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Quick recap: climatology on one page

Climatology: the atmosphere, winds, weather systems and world climates · Quick recap · Fact-checked on · how we check

Written with AI from the sources listed at the bottom of this page, then fact-checked by a separate AI.

All four parts condensed for quick revision: the layers of the atmosphere, insolation, the heat budget and temperature; pressure belts, the forces on winds, planetary and local winds and jet streams; humidity, rainfall, air masses, fronts and cyclones; and the Köppen classification of world and Indian climates.

Climatology follows one chain of cause and effect. The Sun heats the ground, the ground heats the atmosphere from below, uneven heating creates differences of pressure, pressure differences drive the winds, and the winds carry moisture that becomes cloud, rain and storms. Averaged over decades, all of this produces the climates of the world. Part 1 covers the atmosphere and temperature, Part 2 pressure and winds, Part 3 moisture and weather systems, and Part 4 the classification of climates.

Part 1: nitrogen and oxygen make up most of dry air, but it is the changing amounts of water vapour and dust that shape the weather. Weather happens in the troposphere, whose depth is roughly 18 km over the equator but only about 8 km over the poles; within it the air cools by about 6.5 °C for every kilometre of height. The ozone layer in the stratosphere absorbs ultraviolet rays, which is why the stratosphere warms with height; the Montreal Protocol (1987) controls ozone-depleting chemicals. insolation is highest over the subtropical deserts, not the cloudy equator. The atmosphere is heated mainly from below by long-wave radiation from the ground, and greenhouse gases keep the surface far warmer than it would otherwise be. On clear, calm winter nights a temperature inversion can trap fog and smog in valleys.

Part 2: pressure at sea level averages about 1013 hPa and falls quickly with height. Two pressure belts, the equatorial low and the polar highs, are thermal in origin, set by heating and cooling; the other two, the subtropical highs and the subpolar lows, owe their origin to the way air moves. Winds start because of the pressure gradient, are turned by the Coriolis force (to the right in the Northern Hemisphere, to the left in the Southern, and not at all at the equator) and are slowed by friction near the ground. Three belts of planetary winds (the trade winds, the westerlies and the polar easterlies) are driven by three circulation cells: the Hadley, the Ferrel and the polar cell. Monsoons reverse with the seasons, land and sea breezes with day and night. Near the tropopause, fast jet stream winds steer western disturbances in winter and accompany the Indian monsoon.

Part 3: relative humidity is the vapour present as a share of what the air could hold, and it falls when air warms. Evaporation absorbs latent heat and condensation releases it, which is the fuel of thunderstorms and cyclones. Rain is convectional, orographic (with a rain-shadow on the leeward side) or frontal. Air masses are maritime or continental, tropical or polar, and meet along fronts. For a tropical cyclone to form, the sea surface must be warm (roughly 26–27 °C), the Coriolis force must be strong enough and the winds must change little with height; it has a calm eye and causes storm surges on low coasts. The Bay of Bengal sees more cyclones than the Arabian Sea, and IMD's regional centre in New Delhi names them. Temperate cyclones form on the polar front, have fronts but no eye, and travel west to east.

Part 4: the Köppen climate classification uses temperature and rainfall limits matched to vegetation. Group A has every month above 18 °C: equatorial rainforest (Af), monsoon (Am) and savanna (Aw). Group B is dry: hot deserts lie on the western margins of continents between about 15° and 30°, where sinking air, offshore trade winds and cold currents combine. Group C includes the Mediterranean type with dry summers and winter rain, the humid China type and the marine west coast type. The D group, home of the taiga, is found in the Northern Hemisphere alone; the E group covers tundra and ice-cap climates. Applied to India, the scheme gives Amw for the west coast, As for the Coromandel coast, Aw for most of the plateau, Cwg for the Ganga plain, BWhw for western Rajasthan and E for the high Himalaya.

Climatology at a glance
TopicMust-know factsCommon trap
LayersTroposphere about 18 km at equator, 8 km at poles; ozone in stratosphere; mesosphere coldestTroposphere is thickest at the equator, not the poles
HeatingAir heated from below by long-wave radiation; insolation highest over subtropical desertsMaximum insolation is not at the equator
Pressure beltsEquatorial low and polar highs thermal; subtropical highs and subpolar lows dynamicDo not swap thermal and dynamic belts
Coriolis forceRight in NH, left in SH, zero at equator; changes direction, not speedNo cyclones form right on the equator
CyclonesTropical: warm sea, eye, storm surge; temperate: fronts, no eye, west to eastFronts belong to temperate, not tropical, cyclones
KöppenA >18 °C every month; B dry; C mild winter; D cold, NH only; E warmest month <10 °Cs = dry summer, w = dry winter

Exam traps

Recap

Practice questions

Q1. In which layer of the atmosphere does temperature increase with height because of the absorption of ultraviolet radiation by ozone?

  1. Troposphere
  2. Stratosphere
  3. Mesosphere
  4. Exosphere
Show answer

Answer: B. Stratosphere

The ozone layer lies in the stratosphere, and the ultraviolet energy it absorbs warms that layer with height. Temperature falls with height in the troposphere and the mesosphere, and the exosphere is the thin outermost fringe where ozone plays no such role.

Q2. Consider the following statements:
1. Subtropical deserts receive more insolation at the ground than any other region.
2. The atmosphere is heated mainly by incoming short-wave solar radiation passing through it.
Which of the statements given above is/are correct?

  1. 1 only
  2. 2 only
  3. Both 1 and 2
  4. Neither 1 nor 2
Show answer

Answer: A. 1 only

Statement 1 is correct because clear skies over the subtropical deserts let more sunlight reach the ground than over the cloudy equator. Statement 2 is wrong: the air gets most of its heat from the ground beneath it, which absorbs sunlight and gives it back as long-wave radiation. So '2 only' and 'both' are wrong, and 'neither' rejects a true statement.

Q3. Which of the following pressure belts are dynamic rather than thermal in origin?
1. Equatorial low
2. Subtropical high
3. Subpolar low
4. Polar high
Select the correct answer using the code given below:

  1. 1 and 4 only
  2. 2 and 3 only
  3. 1, 2 and 3 only
  4. 2, 3 and 4 only
Show answer

Answer: B. 2 and 3 only

The subtropical highs form where air piles up and sinks, and the subpolar lows where air is forced to rise; both are dynamic. The equatorial low (heating) and polar high (cooling) are thermal, so any option including 1 or 4 is wrong.

Q4. Match the local winds with their nature:
A. Loo — 1. Cold wind of the Adriatic coast
B. Chinook — 2. Hot, dry summer wind of north India
C. Bora — 3. Warm, dry wind on the lee of the Rockies
Select the correct code:

  1. A-2, B-3, C-1
  2. A-3, B-2, C-1
  3. A-2, B-1, C-3
  4. A-1, B-3, C-2
Show answer

Answer: A. A-2, B-3, C-1

The Loo blows hot and dry across the plains of north India in summer; the Chinook comes down the eastern slopes of the Rockies as a warm, dry wind; the Bora is a cold wind of the Adriatic coast: A-2, B-3, C-1. The other codes call the Loo or Chinook cold, or place the Chinook in India, which is wrong.

Q5. Which one of the following conditions is NOT favourable for the formation of a tropical cyclone?

  1. Sea surface temperature of about 27 °C
  2. Location right on the equator
  3. Low vertical wind shear
  4. Release of latent heat by condensation
Show answer

Answer: B. Location right on the equator

On the equator the Coriolis force is zero, so the air cannot start spinning; cyclones form a few degrees away from it. A warm sea surface, low wind shear and latent heat released by condensation are all needed for a cyclone to form and grow.

Q6. Consider the following statements about temperate cyclones:
1. They form along the polar front where warm and cold air masses meet.
2. They generally move from east to west.
Which of the statements given above is/are correct?

  1. 1 only
  2. 2 only
  3. Both 1 and 2
  4. Neither 1 nor 2
Show answer

Answer: A. 1 only

Statement 1 is correct: temperate cyclones develop on the polar front between tropical and polar air. Statement 2 is wrong: carried by the westerlies, they move from west to east. So '2 only' and 'both' are wrong, and 'neither' rejects a true statement.

Q7. A region has hot, dry summers and mild, rainy winters and lies on the western side of a continent at about 35° latitude. Its climate is most likely:

  1. Mediterranean
  2. Humid subtropical (China type)
  3. Tropical savanna
  4. Taiga
Show answer

Answer: A. Mediterranean

Dry summers, winter rain and a west-coast position near 35° define the Mediterranean (Cs) climate. The China type lies on eastern margins with summer rain; the savanna is tropical with summer rain and a dry winter; the taiga is a cold forest of high northern latitudes.

Q8. In the Köppen classification, the Ganga plain falls under which climatic type?

  1. Amw
  2. As
  3. Cwg
  4. BWhw
Show answer

Answer: C. Cwg

The Ganga plain is Cwg: winters are dry, and the warmest month arrives ahead of both the summer solstice and the monsoon rains. Amw is the west coast south of Goa, As the Coromandel coast of Tamil Nadu, and BWhw the hot desert of extreme western Rajasthan.

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