Atmosphere – NDA Geography PYQs

NDA 2019-II

Q. 1. In which layer of the atmosphere do satellites orbit the Earth?

(a) Exosphere

(b) Thermosphere

(c) Mesosphere

(d) Stratosphere

Answer: (a) Exosphere

Explanation: Most satellites orbit Earth in the exosphere or at the upper boundary of the thermosphere/exosphere. The exosphere is the outermost layer of the atmosphere (above ~700 km) where the atmosphere is so thin that gas molecules can escape into space without colliding with other molecules. Low Earth Orbit (LEO) satellites (like the ISS at ~400 km) are technically in the thermosphere. Geostationary satellites are at ~36,000 km: well within the exosphere. The source answer is (a) Exosphere: the outermost layer where satellites orbit. Concept Tested: Satellites orbit primarily in the exosphere (outermost layer); ISS is technically in upper thermosphere/lower exosphere

NDA 2018-II

Q. 2. In which layer of the atmosphere does the Aurora Borealis (Northern Lights) occur?

(a) Troposphere

(b) Stratosphere

(c) Mesosphere

(d) Thermosphere

Answer: (d) Thermosphere

Explanation: Aurora Borealis (Northern Lights) and Aurora Australis (Southern Lights) occur in the thermosphere, at altitudes of approximately 100–300 km. They are caused when charged particles from the Sun (solar wind) interact with gas molecules in the thermosphere along Earth’s magnetic field lines. This excites the gas molecules, causing them to emit light of different colours (green from oxygen, red from high-altitude oxygen, blue/purple from nitrogen). Concept Tested: Aurora Borealis: occurs in the thermosphere (~100–300 km); solar wind particles excite atmospheric gases

NDA 2018-I

Q. 3. Which of the following gases absorbs the long wave (terrestrial) radiation and keeps the Earth warm?

(a) Oxygen

(b) Carbon dioxide

(c) Nitrogen

(d) Helium

Answer: (b) Carbon dioxide

Explanation: Carbon dioxide (CO₂) is a greenhouse gas that absorbs outgoing longwave (infrared) radiation from Earth’s surface and re-radiates it back to Earth, trapping heat in the lower atmosphere: the greenhouse effect. This keeps Earth’s surface about 33°C warmer than it would otherwise be. Other greenhouse gases include water vapour (most important by volume), methane (CH₄), nitrous oxide (N₂O), and ozone (O₃). Nitrogen and Oxygen are the main atmospheric gases but are NOT greenhouse gases: they do not absorb significant longwave radiation. Concept Tested: CO₂ absorbs longwave (terrestrial/infrared) radiation; greenhouse effect keeps Earth warm

NDA 2016-I

Q. 4. The Jet Streams are associated with which layer of the atmosphere?

(a) Stratosphere

(b) Troposphere

(c) Mesosphere

(d) Exosphere

Answer: (b) Troposphere

Explanation: Jet streams are narrow, fast-flowing air currents in the upper troposphere and lower stratosphere, at altitudes of approximately 9–16 km. They flow from west to east at speeds typically between 100–400 km/h. They are associated primarily with the troposphere (upper portion), where the temperature gradient between the tropics and poles is strongest. Jet streams influence weather patterns, and aviation routes often use them to reduce fuel consumption (following the jet stream going east, avoiding it going west). Concept Tested: Jet streams: fast air currents in upper troposphere (9–16 km); flow west to east; influence weather and aviation

NDA 2015-II

Q. 5. Which one of the following statements about the atmosphere is NOT correct?

(a) The composition of the atmosphere is uniform up to an altitude of about 80 km.

(b) Above about 80 km, the atmospheric composition is not uniform.

(c) Water vapour is found up to 12 km.

(d) Below about 100 km, the different gases become thoroughly mixed.

Answer: (b) Above about 80 km, the atmospheric composition is not uniform.

Explanation: Option (b) is marked as NOT correct in the source. However, this statement is actually scientifically accurate: above approximately 80 km (the turbopause), gases ARE stratified by molecular weight (heterosphere), so the composition is NOT uniform. Option (a) is the complementary statement: below ~80 km, the composition IS uniform (homosphere: gases are thoroughly mixed by turbulence). Option (d) says below 100 km gases are mixed: this is approximately correct (more precisely below 80 km). Option (c) Water vapour up to 12 km: essentially correct; most water vapour is confined to the troposphere. Students should verify this answer against the official NDA 2015-II answer key, as options (a) and (b) together describe the homosphere/heterosphere boundary and may have been marked differently. Concept Tested: Atmospheric uniformity: homosphere (below ~80 km, uniform); heterosphere (above ~80 km, stratified by mass)

NDA 2013-II

Q. 6. Which of the following statements is/are correct?

1. Temperature decreases with increase in altitude in the troposphere.

2. In the stratosphere, temperature increases with altitude due to presence of ozone.

3. Thermosphere is the coldest region of the atmosphere.

Select the correct answer using the code given below:

(a) 1 only

(b) 1 and 2 only

(c) 2 and 3 only

(d) 1, 2 and 3

Answer: (b) 1 and 2 only

Explanation: Statement 1: Correct. In the troposphere, temperature decreases with altitude at the normal lapse rate of ~6.5°C/km. Statement 2: Correct. In the stratosphere, ozone absorbs UV radiation, generating heat. This causes temperature to increase with altitude (temperature inversion). This is why the stratosphere is relatively stable. Statement 3: Incorrect. The thermosphere is actually the HOTTEST region in terms of gas temperature (up to 2,000°C or more), because it absorbs high-energy X-rays and extreme UV from the Sun. The MESOSPHERE is the coldest layer (temperature drops to about −90°C at the mesopause). Concept Tested: Temperature profiles: troposphere (decrease), stratosphere (increase due to ozone); thermosphere is HOTTEST not coldest

NDA 2012-I

Q. 7. Which of the following statements with regard to the ozone layer is/are correct?

1. It absorbs ultraviolet radiation from the sun.

2. Chlorofluorocarbons are serious threat to the ozone layer.

3. The ozone layer is found between 15 km and 35 km in the stratosphere.

Select the correct answer using the code given below:

(a) 1 and 2 only

(b) 2 and 3 only

(c) 1 and 3 only

(d) 1, 2 and 3

Answer: (d) 1, 2 and 3

Explanation: Statement 1: Correct. The ozone layer absorbs most of the Sun’s harmful ultraviolet radiation. Statement 2: Correct. Chlorofluorocarbons (CFCs): used in refrigerants, aerosol sprays, and foam agents: release chlorine atoms in the stratosphere that catalytically destroy ozone molecules. Each chlorine atom can destroy thousands of ozone molecules. Statement 3: Correct. The ozone layer is concentrated between 15 and 35 km altitude in the stratosphere (peak concentration at about 20–25 km). All three statements are correct. Concept Tested: Ozone layer facts: absorbs UV, CFCs are a threat, found 15–35 km in stratosphere: all three correct

NDA 2011-II

Q. 8. Which layer of the atmosphere enables radio waves to be reflected back to the Earth’s surface?

(a) Troposphere

(b) Ionosphere

(c) Stratosphere

(d) Mesosphere

Answer: (b) Ionosphere

Explanation: The Ionosphere is a region of the upper atmosphere (from about 60–1,000 km) where solar radiation ionises gas molecules, creating electrically charged ions and free electrons. These free electrons act as a conducting layer that reflects radio waves (particularly medium wave and short wave frequencies) back to Earth’s surface: enabling long-distance radio communication. Without the ionosphere, radio waves would pass straight into space. The ionosphere overlaps with parts of the mesosphere and thermosphere. Concept Tested: Ionosphere reflects radio waves back to Earth’s surface; located 60–1,000 km; enables long-distance communication

NDA 2010-I

Q. 9. What is the correct name for the boundary between the Troposphere and Stratosphere?

(a) Mesopause

(b) Stratopause

(c) Tropopause

(d) Ionopause

Answer: (c) Tropopause

Explanation: The boundaries between atmospheric layers are named with the suffix “-pause”: Tropopause: boundary between troposphere and stratosphere (at ~12 km on average; varies from ~8 km at poles to ~16 km at equator). Stratopause: boundary between stratosphere and mesosphere (~50 km). Mesopause: boundary between mesosphere and thermosphere (~80 km); the coldest point in the atmosphere. There is no standard “Ionopause” in this context; the ionosphere overlaps with the mesosphere/thermosphere. Concept Tested: Tropopause: boundary between troposphere and stratosphere (~12 km); named with the “-pause” suffix

NDA 2009-II

Q. 10. In which one of the following is the composition of atmospheric air given in the correct descending order?

(a) Nitrogen, Oxygen, Argon, Carbon dioxide

(b) Nitrogen, Oxygen, Carbon dioxide, Argon

(c) Oxygen, Nitrogen, Argon, Carbon dioxide

(d) Oxygen, Nitrogen, Carbon dioxide, Argon

Answer: (a) Nitrogen, Oxygen, Argon, Carbon dioxide

Explanation: The composition of dry air by volume (descending order): Nitrogen (N₂): approximately 78.09% Oxygen (O₂): approximately 20.95% Argon (Ar): approximately 0.93% Carbon dioxide (CO₂): approximately 0.04% (and rising) The correct descending order is: Nitrogen > Oxygen > Argon > Carbon dioxide. Concept Tested: Atmospheric composition: N₂ (78%) > O₂ (21%) > Ar (0.93%) > CO₂ (0.04%)

NDA 2009-I

Q. 11. Which one of the following statements about the troposphere is not correct?

(a) It is the lowermost layer of the atmosphere.

(b) Weather occurs only in this layer.

(c) Temperature increases with an increase in altitude.

(d) Air is densest in this layer.

Answer: (c) Temperature increases with an increase in altitude

Explanation: Option (c) is NOT correct: it is false. In the troposphere, temperature DECREASES with increasing altitude at an average rate of about 6.5°C per km (the environmental lapse rate). This is the opposite of what option (c) states. All other options are correct: (a) The troposphere is indeed the lowermost layer (0–12 km altitude on average). (b) All weather phenomena (clouds, rain, thunderstorms, cyclones) occur in the troposphere. (d) Air is densest at the surface and becomes progressively less dense with altitude; the troposphere contains about 75–80% of the total atmospheric mass. Concept Tested: Troposphere: temperature DECREASES with altitude (lapse rate); weather occurs here; densest air layer

★ JOVIK Exam Insight Atmospheric layers and temperature behaviour for NDA: Troposphere (0–12 km): Temp DECREASES with altitude. Weather here. Stratosphere (12–50 km): Temp INCREASES with altitude (due to ozone absorbing UV). Mesosphere (50–80 km): Temp DECREASES with altitude. Coldest layer (~-90°C at top). Thermosphere (80–700 km): Temp INCREASES dramatically with altitude (absorbs X-rays/EUV). Hottest layer. Exosphere (700+ km): Outermost layer; molecules escape to space.

NDA 2008-II

Q. 12. The ozone layer in the stratosphere absorbs:

(a) X-rays

(b) Ultraviolet radiation

(c) Infrared radiation

(d) Gamma rays

Answer: (b) Ultraviolet radiation

Explanation: The ozone (O₃) layer is concentrated in the stratosphere, primarily between 15 and 35 km altitude. It acts as a shield that absorbs most of the harmful ultraviolet (UV) radiation from the Sun: particularly UV-B (280–315 nm) and UV-C (100–280 nm). UV radiation from the Sun would damage DNA, cause skin cancer, and harm ecosystems if it reached Earth’s surface in full strength. Infrared radiation from Earth is absorbed by greenhouse gases (CO₂, water vapour, methane): not by ozone. X-rays and gamma rays are absorbed at much higher altitudes by the ionosphere. Concept Tested: Ozone layer absorbs UV radiation from the Sun; located in stratosphere at 15–35 km altitude

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