CDS Geography · Climatology, Atmosphere and Weather
Insolation and Temperature
How sunlight reaches and heats the Earth, why some surfaces reflect more of it, how temperature falls with height, and when that fall turns round into an inversion.
Why this matters
Nine CDS questions from 2018 to 2026, all MODERATE. Four are about temperature inversion (2022, 2025 and 2026), so inversion is the one idea on this page to know in full: when it forms, what it does to the air and why it traps smog.
Concept 1 of 3: Insolation, albedo and global dimming
The Sun is very hot, so it sends its energy as short waves, mostly visible light. The Earth's round surface spreads that energy unevenly: rays fall straight down near the equator and at a slant near the poles. Some of the light is bounced straight back, and how much depends on the surface.
Definition
The terms:
- Insolation (incoming solar radiation): the Sun's energy received by the Earth. It is mostly short-wave radiation, much of it visible light.
- It is unevenly spread: because the Earth is curved, rays strike low latitudes nearly vertically and high latitudes obliquely, spreading the same energy over a larger area.
- The Earth gives heat back out as long-wave (terrestrial) radiation, which greenhouse gases trap.
- Albedo: the share of incoming light a surface reflects. Of every 100 units of sunlight reaching the top of the atmosphere, about 35 are reflected back to space (the Earth's albedo).
- Global dimming: a fall in the sunlight reaching the ground because particulates (dust, smoke, aerosols) scatter and absorb it. Its effect is a fall in surface temperature.
| Surface | Share of sunlight it reflects |
|---|---|
| Fresh snow | About 80 to 90 per cent, the highestQ CDS 2022 (I): a snow covered mountain has the highest albedo, above a sandy desert, bare rock or a mangrove forest. |
| Desert sand | About 30 to 40 per cent |
| Grassland | About 20 to 25 per cent |
| Dense forest | About 5 to 15 per cent |
| Open water with the Sun high | Under 10 per cent |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2022 · CDS (I) 2022 — General Knowledge · Q88Moderate
Example 1 · Climatology, Atmosphere and Weather · Insolation and Temperature
Which one of the following land surfaces has the highest albedo ?
The curve makes insolation uneven
A statement that insolation is evenly spread because the Earth is curved is false. The curve is exactly why it is uneven: straight rays at the equator, slanting rays near the poles.
Dimming cools, it does not change pressure
Global dimming is about less sunlight and a fall in temperature caused by particulates. Options that tie it to a rise or fall in air pressure, or to a rise in temperature, are wrong.
Concept 2 of 3: Lapse rate and adiabatic change
Air near the ground is heated by the ground, so it gets colder as you go up: about 6.5 °C for every kilometre. A hill station is cooler than the plain below for this reason alone. A rising parcel of air cools in a different way: it has less air pushing on it, so it expands, and expanding air cools without losing any heat to its surroundings.
Definition
The rules:
- Normal lapse rate: temperature falls by about 6.5 °C per 1,000 m of height in the troposphere.
- Adiabatic change: a temperature change with no heat added or removed. Rising air expands and cools; sinking air is compressed and warms.
- Because hills are cooler, the India Meteorological Department uses lower limits for them: a heat wave is a maximum of 40 °C or more in the plains or 30 °C or more in the hills; a cold wave is a minimum of 10 °C or less in the plains or 0 °C or less in the hills.
Temperature at a height, using the normal lapse rate
- temperature at the lower place, in °C
- height climbed, in kilometres
- temperature at the higher place, in °C
Worked example
It is 30 °C at a town at sea level. Using the normal lapse rate, what temperature would you expect at a hill station 2,000 m up?
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2018 · CDS (I) 2018 — General Knowledge · Q24Moderate
Example 2 · Climatology, Atmosphere and Weather · Insolation and Temperature
Which of the following causes adiabatic temperature changes in atmosphere?
Adiabatic means no heat is exchanged
In an adiabatic change the air's temperature changes only because it expands (cools) or is compressed (warms). Latent heat, absorption of sunlight and advection are other ways of changing temperature, not adiabatic ones.
Different heat wave limits for hills
The heat wave limit is 40 °C for the plains but 30 °C for the hills, because hills are cooler to begin with. A statement giving both limits correctly is true.
Concept 3 of 3: Temperature inversion
Normally the air gets colder as you go up. On a long, clear, calm winter night the ground loses its heat fast and chills the air touching it, so for a while the air near the ground is colder than the air above. The usual order is turned upside down. Cold heavy air sitting under warm air cannot rise, so the air goes still and holds whatever is in it: fog, frost, smoke.
Definition
Inversion of temperature:
- Temperature rises with height instead of falling: the normal lapse rate is reversed (a negative lapse rate).
- Ideal conditions: a long winter night, clear skies and still air.
- Surface inversion forms as the ground loses heat by radiation at night, so it is also called radiation inversion. It usually lasts a few hours, until the Sun heats the ground again.
- It makes the lower air stable: cold air below cannot rise, so there is no mixing.
- So it traps pollution under an inversion lid, the main cause of smog, and helps fog and frost form.
- Valley inversion: at night cold air slides down the slopes and collects in the valley floor, so the valley is colder than the slopes (frost pockets).
| Feature | Inversion |
|---|---|
| Temperature with height | Rises |
| Effect on the lower air | Stable, no vertical mixingQ CDS 2025 (I): surface inversion causes stability, not instability, and lasts a few hours until sunrise. |
| Main reason smog lingers | The inversion lid holds pollutants near the groundQ CDS 2022 (I): the inversion lid is the most important factor, ahead of long nights or many pollution sources. |
| Other name for surface inversion | Radiation inversion |
| Ideal conditions | Long winter night, clear sky, still air |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2025 · CDS (I) 2025 — General Knowledge · Q4Moderate
Example 3 · Climatology, Atmosphere and Weather · Insolation and Temperature
Which of the following statements with reference to Surface inversion of temperature is/are correct ?
1. It causes instability in the lower layers of the atmosphere.
2. This inversion commonly lasts for a few hours until the Sun comes up.
Select the answer using the code given below :
Inversion means stability
Cold, heavy air under warm air cannot rise, so an inversion makes the lower air stable. A statement that it causes instability is false.
A fast fall with height spreads smog
Smog builds up under an inversion lid. A rapid fall of temperature with height (a steep normal lapse rate) does the opposite: it makes air rise and carries pollutants away.
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.
Formulas (1)
- Lapse rate and adiabatic change
Temperature at a height, using the normal lapse rate
Reference tables (2)
Insolation, albedo and global dimming5 rows
| Surface | Share of sunlight it reflects |
|---|---|
| Fresh snow | About 80 to 90 per cent, the highestQ CDS 2022 (I): a snow covered mountain has the highest albedo, above a sandy desert, bare rock or a mangrove forest. |
| Desert sand | About 30 to 40 per cent |
| Grassland | About 20 to 25 per cent |
| Dense forest | About 5 to 15 per cent |
| Open water with the Sun high | Under 10 per cent |
Temperature inversion5 rows
| Feature | Inversion |
|---|---|
| Temperature with height | Rises |
| Effect on the lower air | Stable, no vertical mixingQ CDS 2025 (I): surface inversion causes stability, not instability, and lasts a few hours until sunrise. |
| Main reason smog lingers | The inversion lid holds pollutants near the groundQ CDS 2022 (I): the inversion lid is the most important factor, ahead of long nights or many pollution sources. |
| Other name for surface inversion | Radiation inversion |
| Ideal conditions | Long winter night, clear sky, still air |
Watch out for (6)
- The curve makes insolation uneven→ Insolation, albedo and global dimming
- Dimming cools, it does not change pressure→ Insolation, albedo and global dimming
- Adiabatic means no heat is exchanged→ Lapse rate and adiabatic change
- Different heat wave limits for hills→ Lapse rate and adiabatic change
- Inversion means stability→ Temperature inversion
- A fast fall with height spreads smog→ Temperature inversion
Test yourself on Climatology, Atmosphere and Weather
15 past CDS questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.