Climatology studies the atmosphere and long-term weather patterns. This article covers the key concepts behind global weather and climate.
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World Geography · WorldGeo0002
Climatology: Atmosphere, Weather & Climate
Five Atmospheric Layers
Troposphere — up to ~13 km, all weather happens here
Stratosphere — up to ~50 km, holds the ozone layer
Mesosphere — up to ~80 km, meteors burn up here
Thermosphere — ionosphere reflects radio waves
Exosphere — outermost, fades into space
Three Circulation Cells
Hadley Cell — equator to ~30°, gives trade winds
Ferrel Cell — mid-latitudes, gives the westerlies
Polar Cell — poles to ~60°, gives polar easterlies
Together — move heat from equator toward poles
Four Pressure Belts
Equatorial low — hot rising air, calm winds
Subtropical high — near 30°N/S, sinking dry air
Sub-polar low — near 60°N/S, stormy belt
Polar high — cold, dense, sinking air
Forces Behind Wind
Pressure gradient — pushes air high to low
Coriolis force — deflects wind, zero at equator
Friction — slows wind near the surface
Result — geostrophic wind, parallel to isobars
One connected system
Layers trap heat, pressure belts push air, circulation cells carry it toward the poles — three separate-looking topics that are really one heat-distribution machine.
✊ Must Know
1. Atmosphere, Heat, and Water Vapour
- Atmospheric Layers The atmosphere is made up of distinct layers, each with different temperature and pressure characteristics.
- Composition Air is a mixture of gases: nitrogen makes up about 78%, oxygen about 21%, with carbon dioxide, argon, helium, ozone and hydrogen making up the remaining roughly 1%, plus tiny dust particles. Plants cannot take nitrogen directly — soil bacteria convert it for them. Oxygen from green plants balances the oxygen used in breathing.
- Greenhouse & Warming Carbon dioxide acts as a greenhouse gas by trapping heat radiated from the earth; without it the earth would be too cold, but extra CO2 from factory smoke and car fumes raises the temperature (global warming), which melts polar snow, raises sea level and may flood coasts.
- Layers The atmosphere has five layers: the troposphere (most important, average height ~13 km, where almost all weather like rain, fog and hail occurs), stratosphere (to ~50 km, cloud-free and ideal for planes, holding the ozone layer), mesosphere (to ~80 km, where meteorites burn up), thermosphere (temperature rises fast; the ionosphere part, ~80-400 km, reflects radio waves) and the topmost exosphere.
- Exact Numbers In the troposphere, temperature drops about 1°C every 165 m of height. The tropopause sits near -80°C over the equator, but only -45°C over the poles, since the equatorial troposphere is so much taller.
- Where Gases Run Out Oxygen becomes negligible above 120 km. Carbon dioxide and water vapour are both gone by 90 km. Nearly 99% of the atmosphere’s total mass sits within just 32 km of the surface.
- Dust Particles These come from sea salt, fine soil, smoke-soot, ash, pollen, and even bits of burnt-up meteors. They sit mostly in the lower atmosphere, but strong convection currents can carry them much higher.
- Hygroscopic Nuclei Dust and salt particles are not just passive passengers in the air. Water vapour actually condenses around them, which is exactly how clouds get their start.
- Greenhouse Effect Global warming refers to the rise in Earth’s average temperature. The greenhouse effect is the natural process that traps heat and makes this warming possible.
- Wind & Pressure Belts Global and local wind and pressure belts drive much of the world’s weather, moving heat and moisture across the planet.
- Climate Classification Climate types vary systematically by latitude and region, from tropical to polar, each shaping the ecosystems and human activity found there.
- Data Water vapour is a gas whose amount decreases with altitude, since colder air higher up holds less moisture. About 90% of atmospheric moisture sits within 6 km of the surface. Its percentage is highest near the equator, not at the poles, since warmer tropical air can hold far more moisture than the cold, dry polar air.
- Insolation Insolation is the incoming solar energy the earth intercepts. It decreases from the equator toward the poles, which is why temperature falls the same way.
- Three Types of Rainfall Convectional rainfall forms when warm, moist air rises and cools on its own. Orographic rainfall occurs when moist wind is forced up and over a mountain barrier. Cyclonic rainfall comes from the rising air within a cyclone’s low-pressure system.
📘 Good to Know
2. Timescales, and Latitude Effects
- Instead Weather and climate describe different timescales. Weather is the short-term atmospheric condition. Climate is the long-term pattern averaged over many years.
- Precipitation & Humidity Precipitation, humidity, and related phenomena like thunderstorms result from the interaction of temperature, moisture, and pressure systems.
- Latitudinal Variation Latitudinal changes drive much of the variation in weather systems. The angle and intensity of incoming sunlight varies systematically from the equator to the poles.
- Data Atmospheric dust is more concentrated over subtropical and temperate areas than over equatorial or polar regions. Frequent dry winds in these zones pick up loose surface material and keep it suspended, unlike the wetter equatorial belt or the snow-covered poles.
- Data The troposphere is far thicker at the equator, about 18 km, than at the poles, about 8 km. Intense equatorial solar heating drives strong convectional currents that carry heat to great heights, pushing the tropopause boundary much higher there.
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💎 Great to Know
3. Why the Atmosphere Warms the Way It Does
- Why The greenhouse effect is a case of a natural process becoming a policy problem at a different scale. In moderate amounts, it makes Earth’s surface warm enough to support life. In excess, driven by rising greenhouse gas concentrations, the same basic mechanism drives disruptive climate change.
- Instead Without an atmosphere, Earth’s surface would sit well below freezing everywhere. Gases like carbon dioxide are especially good at absorbing and trapping outgoing radiation, and that trapped heat is what keeps the planet’s average temperature habitable.
- Instead The atmosphere is actually heated more by outgoing terrestrial (long-wave) radiation than by incoming solar (short-wave) radiation. Most sunlight passes straight through the atmosphere and heats the ground instead. It is the ground’s own re-emitted long-wave radiation that carbon dioxide and other greenhouse gases absorb well, which is what actually warms the air.
4. Wind, Rotation, and the Sun’s Angle
- Why Wind and pressure belts function as a global heat-distribution system. Without this constant redistribution of heat from the equator toward the poles, equatorial regions would be far hotter and polar regions far colder than they actually are.
- Coriolis Force The Coriolis force deflects moving air due to Earth’s rotation. It grows stronger as wind velocity increases, and varies by latitude too — it is at its maximum at the poles and effectively absent right at the equator.
- Data On the Northern Hemisphere’s summer solstice, 21 June, day length varies sharply by latitude. The equator stays at a fixed 12 hours. The Tropic of Cancer sees about 13.5 hours of daylight, and the Arctic Circle gets a full 24 hours of continuous “midnight sun.” The Tropic of Capricorn, in the Southern Hemisphere, gets under 12 hours that day.
5. Regional Climate Patterns
- Instead Climate classification systems group regions by shared temperature and precipitation patterns, but real conditions on the ground can vary sharply even within one broad climate type, shaped by elevation, ocean proximity, and local terrain.
- Data The Marine West Coast climate has small annual and daily temperature ranges, thanks to moderating ocean influence, plus year-round precipitation of roughly 50 to 250 cm. Nearby ocean currents and westerly winds carry moisture inland throughout the year.
- Why Land and ocean heat and cool at different rates, since water has a much higher specific heat capacity. In January, Northern Hemisphere land cools faster than the ocean, so isotherms (lines of equal temperature) bend equatorward over land and poleward over the relatively warmer oceans.
- Ocean Currents The Gulf Stream and North Atlantic Drift are warm ocean currents, not cold ones. They keep the North Atlantic warmer than its latitude would suggest, which is exactly why January isotherms bend so far north over that ocean.
Pressure Belts and the Forces Behind Wind
- The Basic Rule Air always moves from a high-pressure area to a low-pressure area, and this moving air is what we call wind. An anemometer measures wind speed.
- Four Pressure Belts Earth has four pressure belts: the equatorial low, the subtropical highs near 30°N/S, the sub-polar lows near 60°N/S, and the polar highs. These belts shift with the sun’s seasonal path.
- Three Forces on Wind Wind direction and speed come from three forces together. These are the pressure gradient force (stronger where isobars sit close together), friction (strongest near the surface), and the Coriolis force.
- Geostrophic Wind High above the friction layer, when isobars run straight, the pressure gradient force and Coriolis force balance out. The wind that results blows parallel to the isobars, and this is called the geostrophic wind.
- Cyclones vs Anticyclones Around a low-pressure centre in the Northern Hemisphere, wind circulates anticlockwise; around a high, it circulates clockwise. Both patterns reverse in the Southern Hemisphere.
The Three-Cell General Circulation
- Hadley Cell Air rises at the ITCZ near the equator, then flows poleward aloft. It sinks back down around 30°N/S as the subtropical high, then returns to the equator at the surface as trade winds. This loop is the Hadley cell.
- Ferrel Cell In the middle latitudes, surface winds called westerlies blow from the subtropical high toward the poles. This middle loop is the Ferrel cell.
- Polar Cell Cold, dense air sinks near the poles. It then flows toward the middle latitudes at the surface as the polar easterlies, completing the third loop.
Local Winds: Breezes, Mountains, and Foehn
- Land and Sea Breeze By day, land heats faster than the sea, so wind blows from sea to land as a sea breeze. By night, land cools faster, reversing the flow into a land breeze.
- Mountain and Valley Winds By day, heated slopes push air upward as a valley breeze. By night, cooling slopes send dense air flowing back down as a mountain wind.
- Katabatic Wind This is cold, dense air draining downhill from high plateaus or ice fields, a stronger, colder cousin of the ordinary mountain wind.
- Warm Leeward Winds Moist air drops its rain while crossing a mountain range. Coming down the far side, it warms up further through compression, and can melt snow quickly.
📝 Previous Year Questions
0. CDS II 2025 GK
- Prelims PYQ On water vapour’s role in the atmosphere, asked as: “Which of the following statements with reference to water vapour in the atmosphere is/are correct?” Both statements were correct: water vapour affects air stability, and it also traps outgoing heat, moderating the Earth’s temperature range. View this question.
1. UPSC CSP 2025 GS Paper I
- Prelims PYQ On dust particles and dry winds, asked as: “Consider the following statements on dust particles in the atmosphere and dry winds.” View this question.
2. UPSC CSP 2025 GS Paper I
- Prelims PYQ On January isotherms bending over land and ocean, asked as: “Consider the following statements on isotherms in the Northern Hemisphere in January.” View this question.
3. UPSC CSP 2025 GS Paper I
- Prelims PYQ On the atmosphere’s greenhouse effect, asked as: “Consider the following statements on the atmosphere and Earth’s temperature.” View this question.
4. UPSC CSP 2024 GS Paper I
- Prelims PYQ On why the atmosphere is heated more by terrestrial than solar radiation, asked as: “Consider the following statements on solar radiation, terrestrial radiation, and greenhouse gases.” View this question.
5. UPSC CSP 2024 GS Paper I
- Prelims PYQ On why the troposphere is thicker at the equator, asked as: “Consider the following statements on troposphere thickness at the equator and poles.” View this question.
6. UPSC CSP 2024 GS Paper I
- Prelims PYQ On isothermal-map inferences and the Gulf Stream, asked as: “Which of the following is/are correct inference/inferences from isothermal maps in the month of January.” View this question.
7. UPSC CSP 2024 GS Paper I
- Prelims PYQ On water vapour and altitude/latitude, asked as: “With reference to water vapour, which of the following statements is/are correct.” View this question.
8. UPSC CSP 2024 GS Paper I
- Prelims PYQ On identifying the Marine West Coast climate type, asked as: “Consider the following description: what is this type of climate.” View this question.
9. UPSC CSP 2024 GS Paper I
- Prelims PYQ On the Coriolis force and its dependence on velocity and latitude, asked as: “With reference to Coriolis force, which of the following statements is/are correct.” View this question.
10. UPSC CSP 2024 GS Paper I
- Prelims PYQ On June 21 day length by latitude, asked as: “On June 21 every year, which of the following latitude(s) experience(s) a sunlight of more than 12 hours.” View this question.
11. UPSC CSP 2023 GS Paper I
- Prelims PYQ On why land-ocean temperature contrast is greater in winter than summer, asked as: “Consider the following statements on temperature contrast between continents and oceans.” View this question.
12. UPSC Mains 2022, GS Paper I, Q17
13. UPSC CSP 2021 GS Paper I
- Prelims PYQ On boundary currents and wind belts, asked as: “Consider the following statements on tropical and temperate ocean current warming.” View this question.
14. UPSC CSP 2019 GS Paper I
- Prelims PYQ On the Sun’s position on 21 June, asked as: “On 21st June, the Sun.” View this question.
15. UPSC CSP 2022 GS Paper I
- Prelims PYQ On when the longest day of the year falls, asked as: “In the northern hemisphere, the longest day of the year normally occurs in the.” View this question.
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