferrel and Polar cells. Next topic: General Circulation of the Atmosphere: Hadley Cell, Ferrel Cell, Polar Cell.

Key People and Terms

Pressure gradient force Force from pressure difference
Coriolis force Apparent deflecting force
Geostrophic wind Wind parallel to isobars
Anemometer Wind speed instrument
Ferrel’s law Deflection right in north, left in south
Boundary layer Friction layer near the ground

Exam Corner

Points to Remember

Coriolis deflects right in the north, left in the south.
Coriolis force is zero at the equator.
Pressure gradient force acts across the isobars.
Geostrophic wind blows parallel to isobars.
Northern low pressure rotates anticlockwise.

Do Not Confuse With

Pressure gradient and Coriolis force Starts the wind versus turns it
Geostrophic and surface wind Parallel to isobars versus crossing them
Cyclone and anticyclone Low versus high

Memory Hook

Gradient pushes, Coriolis turns, friction drags; parallel to isobars is geostrophic.

Mains Angle

Questions ask for forces and their effect on winds. These points add depth.

Forces Gradient, Coriolis, friction
Pattern Cyclone and anticyclone
Limit Coriolis fails at the equator

A Question You May Face

An original practice question, not a past paper question.

“Explain the forces that act on winds and show how they produce the circulation around low and high pressure systems.”

How to Answer

Gradient Starts the flow.
Coriolis Deflects.
Friction Slows and crosses isobars.
Circulation Cyclone and anticyclone.

GS Relevance

Prelims: direction of rotation of cyclones and anticyclones, weather features. GS Paper 1: atmospheric circulation.

Frequently Asked Questions

What is the geostrophic wind?

The geostrophic wind is the wind that blows parallel to the isobars above the friction layer, where the pressure gradient force is balanced by the Coriolis force. Upper-level winds, including the jet streams, are close to geostrophic.

Why do winds spiral around a low pressure area?

Air moves toward the low pressure under the gradient force, but the Coriolis force deflects it. In the northern hemisphere it is deflected to the right, so the winds spiral inward in an anticlockwise direction around a low, and clockwise in the southern hemisphere.

What is cyclonic circulation?

Cyclonic circulation is the flow of air around a centre of low pressure. Air converges towards the centre, rises, and cools, forming cloud and rain, and the Coriolis force makes it circle anticlockwise in the northern hemisphere and clockwise in the southern hemisphere.

What is anticyclonic circulation?

Anticyclonic circulation is the flow of air around a centre of high pressure. Air sinks from above and flows outward at the surface, circling clockwise in the northern hemisphere and anticlockwise in the southern. The sinking air warms and dries, giving clear skies and settled weather.

What weather do anticyclones bring?

Anticyclones usually bring clear skies, dry air and light winds, because the descending air warms and prevents cloud formation. In winter they can bring frost, fog and smog by trapping cold air and pollution near the surface, and persistent ones cause heat waves or droughts.

PYQ Practice — Statement Analysis

1 Friction is greater over the sea than over the land.

False

Friction is greater over rough land.

2 In the northern hemisphere air circulates clockwise around a low pressure centre.

False

It circulates anticlockwise.

3 Anticyclones usually bring clear weather.

True

The descending air is dry.

4 Air converges at the surface in a cyclone.