Atmospheric Pressure & Planetary Winds – NDA Geography Notes

Introduction

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.

This chapter explains why pressure belts form where they do, why winds blow in the directions they do, what the Coriolis Effect is, what the jet stream is, and what the trade winds and westerlies are. These are the planetary-scale wind systems that control weather and climate across the entire globe.

NDA tests this chapter through questions on specific pressure belts, 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 here.

Understand the Topic

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. [NDA tested via CDS]

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 decreases with increase in altitude — TRUE. [NDA tested via CDS]

Pressure and temperature:
Temperature and pressure are inversely related in general — 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. [NDA tested via CDS]

Horizontal Distribution of Pressure — Pressure Belts

Because temperature varies systematically with latitude, pressure also varies systematically. The Earth’s surface can be divided into alternating pressure belts — bands of high and low pressure encircling the globe at specific latitudes.

The major pressure belts from the equator to the poles are:

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1. Equatorial Low Pressure Belt (0°–5°N/S)

At the equator, the Sun is nearly overhead throughout the year. Intense heating causes the air to expand, become less dense, and rise. This rising air creates a permanent low pressure zone at the equator.

This belt is also called the doldrums — a zone of calm, weak, variable winds. Sailors in the age of sail dreaded the doldrums because ships could be becalmed for weeks with no wind.

The equatorial belt is a zone of low pressure because of thermal heating — the surface heats intensely, air rises, and pressure falls. [NDA 2009-II]

2. Subtropical High Pressure Belts (25°–35°N and S)

The air that rose at the equator moves poleward at high altitude. As it moves away from the equatorial heat source and reaches about 25°–35° latitude, it cools, becomes denser, and sinks. This sinking air creates 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 had to throw horses overboard to save water. [NDA tested via CDS]

The subtropical high is caused by dynamic factors (sinking air caused by atmospheric circulation) rather than purely thermal factors. [NDA 2009-II]

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3. 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 — the meeting of contrasting air masses creates turbulent weather.

4. Polar High Pressure Caps (90°N and S)

At the poles, temperatures are extremely low. Cold dense air sinks, creating high pressure.

Polar air is intensely cold and dense — its weight creates the polar high. This is a thermally induced high — caused by extreme cold and dense air. [NDA 2009-II]

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Why Equatorial Low and Polar High are Different in Origin:

The equatorial low is thermally induced — caused by intense heating.
The subtropical high is dynamically induced — caused by sinking air returning from the equatorial rising current.
The polar high is thermally induced — caused by intense cooling. [NDA 2009-II]

The Coriolis Effect

When air starts flowing from high pressure to low pressure, it does not flow in a straight line. The Earth’s rotation causes moving air (and water) to be deflected from its path.

In the Northern Hemisphere, moving objects (including wind) are deflected to the right.
In the Southern Hemisphere, they are deflected to the left.

This deflection is called the Coriolis Effect — after French mathematician Gaspard-Gustave de Coriolis who described it mathematically. [NDA 2009-I | NDA tested via CDS]

The Coriolis Effect is caused by the rotation of the Earth. [NDA 2009-I]

The Coriolis Effect is zero at the equator and maximum at the poles. [NDA tested via CDS]

At the equator, the Earth’s surface moves parallel to the Earth’s axis — there is no component of rotation perpendicular to the surface, so there is no Coriolis deflection. At the poles, the surface is perpendicular to the axis of rotation — maximum Coriolis deflection.

Geostrophic Wind:
When the Coriolis Effect balances the pressure gradient force exactly, wind blows parallel to isobars — not across them. This is called a geostrophic wind. [NDA tested via CDS]

Buys Ballot’s Law:
A law that describes the relationship between wind direction and pressure. 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. [NDA tested via CDS]

Planetary Winds — Global Wind Patterns

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

1. Trade Winds

Trade winds blow from the subtropical high pressure belts (25°–35°) toward the equatorial low (0°–5°).

Without Coriolis: they would blow straight toward the equator (north-south).
With Coriolis deflection:

• In the Northern Hemisphere: deflected to the right → blow from NE to SW — called Northeast Trade Winds
• In the Southern Hemisphere: deflected to the left → blow from SE to NW — called Southeast Trade Winds

Trade winds are steady, consistent, and reliable — they blow almost continuously in the same direction throughout the year. This made them extremely useful for sailing ships during the age of exploration — ships used them to cross from Europe to the Americas. The word “trade” originally meant course or track — these were the winds that enabled regular trade routes. [NDA tested via CDS]

Trade winds are stronger in winter and weaker in summer. [NDA tested via CDS]

ITCZ — Intertropical Convergence Zone:

Where the northeast and southeast trade winds from both hemispheres meet near the equator, they converge — the air is forced upward. This convergence zone is called the ITCZ (Intertropical Convergence Zone). [NDA tested via CDS]

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. This seasonal migration of the ITCZ is what drives the Indian monsoon. [NDA tested via CDS]

2. Westerlies

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

Without Coriolis: they would blow straight toward the poles.
With Coriolis deflection:

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

Westerlies in the Southern Hemisphere blow across open ocean without any land to interrupt them. This makes them very strong and consistent. The zone around 40°–60°S is famous for strong westerly winds — called:

• Roaring Forties (40°–50°S)
• Furious Fifties (50°–60°S)
• Screaming Sixties (60°–70°S)
  [NDA 2011-I | NDA tested via CDS]

Westerlies are not as regular as trade winds — they are more variable and associated with temperate cyclones. [NDA tested via CDS]

3. Polar Easterlies

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

With Coriolis deflection:

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

Polar easterlies are cold, dry, and irregular. [NDA tested via CDS]

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Summary of Planetary Winds:

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

Jet Streams

A jet stream is a fast-moving, narrow band of wind found in the upper troposphere and lower stratosphere — 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 | NDA tested via CDS]

Types of Jet Streams:

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

Subtropical Jet Stream:
Located at about 30° latitude. Less variable than the polar front jet.

Tropical Easterly Jet (TEJ):
A jet stream flowing from east to west at about 150 mb pressure level over India and Africa in summer. It is associated with the Indian summer monsoon. The onset of the tropical easterly jet over India helps establish the monsoon pattern. [NDA 2012-II]

Characteristics of Jet Streams:

All four of these statements are correct: [NDA 2012-II]

1. They are found in the upper troposphere — TRUE
2. Their speed varies between 110 and 184 km/h on average — TRUE
3. They generally move from west to east — TRUE
4. They influence the movement of cyclones — TRUE

Jet streams move from west to east in the upper atmosphere. [NDA 2012-II]

The Subtropical Jet Stream is found at an altitude of 12 km at the tropopause level. [NDA tested via CDS]

The formation of jet stream requires a large contrast between temperature of air masses. [NDA tested via CDS]

The Indian Monsoon Connection:

The Indian summer monsoon is closely linked to jet streams. The subtropical westerly jet stream blows over northern India in winter, keeping cold air to the north of the Himalayas. In summer, as the landmass heats up, this jet stream shifts northward — allowing the southwest monsoon to penetrate India. The tropical easterly jet simultaneously establishes itself over peninsular India, reinforcing the monsoon circulation. [NDA 2012-II | NDA tested via CDS]

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Local Pressure Differences and Wind

At the local scale, pressure differences are caused by differential heating rather than global pressure belts.

Sea Breeze and Land Breeze (covered in Chapter 15) are examples of local winds caused by temperature differences between land and sea.

Mountain Breeze and Valley Breeze are caused by temperature differences between valley floors (warmer) and mountain slopes (cooler).