- Climatology and oceanography make up the second half of NCERT's physical geography and are the background to everything from the Indian monsoon to cyclone warnings and fisheries.
- The atmosphere and oceans work as one system: the unequal heating of the Earth creates pressure differences, which drive winds, which in turn drive ocean currents.
- For exams, the essentials are the layers of the atmosphere, the pressure belts and wind system, cyclones, and warm vs cold currents.
Composition
Dry air is about 78% nitrogen, 21% oxygen and 0.93% argon; carbon dioxide is only about 0.04% (around 420 parts per million) but traps heat. Water vapour and dust are variable but crucial for clouds and rain.
Layers of the Atmosphere
| Layer | Height | Key features |
|---|---|---|
| Troposphere | About 8 km at the poles to 18 km at the equator | All weather happens here; temperature falls about 6.5°C per km (the normal lapse rate) |
| Stratosphere | To about 50 km | Ozone layer (mainly 15–35 km) absorbs UV, so temperature rises with height; calm air suits jet aircraft |
| Mesosphere | To about 80–85 km | Coldest layer (about −100°C); meteors burn up here |
| Thermosphere (ionosphere) | About 80–400 km | Charged layers reflect radio waves; auroras |
| Exosphere | Outermost | Air gradually thins into space |
Heat Concepts
- Insolation — incoming solar radiation; highest in the tropics.
- Albedo — the share of sunlight reflected; fresh snow reflects the most.
- Temperature inversion — temperature rises with height instead of falling; common on long, clear winter nights and in valleys, trapping fog and smog.
Pressure Belts and Planetary Winds
| Belt | Latitude | Notes |
|---|---|---|
| Equatorial low | 0°–5° | Calm, rising air — the doldrums |
| Subtropical high | About 30° N and S | Sinking, dry air — the horse latitudes; hot deserts lie here |
| Subpolar low | About 60° N and S | Meeting zone of westerlies and polar air |
| Polar high | 90° | Cold, dense air |
Winds blow from high to low pressure: trade winds (easterlies) from the subtropical highs to the equator; westerlies from the subtropical highs to the subpolar lows; and polar easterlies. In the Southern Hemisphere, the strong westerlies are called the Roaring Forties, Furious Fifties and Shrieking Sixties. The Coriolis force deflects winds to the right in the Northern Hemisphere and to the left in the Southern; it is zero at the equator.
Nitrogen in air≈78%
Ozone layerStratosphere
Coldest layerMesosphere
Average ocean salinity≈35‰
Deepest pointMariana Trench (~11 km)
Spring tidesNew & full moon
Local Winds of the World
| Wind | Region | Nature |
|---|---|---|
| Chinook | Rockies (USA, Canada) | Warm, dry wind down the eastern slopes; called the "snow eater" |
| Foehn | Alps | Warm, dry wind on the lee side |
| Sirocco | Sahara → southern Europe | Hot, dusty; picks up moisture over the Mediterranean |
| Harmattan | West Africa | Dry, dusty; called the "doctor" because it brings relief from humidity |
| Mistral | Rhône valley, France | Cold, from the Alps |
| Bora | Adriatic coast | Cold, gusty |
| Santa Ana | California | Hot, dry; spreads wildfires |
For Indian local winds such as the Loo, mango showers and Kalbaisakhi, see our Climate & Monsoon notes.
Cyclones
| Tropical cyclone | Temperate cyclone | |
|---|---|---|
| Origin | Over warm oceans (above about 27°C), roughly 5°–20° latitude | Where warm and cold air masses meet (fronts), in mid-latitudes |
| Energy | Latent heat from condensation; weakens over land | Contrast between air masses; forms over land and sea |
| Structure | Calm eye surrounded by the eyewall with the strongest winds | No eye; large, with warm and cold fronts |
| Names | Hurricanes (Atlantic, eastern Pacific), typhoons (western Pacific), cyclones (Indian Ocean) | Western disturbances that bring winter rain to north-west India |
Cyclones rotate anticlockwise in the Northern Hemisphere and clockwise in the Southern. They do not form within about 5° of the equator because the Coriolis force there is too weak. North Indian Ocean cyclones are named from a list prepared by a panel of 13 countries, coordinated by the IMD in New Delhi.
Clouds
Cirrus (high, thin, feathery), cumulus (cotton-wool heaps, fair weather), stratus (grey layers) and nimbus (dark, rain-bearing). Cumulonimbus clouds bring thunderstorms, lightning and hail.
The Oceans
- The Pacific is the largest and deepest ocean; its Mariana Trench (Challenger Deep) is about 11 km deep. The Atlantic is S-shaped with the Mid-Atlantic Ridge. The Indian Ocean is the only ocean named after a country.
- Salinity averages about 35 parts per thousand. It is high in enclosed seas in hot, dry regions (the Red Sea) and low where rivers and melting ice add fresh water (the Baltic Sea).
- Coral reefs need warm (above about 20°C), shallow, clear, salty water. Types: fringing, barrier (the Great Barrier Reef) and atolls (Lakshadweep).
Ocean Currents
| Warm currents | Cold currents |
|---|---|
| Gulf Stream and North Atlantic Drift — keep north-west European ports ice-free | Labrador (off Canada) |
| Kuroshio (Japan) | Oyashio (Kuril) |
| Brazil, Agulhas, East Australian | Humboldt (Peru), Benguela, California, Canary, West Australian |
Warm currents generally flow along the east coasts of continents in low latitudes and cold currents along the west coasts. Cold currents help create coastal deserts such as the Atacama (Humboldt) and the Namib (Benguela). Where warm and cold currents meet, plankton thrives and rich fishing grounds form — the Grand Banks off Newfoundland (Gulf Stream + Labrador) and the waters off Japan (Kuroshio + Oyashio).
Tides
Tides are caused mainly by the Moon's gravity, helped by the Sun. Spring tides (highest range) occur at new moon and full moon, when the Sun, Moon and Earth are in line; neap tides (lowest range) occur at the quarter moons, when they are at right angles. The Bay of Fundy (Canada) has the world's highest tidal range; in India, the Gulf of Khambhat has the highest.
- Layer ↔ feature: ozone (stratosphere), weather (troposphere), meteors (mesosphere), radio waves (ionosphere).
- Current classification — warm vs cold, and the deserts and fishing grounds they explain.
- Cause-and-effect statements: why cyclones avoid the equator, why spring tides happen, why deserts lie at 30°.
Q1. The ozone layer lies mainly in the:
✅ Answer: (b) Stratosphere
💡 Explanation: Ozone is concentrated at about 15–35 km, in the stratosphere.
Q2. Tropical cyclones do not form near the equator mainly because:
✅ Answer: (c) The Coriolis force is negligible there
💡 Explanation: Without the Coriolis force, the rotating circulation of a cyclone cannot develop.
Q3. Which of the following is a cold ocean current?
✅ Answer: (d) Humboldt
💡 Explanation: The Humboldt (Peru) current is cold; the others are warm currents.
Q4. Spring tides occur when:
✅ Answer: (a) The Sun, Moon and Earth are in a straight line
💡 Explanation: At new moon and full moon, the Sun's and Moon's pulls combine to produce the highest tides.
Q5. The Chinook, called the "snow eater", blows on the slopes of the:
✅ Answer: (d) Rocky Mountains
💡 Explanation: The Chinook is a warm, dry wind on the eastern slopes of the Rockies.
🔁 Revise in 60 Seconds
- Air: N₂ 78%, O₂ 21%, Ar 0.93%, CO₂ ~0.04%. Troposphere 8 km (poles)–18 km (equator); lapse rate 6.5°C/km.
- Ozone — stratosphere; coldest — mesosphere; radio waves — ionosphere.
- Doldrums (0°), horse latitudes (30°), subpolar low (60°); trades, westerlies, polar easterlies; Coriolis right in NH.
- Warm: Gulf Stream, Kuroshio; cold: Labrador, Humboldt, Benguela. Grand Banks fishing. Spring tide = new/full moon.
📌 Prelims Traps to Avoid
- ✘ The troposphere is thicker at the equator than at the poles.
- ✘ Temperature rises with height in the stratosphere because ozone absorbs UV.
- ✘ Cyclones spin anticlockwise in the Northern Hemisphere.
Why are hot deserts found on the western coasts of continents?
Most hot deserts lie near 30° latitude, under subtropical high pressure where dry air sinks. Cold ocean currents along western coasts, such as the Humboldt and Benguela, cool the air above them so it holds little moisture, creating deserts like the Atacama and the Namib.
What is the difference between spring tides and neap tides?
Spring tides, with the highest high tides and lowest low tides, happen at new and full moon when the Sun, Moon and Earth are in line. Neap tides, with the smallest range, happen at the first and last quarter moons when the Sun and Moon are at right angles to the Earth.