Overview
Previous Year UPSC-CSE Questions By the end you will be able to draft model answers for the following UPSC questions. Each question carries a collapsible framework showing how to approach it in the exam.
- UPSC Mains 2013 GS-IWhat do you understand about the phenomenon of temperature inversion in meteorology? How does it affect the weather and the habitants of the place?
How to structure the answer in the exam
Introduction: Define a temperature inversion as a reversal of the normal fall of temperature with height.
Body (sub-themes to develop):
- Normal lapse versus the inversion: warm air over cold air.
- Formation: clear, calm winter nights; radiation cooling; cold-air drainage in valleys.
- Weather effects: stable air, fog, frost and trapped smog.
- Effects on people: poor visibility, air pollution and health, frost damage to crops.
Conclusion: Conclude that inversion is a stable, low-level reversal with sharp local effects on weather and health.
The heat budget of the earth is the balance between the incoming short-wave solar radiation (insolation) the planet absorbs and the outgoing long-wave radiation it returns to space; because the Sun heats the earth unevenly, this budget also shapes the global distribution of temperature through latitude, altitude, the sea and the winds.
Insolation and the heat budget
Insolation: the incoming solar radiation that drives weather
Almost all the energy that drives the earth's weather comes from the Sun as insolation, the incoming solar radiation. The earth absorbs this short-wave energy by day and radiates long-wave energy back to space, and the balance between the two is the heat budget.
That balance is what keeps the planet's average temperature roughly steady. Over a year the energy the earth receives from the Sun and the energy it loses to space are nearly equal, so the global temperature neither runs away nor collapses, a balance now disturbed by greenhouse gases.
The earth's energy budget: incoming and outgoing radiation
At the top of the atmosphere the earth receives about 340 watts per square metre of solar energy, averaged over the globe and the year. What happens to this incoming radiation can be set out in three parts.
- About 30 per cent is reflected straight back to space by clouds, the air and bright surfaces; this share is the albedo.
- The remaining roughly 70 per cent (about 240 watts per square metre) is absorbed and warms the surface and the air.
- The absorbed energy is returned to space as long-wave radiation, so that incoming and outgoing balance.
This is why it is called a budget. Just as money in must match money out, the energy the earth takes in as short-wave sunlight must be matched by the long-wave energy it sends back, or the planet would steadily heat or cool.
The greenhouse effect and surface temperature
How greenhouse gases keep the surface warm
The atmosphere does not let all the long-wave radiation escape. Greenhouse gases, mainly water vapour and carbon dioxide, absorb part of the outgoing long-wave energy and radiate some of it back down, keeping the surface far warmer than bare sunlight alone would.
This natural greenhouse effect makes the earth habitable. The concern of climate change is that adding more greenhouse gases traps additional long-wave energy, tilting the budget towards a warming imbalance, the theme of the cyclones-and-climate-change discussion elsewhere in our geography notes.
Latitudinal heat balance: tropical surplus and polar deficit
The latitudinal heat balance and poleward heat transfer
The Sun does not heat the earth evenly. The tropics receive the Sun's rays nearly overhead and gain more energy than they lose, while the polar regions receive them at a low slant and lose more than they gain.
The result is a permanent surplus in the low latitudes and a deficit towards the poles. Because the planet does not keep heating at the equator, winds and ocean currents carry the surplus heat poleward, and this transfer is the engine behind the world's pressure belts, winds and storms.
The global distribution of temperature
Factors controlling temperature: latitude, altitude, continentality and currents
The heat budget sets the broad pattern, but the temperature of any one place depends on several factors acting together. The most important is latitude, which fixes how directly the Sun's rays strike, but others modify it strongly.
Among these, altitude cools the air as one climbs, distance from the sea makes interiors hotter in summer and colder in winter, and warm or cold ocean currents, prevailing winds and cloud cover all push local temperatures up or down.
Isotherms and the equator-to-pole temperature gradient
On a map, temperature is shown by isotherms, lines joining places of equal temperature. Reading them reveals the great patterns of the earth's surface heat, and a few features recur on every world map of temperature.
| Feature of the isotherms | What it shows |
|---|---|
| Roughly east-west alignment | Temperature is governed first by latitude |
| Equator-to-pole decrease | The latitudinal heat surplus falls away towards the poles |
| Bending over land and sea | Continents heat and cool faster than oceans |
| Seasonal shift (January and July) | The zone of greatest heat follows the overhead Sun |
The bending is the most revealing. Because land heats and cools faster than water, isotherms bend poleward over warm summer continents and towards the equator over cold winter ones, tracing the contrast between maritime and continental climates.
Temperature inversion: formation, types and effects
How a temperature inversion forms and what it affects
Normally the air grows colder with height. A temperature inversion reverses this, with a layer of warmer air sitting over cooler air below, so that temperature for a time rises with height instead of falling.
The commonest type forms on a clear, calm winter night, when the ground radiates away its heat and chills the air just above it, an effect sharpened in valleys where cold air drains and settles. The warm cap is stable and traps smog and fog near the surface, harming health and visibility.
How this appears in the UPSC exam
What the UPSC exam asks on the heat budget
This topic underpins all of climatology, so it appears in GS Paper I and the geography optional. The 2013 question on temperature inversion is answered directly here, and the heat budget and latitudinal balance explain winds, pressure belts and the monsoon.
A strong answer keeps the logic in order. Start from the budget, move to the unequal heating that creates surplus and deficit, then to the factors that shape local temperature, and use inversion and isotherms as precise, well-labelled examples.
Prelims MCQ practice
Each question below tests one specific concept on the topic. Click to reveal the answer and a full option-wise explanation.
Q1. The term 'insolation' refers to:
- the heat radiated by the earth to space
- the incoming short-wave solar radiation received by the earth
- the insulation provided by the atmosphere
- the reflection of sunlight by clouds
Show answer and explanation
Answer: the incoming short-wave solar radiation received by the earth
Explanation.
Insolation is the incoming solar (short-wave) radiation received by the earth. Option (a) is outgoing terrestrial radiation, (c) is a play on the word, and (d) is the albedo. Hence (b).
Q2. With reference to the earth's heat budget, consider the following statements:
- About 30 per cent of incoming solar radiation is reflected back to space (the albedo).
- Over a year, the energy received from the Sun and the energy lost to space are nearly balanced.
- The earth loses energy to space mainly as short-wave radiation.
Which of the statements given above is/are correct?
- 1 and 2 only
- 2 and 3 only
- 1 and 3 only
- 1, 2 and 3
Show answer and explanation
Answer: 1 and 2 only
Explanation.
Statements 1 and 2 are correct: about 30 per cent is reflected, and incoming and outgoing energy are nearly balanced. Statement 3 is wrong: the earth loses energy as long-wave (terrestrial) radiation, not short-wave. Hence 1 and 2 only.
Q3. The 'albedo' of the earth refers to:
- the fraction of incoming solar radiation reflected back to space
- the total solar radiation absorbed by the oceans
- the long-wave radiation emitted by the surface
- the temperature of the upper atmosphere
Show answer and explanation
Answer: the fraction of incoming solar radiation reflected back to space
Explanation.
Albedo is the fraction of incoming solar radiation that is reflected back to space (about 0.3, or 30 per cent, for the earth). The other options are unrelated. Hence (a).
Q4. With reference to the latitudinal heat balance, consider the following statements:
- The tropics receive more energy than they radiate, giving a surplus.
- The polar regions radiate more energy than they receive, giving a deficit.
- This surplus and deficit are evened out by winds and ocean currents.
Which of the statements given above is/are correct?
- 1 and 2 only
- 2 and 3 only
- 1 and 3 only
- 1, 2 and 3
Show answer and explanation
Answer: 1, 2 and 3
Explanation.
All three are correct: the tropics run a surplus, the poles a deficit, and winds and ocean currents transport heat poleward to even out the imbalance. Hence all three.
Q5. A temperature inversion is a condition in which:
- temperature decreases steadily with height
- temperature increases with height, with warm air over cooler air
- temperature is uniform at all heights
- the surface is warmer than the upper atmosphere by day
Show answer and explanation
Answer: temperature increases with height, with warm air over cooler air
Explanation.
In a temperature inversion the normal fall of temperature with height is reversed, so warmer air lies over cooler air and temperature increases with height. Option (a) is the normal condition. Hence (b).
Q6. Which one of the following best describes a common effect of a temperature inversion?
- it disperses air pollution quickly
- it traps fog and smog near the ground, reducing visibility and air quality
- it triggers heavy convectional rainfall
- it raises the surface temperature sharply
Show answer and explanation
Answer: it traps fog and smog near the ground, reducing visibility and air quality
Explanation.
An inversion creates a stable cap that inhibits vertical mixing, trapping fog and smog near the surface and worsening visibility and air quality. It suppresses, not triggers, convection. Hence (b).
Sources and Further Reading
- Wikipedia: Earth's energy budget
- Wikipedia: Inversion (meteorology)
- Wikipedia: Solar irradiance (insolation)
- NASA Earth Observatory: Earth's energy budget
- NOAA: climate and the energy balance
- India Meteorological Department
- WMO (World Meteorological Organization)
- NCERT: Geography (fundamentals of physical geography)
Editorial Disclaimer
This article explains the earth's heat budget and the distribution of temperature for UPSC preparation, drawing on standard climatology and earth-science sources. Figures and mechanisms reflect the cited authorities.
