UPSC Prelims 2024 Q2: Troposphere Thickness at Equator vs Poles
Consider the following statements:
Statement-I:
Thickness of the troposphere at the equator is much greater as compared to poles.
Statement-II:
At the equator, heat is transported to great heights by strong convectional currents.
Which one of the following is correct in respect of the above statements?
- Both Statement-I and Statement-II are correct and Statement-II explains Statement-I
- Both Statement-I and Statement-II are correct, but Statement-II does not explain Statement-I
- Statement-I is correct, but Statement-II is incorrect
- Statement-I is incorrect, but Statement-II is correct
Answer: (a) Both Statement-I and Statement-II are correct and Statement-II explains Statement-I
Statement-wise explanation
- Statement-I: correct — NCERT Class 11 (Fundamentals of Physical Geography, 'Composition and Structure of Atmosphere') says the troposphere averages 13 km in height. It reaches about 18 km at the equator and only about 8 km near the poles. NOAA's JetStream gives a similar picture: 18–20 km at the equator and about 6 km at the poles. So at the equator the troposphere is roughly twice as thick as at the poles, which makes 'much greater' accurate.
- Statement-II: correct — The equator gets intense sunlight all year. This heats the surface strongly, and the warm, moist air rises in powerful convection currents that carry heat high into the atmosphere. NCERT gives this as the reason in the same passage: the troposphere is thickest at the equator 'because heat is transported to great heights by strong convectional currents'. Statement-II is the direct cause of Statement-I, so it explains Statement-I.
How to eliminate
Statement-I can be confirmed from basic NCERT knowledge: about 18 km at the equator against about 8 km at the poles. That rules out the option that calls Statement-I incorrect. Statement-II is also safe from general knowledge, because the equatorial belt is famous for convection, rising air, tall thunderclouds and afternoon rain. Once both statements are known to be true, the only question is whether II explains I. Use the 'because' test: read the pair as 'the troposphere is thicker at the equator because convection carries heat to great heights'. If the sentence describes a real cause and effect, II explains I. Here it does, and NCERT links the two in a single sentence. The fact to remember is 18 km at the equator, 8 km at the poles, 13 km on average, with strong convection as the reason.
Know more
Why the troposphere's thickness changes with latitude
The troposphere is the lowest layer of the atmosphere. It holds almost all the water vapour and dust, and nearly all weather happens in it. Temperature here falls with height at about 1°C per 165 m (the normal lapse rate, NCERT). The layer ends at the tropopause, where temperature stops falling.
The top of the troposphere is set by how high surface heat can be carried upward. At the equator, strong heating drives deep convection. Rising air columns, often visible as tall cumulonimbus clouds, push the tropopause up. At the poles, the surface is cold and the air is dense. Vertical mixing is weak, so the layer stays shallow.
| Feature | Equator | Poles |
|---|---|---|
| Height of troposphere (NCERT) | ~18 km | ~8 km |
| Surface heating | Intense, all year | Weak |
| Vertical convection | Strong, deep | Weak |
| Temperature at tropopause (NCERT) | ~ −80°C | ~ −45°C |
The counter-intuitive point: the tropopause is colder over the equator than over the poles. The air keeps cooling with height for about 18 km over the equator but only for about 8 km over the poles. So it gets colder before the layer ends.
Layers above: the stratosphere extends to about 50 km and contains the ozone layer. Temperature rises with height there because ozone absorbs ultraviolet radiation. Above it are the mesosphere, the thermosphere (which contains the ionosphere) and the exosphere. The layers are defined by how temperature changes with height.
UPSC often pairs a fact with its cause in Statement-I/II questions on atmospheric structure. Learn each feature together with its 'why'.
NCERT Concepts to Revise
- Troposphere: Both statements are about this layer, so you need to know what it is and what goes on in it.
- Tropopause: The thickness of the troposphere is measured up to the tropopause, whose height changes with latitude.
- Structure of the atmosphere: Shows where the troposphere sits among the five layers, which are defined by how temperature changes with height.
- Cumulonimbus: These tall clouds are the visible sign of the deep convection described in Statement-II.
- Stratosphere: The layer above the tropopause; it helps you picture where the troposphere ends.