Atmospheric Pressure & Planetary Winds – NDA Geography Notes

Air has weight. The weight of the column of air above any point on the Earth’s surface presses down on that surface. This is atmospheric pressure. Differences in pressure between one place and another are what cause wind. Air always flows from high pressure to low pressure, just as water flows downhill.

NDA tests this chapter through questions on specific pressure belts and why they form, the direction of winds in each hemisphere, the jet stream, and the ITCZ. Understanding the physical logic behind these patterns, rather than memorising them as random facts, is the key to scoring well.

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What is Atmospheric Pressure?

Atmospheric pressure is the force per unit area exerted by the weight of the air column above that point. At sea level, the standard pressure is approximately 1013.25 millibars (mb): also expressed as 760 mm of mercury or 1 atmosphere.

Atmospheric pressure is measured by an instrument called a barometer.

Pressure decreases with altitude, because there is less air above as you go higher. At the summit of Mount Everest, pressure is about one-third of sea-level pressure.

Pressure and temperature: Warm air expands, becomes less dense, and rises, creating low pressure. Cold air contracts, becomes denser, and sinks, creating high pressure.

Isobars are lines on a map connecting all points of equal atmospheric pressure at a given time, similar to how isotherms connect points of equal temperature.

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Pressure Belts: From Equator to Pole

Because temperature varies systematically with latitude, pressure also varies systematically. Alternating pressure belts encircle the globe at specific latitudes:

Equatorial Low Pressure Belt (0°–5° N/S): The Doldrums

At the equator, the Sun is nearly overhead throughout the year. Intense heating causes air to expand, become less dense, and rise, creating a permanent low pressure zone. This belt is also called the Doldrums: a zone of calm, weak, variable winds that sailing ships dreaded because ships could be becalmed for weeks. [NDA 2009-II]

Subtropical High Pressure Belts (25°–35° N and S): Horse Latitudes

The air that rose at the equator moves poleward at high altitude. At about 25°–35° latitude, it cools, becomes denser, and sinks, creating high pressure at the surface. These belts are also called the Horse Latitudes: named because sailing ships carrying horses to the Americas sometimes became becalmed here and threw horses overboard to save water. [NDA 2009-II]

Sub-Polar Low Pressure Belts (60°–65° N and S)

At about 60°–65° latitude, relatively warm air from the tropics meets cold polar air. The warm air is forced upward, creating a low pressure belt. This is the zone where temperate cyclones form.

Polar High Pressure Caps (90° N and S)

At the poles, temperatures are extremely low. Cold, dense air sinks, creating high pressure. [NDA 2009-II]

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Origin of Each Pressure Belt: Thermal vs Dynamic

★ IMPORTANT NDA 2009-II tested this precisely. Three statements correct, one wrong (Sub-polar low is dynamic, not thermal). [NDA 2009-II]

Pressure Belt	Latitude	Type	Origin	PYQs
Equatorial Low (Doldrums)	0°–5°	Low	Thermal: intense surface heating causes air to rise	[NDA 2009-II]
Subtropical High (Horse Latitudes)	25°–35°	High	Dynamic: sinking air from the atmospheric circulation	[NDA 2009-II]
Sub-polar Low	60°–65°	Low	Dynamic: convergence of contrasting air masses forces air up	[NDA 2009-II]
Polar High	90°	High	Thermal: extreme cold makes air dense and heavy; it sinks	[NDA 2009-II]

The NDA 2009-II answer was (b) 1, 2 and 4 only, because Statement 3 (sub-polar low is thermally induced) is FALSE. The sub-polar low is dynamically induced, not thermal.

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The Coriolis Effect

When air starts flowing from high pressure to low pressure, Earth’s rotation causes it to be deflected from its straight-line path.

In the Northern Hemisphere: moving air is deflected to the right. In the Southern Hemisphere: moving air is deflected to the left.

This deflection is called the Coriolis Effect: after French mathematician Gaspard-Gustave de Coriolis. [NDA 2009-I]

The Coriolis Effect is caused by the ROTATION of the Earth, NOT revolution. [NDA 2009-I]

The Coriolis Effect is zero at the equator and maximum at the poles. At the equator, there is no component of Earth’s rotation perpendicular to the surface, meaning no deflection. At the poles, the surface is perpendicular to the axis of rotation, giving maximum deflection.

Geostrophic Wind: when the Coriolis Effect exactly balances the pressure gradient force, wind blows parallel to isobars rather than across them.

Buys Ballot’s Law: in the Northern Hemisphere, if you stand with your back to the wind, low pressure is to your left. In the Southern Hemisphere, low pressure is to your right.

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Planetary Winds: Overview

Because of the pressure belts and the Coriolis Effect, the Earth develops large, predictable wind systems called planetary winds or prevailing winds: blowing consistently in the same direction across large areas of the globe.

Wind Belt	Pressure Source	Pressure Sink	Direction (N. Hemisphere)	Direction (S. Hemisphere)
Trade Winds	Subtropical High	Equatorial Low	NE to SW	SE to NW
Westerlies	Subtropical High	Sub-polar Low	SW to NE	NW to SE
Polar Easterlies	Polar High	Sub-polar Low	NE to SW	SE to NW

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Trade Winds

Trade winds blow from the subtropical high pressure belts (25°–35°) toward the equatorial low (0°–5°). The Coriolis Effect deflects them:

• Northern Hemisphere: deflected right → blow from NE to SW (Northeast Trade Winds)
• Southern Hemisphere: deflected left → blow from SE to NW (Southeast Trade Winds)

Trade winds are steady, consistent, and reliable. They blow almost continuously in the same direction throughout the year. They were extremely useful for sailing ships during the age of exploration, enabling regular trade routes across the Atlantic. The word “trade” originally meant course or track.

Trade winds are stronger in winter and weaker in summer.

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Westerlies

Westerlies blow from the subtropical high pressure belts (25°–35°) toward the sub-polar low (60°–65°). With Coriolis deflection:

• Northern Hemisphere: deflected right → blow from SW to NE
• Southern Hemisphere: deflected left → blow from NW to SE

Westerlies in the Southern Hemisphere blow across open ocean without any land to interrupt them, making them very strong and consistent. The famous names for zones of strong Southern Hemisphere westerlies:

• Roaring Forties: 40°–50°S [NDA 2011-I]
• Furious Fifties: 50°–60°S
• Screaming Sixties: 60°–70°S

Westerlies are not as regular as trade winds. They are more variable and associated with temperate cyclones.

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Polar Easterlies

Polar easterlies blow from the polar high (90°) toward the sub-polar low (60°–65°). With Coriolis deflection:

• Northern Hemisphere: blow from NE to SW
• Southern Hemisphere: blow from SE to NW

Polar easterlies are cold, dry, and irregular.

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The ITCZ

Where the northeast and southeast trade winds from both hemispheres meet near the equator, they converge and the air is forced upward. This convergence zone is called the ITCZ: Intertropical Convergence Zone.

The ITCZ migrates north and south with the seasons, following the overhead position of the Sun. In June it is north of the equator; in December it is south. The ITCZ does NOT always lie exactly on the geographical equator. This seasonal migration of the ITCZ is what drives the Indian monsoon.

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Jet Streams

A jet stream is a fast-moving, narrow band of wind found in the upper troposphere at altitudes of about 9–12 km. Jet streams blow from west to east and can reach speeds of 200–400 km/h.

Jet streams form at the boundaries between warm and cold air masses, where the temperature contrast is greatest. [NDA 2012-II]

Types of Jet Streams

Polar Front Jet Stream (Temperate Jet): located at about 60° latitude, at the polar front boundary between cold polar and warm temperate air. Influences the path of temperate cyclones and is the most important jet stream for mid-latitude weather.

Subtropical Jet Stream: located at about 30° latitude, at approximately 12 km altitude (tropopause level). Less variable than the polar front jet.

Tropical Easterly Jet (TEJ): a jet stream flowing from east to west (an easterly) at about 150 mb pressure level over India and Africa in summer. Associated with the Indian summer monsoon. Its establishment over peninsular India helps initiate the monsoon circulation. [NDA 2012-II]

★ IMPORTANT All four jet stream statements tested in NDA 2012-II are correct: (1) Found in upper troposphere (2) Speed 110–184 km/h (3) Generally move west to east (4) Influence movement of cyclones: Answer: all four correct. [NDA 2012-II]

The Tropical Easterly Jet is the major exception: it flows east to west, not west to east. All other major jet streams flow west to east.

Indian Monsoon Connection: The subtropical westerly jet blows over northern India in winter, keeping cold air north of the Himalayas. In summer, as the landmass heats up, this jet shifts northward, allowing the southwest monsoon to penetrate. The TEJ simultaneously establishes itself over peninsular India, reinforcing the monsoon circulation. [NDA 2012-II]

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