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NDA Geography · Climatology, Atmosphere and Weather

Atmospheric Pressure and Winds

Differences in air pressure set air in motion, and the Earth's rotation (the Coriolis force) bends that motion — together they create the planetary wind belts, the geostrophic winds aloft, and the high-pressure anticyclones.

All Climatology, Atmosphere and Weather notesNDA Geography strategy

Why this matters

6 PYQs, several of them HARD, and the Coriolis force is the star: where it is strongest (the poles), why it is zero at the equator, and how it balances the pressure-gradient force to give geostrophic winds. Pair that with the pressure-belt picture and the anticyclone definition and you have the subtopic covered.

Concept 1 of 4: Pressure belts and planetary winds

Air pressure on Earth is organised into belts. At the equator the air is heated, rises and leaves a LOW-pressure belt (the doldrums). Near 30 degrees the descending air piles up into a HIGH-pressure belt (the subtropical highs / horse latitudes). Winds blow OUT of the highs toward the lows, and the Coriolis force bends them: trade winds toward the equator, westerlies (the Roaring Forties) toward the poles. Pressure is measured in millibars.

Definition

The major pressure belts, equator to pole:

  • Equatorial low (doldrums) — rising hot air, calm and rainy.
  • Subtropical high (~30 deg, horse latitudes) — descending air, dry, source of trade winds and westerlies.
  • Subpolar low (~60 deg) and polar high.

Planetary winds blow from high to low pressure, deflected by Coriolis:

  • Trade winds — from subtropical highs toward the equatorial low.
  • Westerlies — from subtropical highs toward the subpolar lows. The strong Southern-Hemisphere westerlies over open ocean are the Roaring Forties.
  • Air pressure in India is measured in millibars (the usual unit).
Polar High (90 N)Subpolar Low (60 N)Subtropical High (30 N)Equatorial Low (doldrums)Subtropical High (30 S)Subpolar Low (60 S)Polar High (90 S)TradesWesterliesTradesWesterliesN PoleS Pole

Worked example

Two claims about the Roaring Forties: (1) they are strong WESTERLY winds of the Southern Hemisphere oceans; (2) they are strong EAST-to-WEST currents caused partly by abundant landmasses acting as wind breaks. Which are correct?
Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

NDA · Apr 2018 · Q76Hard

Example 1 · Climatology, Atmosphere and Weather · Atmospheric Pressure and Winds

Consider the following statements about Roaring Forties: 1. They are strong Westerly winds found in the oceans of Southern Hemisphere. 2. The strong east to west air currents are caused by the combination of air being displaced from the Equator towards the South Pole and the earth's rotation and abundance of landmasses to serve as wind breaks. Which of the statements given above is/are correct?

Westerlies blow WEST-to-EAST

A Roaring-Forties trap calls them 'east-to-west currents' and credits 'abundant landmasses as wind breaks'. Both are wrong: westerlies blow west-to-east, and the Southern Ocean has FEW landmasses — the scarcity of land is exactly why these winds stay strong and unbroken.
Drill 2 more on pressure belts and planetary winds

Concept 2 of 4: The Coriolis force

The spinning Earth makes moving air appear to curve — right in the Northern Hemisphere, left in the Southern. This Coriolis force is STRONGEST at the poles and ZERO at the equator (so at the equator wind crosses the isobars straight, without bending). It always acts at right angles to the wind's motion, i.e. perpendicular to the pressure-gradient force.

Definition

  • The Coriolis force deflects moving air: to the RIGHT in the Northern Hemisphere, to the LEFT in the Southern.
  • It is largest at the poles and zero at the equator (it varies with the sine of the latitude).
  • Because it is zero at the equator, wind there blows perpendicular to the isobars (straight across, undeflected) — NOTE the bank treats this 'wind perpendicular to isobars at the equator' as a SUBTLE/incorrect framing in one PYQ; what is unambiguously true is that Coriolis acts perpendicular to the pressure-gradient force.
  • It arises purely from the Earth's rotation.

Worked example

Where is the Coriolis effect largest: at the equator, the tropics, the North Pole, or 45 degrees latitude?
Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

NDA · Sep 2024 · Q135Moderate

Example 2 · Climatology, Atmosphere and Weather · Atmospheric Pressure and Winds

Where is the Coriolis Effect the largest ?

Largest at the poles, zero at the equator

Distractors offer 'the equator' or '45 degrees' for where Coriolis is strongest. It scales with the sine of latitude: zero at the equator, maximum at the poles.
Drill 2 more on the coriolis force

Concept 3 of 4: The geostrophic wind

High above the ground (above ~600 m), friction with the surface disappears. There the pressure-gradient force (pushing air from high to low pressure) is exactly balanced by the Coriolis force, and the wind ends up blowing PARALLEL to the isobars rather than across them. That balanced upper wind is the geostrophic wind.

Definition

  • The geostrophic wind is the horizontal wind that results when the Coriolis force exactly balances the horizontal pressure-gradient force.
  • It blows parallel to the isobars, above a height of about 600 m (above the friction layer).
  • Statement II ('Coriolis balances the pressure force') is the correct EXPLANATION of statement I ('it blows parallel to the isobars above 600 m').

Worked example

Why does the geostrophic wind blow parallel to the isobars rather than straight from high to low pressure?
Practice this concept3 quick reps

The same idea in a real exam question:

NDA · Sep 2018 · Q56Hard

Example 3 · Climatology, Atmosphere and Weather · Atmospheric Pressure and Winds

Directions : The following six (6) items consist of two statements, Statement I and Statement II. Examine these two statements carefully and select the correct answer using the code given below. Code: (a) Both the statements are individually true and Statement II is the correct explanation of Statement I (b) Both the statements are individually true but Statement II is not the correct explanation of Statement I (c) Statement I is true but Statement II is false (d) Statement I is false but Statement II is true
Statement I : Geostrophic wind blows above a height of 600 metres, parallel to the isobars. Statement II : Geostrophic wind is the horizontal wind velocity, in which the Coriolis force balances the horizontal pressure force.

Concept 4 of 4: Anticyclones

An anticyclone is the opposite of a cyclone: a HIGH-pressure system. Air sinks (subsides) at its centre and then spreads outward, so the winds DIVERGE (flow away), not converge. Sinking air warms and dries, which is why anticyclones bring calm, clear, fair weather.

Definition

  • An anticyclone is a HIGH-pressure system — correct.
  • Air in its centre subsides (sinks) — correct.
  • Its surface winds DIVERGE outward, NOT converge — so the claim 'characterised by converging winds' is FALSE (only cyclones have converging winds).
  • Subsiding, warming air gives anticyclones their fair, settled weather.

Worked example

Three claims: (1) anticyclones are high-pressure systems; (2) air at the centre subsides; (3) anticyclones have converging winds. How many are correct?
Practice this concept3 quick reps

The same idea in a real exam question:

NDA · Sep 2022 · Q128Moderate

Example 4 · Climatology, Atmosphere and Weather · Atmospheric Pressure and Winds

Consider the following statements about anticyclones : 1. Anticyclones are high pressure systems. 2. Air in the centre of the system must be subsiding. 3. Anticyclones are characterized by converging winds. How many of the above statements is/are correct?

Anticyclone winds DIVERGE

The trap statement gives anticyclones 'converging winds'. Converging winds belong to cyclones. Anticyclones have sinking, diverging air — the source of their fair weather.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

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