NDA Geography · Climatology, Atmosphere and Weather
Atmospheric Layers, Composition and Aurora
The atmosphere is built in five shells — troposphere, stratosphere, mesosphere, thermosphere, exosphere — each with its own temperature behaviour, gases and phenomena, from all our weather down low to the ozone shield and the aurora high up.
Why this matters
12 PYQs — one of the largest slices of the chapter, and a steady source of HARD multi-statement traps. Two ideas earn most of the marks: the ORDER of the layers and what happens in each (weather in the troposphere, ozone in the stratosphere, burning meteors in the mesosphere), and the lapse rate (temperature falls ~6.5 degrees C per km in the troposphere). Get the layer sequence and 'which layer does what' cold.
Concept 1 of 5: The five layers of the atmosphere
Rise straight up and you pass through five shells in order. Closest to the ground is the TROPOSPHERE — where all weather happens. Above it the STRATOSPHERE holds the ozone layer. Then the MESOSPHERE (coldest, where meteors burn up), the THERMOSPHERE (hottest, where the aurora glows and the ionosphere reflects radio waves), and finally the wispy EXOSPHERE fading into space. Remember the order by going outward from the surface: Tropo, Strato, Meso, Thermo, Exo.
Definition
Surface upward:
- Troposphere — nearest the ground (~0–12 km). ALL weather and clouds occur here. Temperature FALLS with height. Thickest at the equator, thinnest at the poles.
- Stratosphere — holds the OZONE layer, which absorbs ultraviolet radiation. Temperature RISES with height (because ozone warms it). Smooth, so jets cruise here.
- Mesosphere — the COLDEST layer; meteors burn up here.
- Thermosphere — the HOTTEST layer; contains the ionosphere, which reflects radio waves back to Earth; auroras occur here.
- Exosphere — the outermost, merging into space.
The boundaries are the tropopause (top of troposphere), stratopause, mesopause.
Worked example
Three pairs are offered: (I) Troposphere absorbs ultraviolet radiation; (II) Stratosphere is where weather changes; (III) Mesosphere reflects radio waves. How many are correctly matched?
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
NDA · Apr 2026 · Q120Hard
Example 1 · Climatology, Atmosphere and Weather · Atmospheric Layers, Composition and Aurora
Consider the following pairs:
Layers of Atmosphere | Characteristics
I. Troposphere | Absorbs ultraviolet radiation
II. Stratosphere | Changes in weather and climate
III. Mesosphere | Reflects radio waves back to the Earth
How many pairs are correctly matched?
Ozone is in the STRATOSPHERE, weather in the TROPOSPHERE
A favourite swap matches the troposphere with UV absorption (really the stratosphere's ozone) or the stratosphere with weather (really the troposphere). Keep them straight: weather = troposphere, ozone/UV = stratosphere, radio-wave reflection = thermosphere (ionosphere).
Sequence trap
An option may reorder the shells (e.g. troposphere → stratosphere → thermosphere → mesosphere). The correct order outward is Tropo, Strato, Meso, Thermo, Exo — mesosphere comes BEFORE thermosphere.
Concept 2 of 5: The normal lapse rate and the tropopause
Inside the troposphere, the air gets colder the higher you go — about 6.5 degrees C for every kilometre. This steady cooling is the NORMAL (environmental) lapse rate. It keeps falling until you reach the tropopause, the ceiling of the troposphere, where the temperature stops dropping (and in the stratosphere above, it begins to rise).
Definition
- The normal lapse rate is the rate at which air temperature FALLS with height in the troposphere: about 6.5 degrees C per kilometre (roughly 1 degree F per 165 m).
- Temperature DECREASES with height in the troposphere.
- At the tropopause (top of the troposphere) the falling temperature levels off — this is where the normal lapse rate effectively drops temperature to its tropospheric minimum.
- In the stratosphere above, temperature RISES with height (ozone warming), the reverse of the troposphere.
Worked example
Statement: 'Temperature increases with height in the troposphere at about 6.5 degrees C per km.' Is it correct, and if not, fix it.
Practice this conceptself-check · 3 quick reps
The same idea in a real exam question:
NDA · Sep 2018 · Q76Moderate
Example 2 · Climatology, Atmosphere and Weather · Atmospheric Layers, Composition and Aurora
Which one of the following statements about temperature is correct?
Direction matters
Options often flip 'decreases' to 'increases' for the troposphere, or claim the stratosphere cools with height. Lock it in: troposphere cools upward, stratosphere warms upward.
Concept 3 of 5: Composition of the atmosphere
The lower atmosphere is mostly nitrogen and oxygen, but the NDA tests the trace gases and the very top. Carbon dioxide is the most abundant of the greenhouse gases that humans add; the exosphere, being lightest, is dominated by the lightest gases — hydrogen and helium.
Definition
Facts the NDA tests:
- The bulk of the air is nitrogen (~78%) and oxygen (~21%).
- Among the greenhouse gases, carbon dioxide is present in the largest concentration (more than methane, nitrous oxide or CFCs).
- The exosphere, the outermost and lightest layer, is composed mainly of the lightest gases — helium and hydrogen.
| Where / which | Dominant gas(es) | Note |
|---|---|---|
| Lower atmosphere | Nitrogen ~78%, Oxygen ~21% | Argon + CO2 + trace gases make up the rest |
| Most abundant greenhouse gas | Carbon dioxide | Above methane, nitrous oxide, CFCs NDA 2018 — CO2 is the greenhouse gas in largest concentration. |
| Exosphere (outermost) | Helium and Hydrogen | The lightest gases float to the top NDA 2021 — exosphere = helium + hydrogen. |
Practice this conceptself-check · 3 quick reps
The same idea in a real exam question:
NDA · Apr 2021 · Q57Moderate
Example 3 · Climatology, Atmosphere and Weather · Atmospheric Layers, Composition and Aurora
The composition of gases in exosphere is
Exosphere is light gases, not oxygen-rich
Distractors offer 'neon and oxygen' or 'neon and hydrogen' for the exosphere. The right answer is the two LIGHTEST gases — helium and hydrogen — which is why they reach the outermost shell.
Concept 4 of 5: Aurora and the origin of the Coriolis effect
Two high-atmosphere ideas live here. The AURORA is the coloured glow when charged particles from the solar wind are funnelled by Earth's magnetic field toward the poles and strike atmospheric gases — each gas glowing its own colour. And the CORIOLIS effect, which bends winds, is purely a result of the Earth's ROTATION (not its tilt or its orbit).
Definition
- Aurora: the solar wind reaching Earth is steered toward the two magnetic poles, producing a colourful night-sky display. Different atmospheric gases glow with different colours. (Auroras were even seen from Hanle, Ladakh in 2023.)
- Coriolis effect: an apparent deflection of moving air (and water) caused by the Earth's rotation — NOT by the pressure gradient, the axial tilt, or the Earth's revolution around the Sun.
Worked example
Why does the aurora appear near the magnetic poles and show several colours?
Practice this conceptself-check · 3 quick reps
The same idea in a real exam question:
NDA · Apr 2024 · Q70Moderate
Example 4 · Climatology, Atmosphere and Weather · Atmospheric Layers, Composition and Aurora
Which of the following statements regarding 'aurora' is/are correct ?
1. The solar wind upon reaching the Earth's atmosphere is directed towards two magnetic poles and a colourful display of lights is seen in the night sky.
2. The particles interact with the different gases of the atmosphere and each gas glows with a particular colour.
3. In April 2023, aurora was captured in the night sky in India by the Indian Astronomical Observatory at Hanle, Ladakh.
Select the answer using the code given below :
Coriolis = rotation, not tilt or revolution
Distractors blame the Coriolis effect on the Earth's axial inclination or its revolution around the Sun. It is the rotation (spin) of the Earth that deflects moving air.
Concept 5 of 5: Regional climate-vegetation recall
A couple of pure world-biome recall facts the bank files in this subtopic: the savanna grasslands of South America carry local names (Campos, Llanos), and south-east China is covered by subtropical broadleaf evergreen forest. Memorise the place-to-vegetation pairing — these reward recall, not reasoning.
Definition
- Campos (the Brazilian Highlands) and Llanos (the Orinoco basin of Venezuela/Colombia) are the local names for the tropical savanna grasslands of South America.
- The natural vegetation of south-east China is subtropical broadleaf evergreen forest (a warm, humid monsoon climate).
| Fact | Answer | Note |
|---|---|---|
| Campos / Llanos grasslands | South America | Tropical savanna by local name NDA 2019 — Campos and Llanos are South American savanna grasslands. |
| Natural vegetation of SE China | Subtropical broadleaf evergreen forest | Warm, humid south-east |
Practice this conceptself-check · 2 quick reps
The same idea in a real exam question:
NDA · Apr 2019 · Q68Easy
Example 5 · Climatology, Atmosphere and Weather · Atmospheric Layers, Composition and Aurora
"Campos" and "Llanos", Tropical Savanna grasslands are generally found in
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.
Reference tables (2)
Composition of the atmosphere3 rows
| Where / which | Dominant gas(es) | Note |
|---|---|---|
| Lower atmosphere | Nitrogen ~78%, Oxygen ~21% | Argon + CO2 + trace gases make up the rest |
| Most abundant greenhouse gas | Carbon dioxide | Above methane, nitrous oxide, CFCs NDA 2018 — CO2 is the greenhouse gas in largest concentration. |
| Exosphere (outermost) | Helium and Hydrogen | The lightest gases float to the top NDA 2021 — exosphere = helium + hydrogen. |
Regional climate-vegetation recall2 rows
| Fact | Answer | Note |
|---|---|---|
| Campos / Llanos grasslands | South America | Tropical savanna by local name NDA 2019 — Campos and Llanos are South American savanna grasslands. |
| Natural vegetation of SE China | Subtropical broadleaf evergreen forest | Warm, humid south-east |
Watch out for (5)
- Ozone is in the STRATOSPHERE, weather in the TROPOSPHERE→ The five layers of the atmosphere
- Sequence trap→ The five layers of the atmosphere
- Direction matters→ The normal lapse rate and the tropopause
- Exosphere is light gases, not oxygen-rich→ Composition of the atmosphere
- Coriolis = rotation, not tilt or revolution→ Aurora and the origin of the Coriolis effect
Test yourself on Climatology, Atmosphere and Weather
20 past NDA questions from this chapter, timed at 20 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.