ScienceCBSE Class 9 · Earth as a System: Energy, Matter, and Life

Solar Energy, Albedo, Uneven Heating and the Atmosphere (Class 9)

Updated 17 October 2026 · Fact-checked

Sunlight reaches the Earth as electromagnetic waves, about 99% of it ultraviolet, visible and infrared. The solar constant is about 1.4 kW m⁻² at the top of the atmosphere and at most about 1 kW m⁻² at the ground. Surfaces heat unevenly: albedo is the fraction of sunlight a surface reflects, low-albedo surfaces warm more, and latitude, land and water and the atmosphere (troposphere, stratosphere with its ozone layer, and the greenhouse effect) shape temperature on Earth.

Understand Solar Energy, Albedo and the Atmosphere

Earth as a system. The geosphere, hydrosphere, cryosphere, atmosphere and biosphere are linked, and energy comes mainly from the Sun. A change in one sphere spreads to others: less winter snow means less lake water in summer and less grass for grazing animals.

Solar radiation. Sunlight travels as electromagnetic (EM) waves, which need no medium and move through vacuum at 3 × 10⁸ m s⁻¹. The spectrum, from high frequency to low frequency, is gamma rays, X-rays, ultraviolet (UV), visible, infrared (IR), microwaves, radio waves. Frequency and wavelength are linked by speed = wavelength × frequency, so shorter wavelength means higher frequency and higher energy. About 99% of the energy from the Sun reaching the Earth is UV, visible and IR. UV (100 to 400 nm) is mostly absorbed by the ozone layer, and too much damages skin and eyes. Visible light reaches the surface and powers photosynthesis. IR warms the surface, which then gives out IR, part of which is trapped by greenhouse gases (CO₂, CH₄, water vapour).

Insolation and the solar constant. Insolation is solar radiation reaching the Earth's surface. The solar constant is about 1.4 kW m⁻² at the top of the atmosphere on a surface facing the Sun, and at the ground on a clear day at most about 1 kW m⁻². Power received = intensity × area; energy = power × time.

Uneven heating and albedo. Land heats and cools faster than water, and dark surfaces absorb more sunlight than light ones. Albedo is the fraction of sunlight reflected: fresh snow 0.80 to 0.90, ice 0.50 to 0.70, light soil about 0.3, and black soil and ocean water about 0.05 to 0.15. Absorbed = (1 − albedo) × incident. High albedo keeps a place cooler. There is a feedback: when ice melts, darker water or land is exposed, which absorbs more and melts more ice. All objects re-radiate heat, so concrete houses stay warm at night. An urban heat island is a city warmer than nearby villages because concrete, steel and asphalt absorb and re-radiate more heat and there is less greenery; trees cool by shade and transpiration.

Latitude. Because the Earth is a sphere, rays hit the equator nearly straight on and concentrate on a small area, but hit the poles at a slant and spread over a much larger area. So the equator is warm and the poles are cold; the tilt of the axis adds seasons and changing day length.

The atmosphere. About 78% nitrogen and 21% oxygen. The troposphere extends from 0 to about 12 km (higher over the equator, lower over the poles), holds almost all weather, is heated from the ground and cools about 6.5 °C per km. The stratosphere (12 to 50 km) contains the ozone layer, which absorbs UV, so its temperature rises with height. The atmosphere protects by absorbing incoming radiation (ozone blocks UV) and by trapping outgoing IR (the greenhouse effect). Extra CO₂ strengthens the effect and causes global warming; Venus, with a thick CO₂ atmosphere, is hotter than Mercury though farther from the Sun. CFCs thinned the ozone layer over Antarctica; the Montreal Protocol cut CFCs, and the layer is slowly recovering.

Solar Energy, Albedo and the Atmosphere: formulas and key facts

Wave relation for EM waves: $c = w a v e l e n g t h \times f r e q u e n cy, w i t h c = 3 \times 1 0^{8} m s^{- 1} in v a c uu m$
Power received: $P o w er = in t e n s i t y \times a r e a; e n er g y = p o w er \times t im e$
Absorbed radiation: $A b sor b e d = (1 - a l b e d o) \times in c i d e n t r a d ia t i o n$
Temperature fall in the troposphere: About 6.5 °C per km of height
Solar intensity values: Solar constant about 1.4 kW m⁻² | clear-day ground value at most about 1 kW m⁻²

How to solve Solar Energy, Albedo and the Atmosphere questions

Use this method for radiation, albedo and atmosphere numericals.

1Write the given intensity, area, albedo or height with units, and convert kW to W or h to s only if needed.
2For power on a surface, multiply intensity by area; for energy, multiply power by time.
3For albedo, find absorbed = (1 − albedo) × incident, one surface at a time.
4For temperature at a height, subtract 6.5 °C for every km from the ground value (troposphere only).
5For wavelength or frequency, use c = wavelength × frequency with SI units (m, Hz).
6State the result with units and, in reasoning questions, link it to the sphere or process involved.

Solar Energy, Albedo and the Atmosphere in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Albedo and a hot afternoon

Suppose a terrace with a black asphalt surface (albedo 0.10) and the next terrace with fresh white paint (albedo 0.60) both receive sunlight of 800 W m⁻² at noon.

Find the radiation absorbed per square metre on each terrace and say which one stays cooler, with a reason.

Try it first, then show the solutionSolution

Black asphalt: absorbed = (1 − 0.10) × 800 = 0.90 × 800 = 720 W m⁻².
White paint: absorbed = (1 − 0.60) × 800 = 0.40 × 800 = 320 W m⁻².
The white terrace absorbs less (320 W m⁻² against 720 W m⁻²), so it stays cooler.

Answer: 720 W m⁻² and 320 W m⁻²; the white terrace is cooler.

Memory hook: Light colours reflect, dark colours absorb.

Temperature on a hill

Suppose a hill station at a height of 2.0 km above the plains sits in the troposphere, and the air temperature in the plains at the same time is 32 °C.

Estimate the temperature at the hill station.

Try it first, then show the solutionSolution

The temperature falls by about 6.5 °C per km in the troposphere.
Fall = 6.5 × 2.0 = 13 °C.
Temperature ≈ 32 − 13 = 19 °C.

Answer: About 19 °C.

Memory hook: Six and a half degrees down for every kilometre up.

Tricks to remember Solar Energy, Albedo and the Atmosphere

Short memory aids. Say each one aloud once, then use it on a question the same day.

Gamma to radio order
'Great Xylophones Use Very Interesting Musical Rhythms': Gamma, X-rays, UV, Visible, IR, Microwaves, Radio waves.
Use it when: Writing the electromagnetic spectrum in order.
Albedo = whiteness
Albedo means whiteness. Whiter surface, higher albedo, more reflected, cooler.
Use it when: Albedo comparisons.
Absorbed is one minus albedo
Whatever is not reflected is absorbed: absorbed = (1 − albedo) × incident.
Use it when: Albedo numericals.
Troposphere cools, stratosphere warms
Weather layer: colder as you climb. Ozone layer: warmer as you climb, because ozone absorbs UV.
Use it when: Describing temperature with height.

Solar Energy, Albedo and the Atmosphere: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: The EM spectrum and UV · 3 marks · Short answerBoard level

(a) Write the EM spectrum in order of decreasing frequency. (1)
(b) Which part is mostly absorbed by the ozone layer and why does this matter? (1)
(c) What percentage of the Sun's energy reaching the Earth comes from UV, visible and IR together? (1)

Try it first, then show the solutionSolution

(a) Gamma rays, X-rays, ultraviolet, visible, infrared, microwaves, radio waves. (1)
(b) Ultraviolet. Too much UV damages skin and eyes and can cause cancer. (1)
(c) About 99%. (1)

Answer: (a) gamma to radio; (b) UV, because it can harm skin and eyes; (c) about 99%.

Example 2: Albedo of sea ice and sea water · 3 marks · Short answerBoard level

(a) Define albedo. (1)
(b) Sunlight of 600 W m⁻² falls on sea ice (albedo 0.60) and on open ocean water (albedo 0.10). Find the radiation absorbed per square metre by each. (1)
(c) How many times more does the ocean absorb? What does this suggest about a warming sea losing its ice? (1)

Try it first, then show the solutionSolution

(a) Albedo is the fraction of the incident sunlight that a surface reflects. (1)
(b) Ice: (1 − 0.60) × 600 = 0.40 × 600 = 240 W m⁻². Ocean: (1 − 0.10) × 600 = 0.90 × 600 = 540 W m⁻². (1)
(c) 540 ÷ 240 = 2.25 times. Replacing ice by open water raises the absorbed radiation, so the sea warms more and melts further ice. (1)

Answer: (a) fraction reflected; (b) 240 W m⁻² and 540 W m⁻²; (c) 2.25 times, so melting ice speeds further warming.

Example 3: A rooftop solar panel · 5 marks · Long answerHarder board level

A flat solar panel of area 2.5 m² faces the Sun on a clear day when the radiation reaching it is the maximum value at the ground, 1 kW m⁻². The panel converts 18% of the power falling on it into electricity.
(a) Find the power falling on the panel. (1)
(b) Find the electrical power produced. (1)
(c) Find the energy produced in 6 hours, in kWh. (1)
(d) For how long can a 100 W fan run on this energy? (1)
(e) What fraction of the solar constant (1.4 kW m⁻²) is the 1 kW m⁻² at the ground? Suggest why the ground value is smaller. (1)

Try it first, then show the solutionSolution

(a) Power = intensity × area = 1 kW m⁻² × 2.5 m² = 2.5 kW. (1)
(b) Electrical power = 18% of 2.5 kW = 0.18 × 2.5 = 0.45 kW. (1)
(c) Energy = 0.45 kW × 6 h = 2.7 kWh. (1)
(d) Fan power = 100 W = 0.1 kW, so time = 2.7 ÷ 0.1 = 27 h. (1)
(e) 1 ÷ 1.4 = 0.71, about 71%. The atmosphere absorbs, scatters and reflects part of the sunlight before it reaches the ground. (1)

Answer: (a) 2.5 kW; (b) 0.45 kW; (c) 2.7 kWh; (d) 27 h; (e) about 71%, because the atmosphere absorbs and scatters some sunlight.

Example 4: Temperature with height · 5 marks · Long answerHarder board level

On a certain day the air temperature at sea level is 32 °C. Assume the temperature falls by 6.5 °C per km in the troposphere.
(a) Find the temperature at 2 km and at 10 km. (2)
(b) Find the temperature at a mountain observatory at 4.4 km. (1)
(c) At what height would the temperature reach 0 °C? (1)
(d) Why does the temperature stop falling and begin to rise in the stratosphere? (1)

Try it first, then show the solutionSolution

(a) At 2 km: 32 − 6.5 × 2 = 32 − 13 = 19 °C. At 10 km: 32 − 6.5 × 10 = 32 − 65 = −33 °C. (2)
(b) At 4.4 km: 32 − 6.5 × 4.4 = 32 − 28.6 = 3.4 °C. (1)
(c) Height = 32 ÷ 6.5 = 4.92 km (about 4.9 km). Check: 6.5 × 4.92 = 31.98 °C. (1)
(d) In the stratosphere the ozone layer absorbs ultraviolet radiation, which warms the air there, so temperature rises with height. (1)

Answer: (a) 19 °C and −33 °C; (b) 3.4 °C; (c) about 4.9 km; (d) ozone absorbs UV.

Example 5: Melting ice and the albedo feedback · 4 marks · Case-basedHOTS / competency

Suppose a polar region has 100 equal area units. Initially 80 units are covered with snow (albedo 0.85) and 20 units are open sea (albedo 0.10). The sunlight falling on every unit is 200 W m⁻² in the same time, so use 200 per unit area.
(a) Calculate the total radiation absorbed by the region. (1)
(b) After warming, only 40 units are snow and 60 are sea. Calculate the new total absorbed. (1)
(c) By what factor has the absorbed radiation changed? (1)
(d) Explain why this makes warming self-reinforcing, and give one effect on the hydrosphere. (1)

Try it first, then show the solutionSolution

(a) Snow absorbs (1 − 0.85) × 200 = 30 per unit, sea absorbs (1 − 0.10) × 200 = 180 per unit. Total = 80 × 30 + 20 × 180 = 2400 + 3600 = 6000. (1)
(b) New total = 40 × 30 + 60 × 180 = 1200 + 10 800 = 12 000. (1)
(c) 12 000 ÷ 6000 = 2, so the region absorbs twice as much. (1)
(d) More melting exposes darker sea, which absorbs more heat and warms the region, which melts more ice: a positive feedback. Meltwater raises sea level (hydrosphere). (1)

Answer: (a) 6000; (b) 12 000; (c) it doubles; (d) warming melts ice, exposing low-albedo sea that absorbs more.

Where students lose marks in Solar Energy, Albedo and the Atmosphere

The slips examiners see most often, and what to write instead.

Mistake: Confusing albedo with absorption.
Students use albedo directly as the absorbed fraction.
Wrong working
Snow absorbs 0.85 × 800 = 680 W m⁻².
Right working
Snow reflects 0.85, so it absorbs (1 − 0.85) × 800 = 120 W m⁻².
Do this: Albedo is the reflected fraction. Absorbed = 1 − albedo.
Usually costs: 2 marks.
Mistake: Applying the 6.5 °C per km rule above the troposphere.
The rule is for the troposphere only.
Wrong working
Temperature at 30 km = 32 − 195 = −163 °C
Right working
The rule applies up to about 12 km; the stratosphere warms with height.
Do this: Check the height is below the top of the troposphere.
Usually costs: 1 to 2 marks.
Mistake: Mixing up the ozone layer and the greenhouse effect.
Both involve the atmosphere and radiation.
Wrong working
Ozone traps heat, causing global warming.
Right working
Ozone absorbs UV; greenhouse gases such as CO₂ and CH₄ trap outgoing IR.
Do this: Ozone: UV shield. Greenhouse gases: heat trap.
Usually costs: 1 mark.
Mistake: Forgetting to convert units in c = wavelength × frequency.
nm values are used directly.
Wrong working
Frequency = 3 × 10⁸ ÷ 500 = 6 × 10⁵ Hz
Right working
Frequency = 3 × 10⁸ ÷ 5 × 10⁻⁷ = 6 × 10¹⁴ Hz
Do this: Convert nm to m (1 nm = 10⁻⁹ m).
Usually costs: 2 marks.
Mistake: Saying the Sun heats the air directly in the troposphere.
The air is mostly heated from the ground below.
Wrong working
The troposphere is cooler higher up because it is farther from the Sun.
Right working
The troposphere is heated mainly from the ground, so air is cooler at greater height.
Do this: Say 'heated from below'.
Usually costs: 1 mark.

Exam tips

State the relation (power = intensity × area, absorbed = (1 − albedo) × incident) before substituting.
In an albedo question, always compare two surfaces and give the higher/lower comparison in words.
Name the spheres involved when asked how a change spreads, such as cryosphere to hydrosphere to biosphere.
Keep the ozone layer (UV shield in the stratosphere) and greenhouse gases (heat trap in the troposphere) separate in answers.
Use 'about' for the quoted values like 6.5 °C per km and 1.4 kW m⁻².

Practice questions on Solar Energy, Albedo and the Atmosphere

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 2 marks · Very short answerBoard level
Give two reasons why the equator is warmer than the poles.
Hint
Think of how the rays strike a spherical Earth.
Answer
At the equator the Sun's rays fall nearly straight, so the energy is concentrated on a small area (1 mark). At the poles the rays strike at a slant and the same energy is spread over a much larger area, so each unit of area receives less (1 mark).
Q2 · 3 marks · Short answerHarder board level
Visible light of wavelength 500 nm and ultraviolet light of wavelength 300 nm travel in vacuum (c = 3 × 10⁸ m s⁻¹). Find the frequency of each and say which carries more energy per photon of its kind.
Hint
Frequency = speed ÷ wavelength; convert nm to m.
Answer
500 nm = 5 × 10⁻⁷ m: frequency = 3 × 10⁸ ÷ 5 × 10⁻⁷ = 6 × 10¹⁴ Hz (1 mark). 300 nm = 3 × 10⁻⁷ m: frequency = 3 × 10⁸ ÷ 3 × 10⁻⁷ = 1 × 10¹⁵ Hz (1 mark). The UV has the higher frequency (shorter wavelength) and so carries more energy (1 mark).
Q3 · 2 marks · Assertion-reasonBoard level
Assertion (A): Venus is hotter than Mercury although it is farther from the Sun. Reason (R): Venus has a thick atmosphere of carbon dioxide that traps heat. Choose: (a) A and R both true, R explains A; (b) A and R both true, R does not explain A; (c) A true, R false; (d) A false, R true.
Hint
Think of the greenhouse effect.
Answer
(a). Both are true; the thick CO₂ atmosphere of Venus traps outgoing infrared radiation (greenhouse effect), which makes it hotter than Mercury.
Q4 · 3 marks · Short answerHarder board level
A roof of area 40 m² receives an effective 0.8 kW m⁻² of sunlight. Solar cells covering the roof turn 15% of the incident power into electricity. Find (a) the power falling on the roof, (b) the electrical power, and (c) the energy produced in 5 hours of such sunshine in kWh.
Hint
Intensity times area gives power.
Answer
(a) 0.8 × 40 = 32 kW (1 mark). (b) 15% of 32 kW = 0.15 × 32 = 4.8 kW (1 mark). (c) 4.8 × 5 = 24 kWh (1 mark).
Q5 · 4 marks · Case-basedHOTS / competency
A city roof has albedo 0.20. A group paints its 100 m² roof white, which raises the albedo to 0.60. The sunlight at noon is 900 W m⁻². (a) Find the radiation absorbed per m² before and after. (b) Find the total reduction in absorbed power for the roof. (c) Name two other measures that reduce an urban heat island and give the reason each works. (d) Why does a concrete building stay warm at night?
Hint
Absorbed = (1 − albedo) × incident.
Answer
(a) Before: 0.80 × 900 = 720 W m⁻²; after: 0.40 × 900 = 360 W m⁻² (1 mark). (b) Reduction = (720 − 360) × 100 = 36 000 W = 36 kW (1 mark). (c) Planting trees (shade and transpiration cool the air); using light-coloured or reflective surfaces for roads and walls (higher albedo means less absorption) (1 mark). (d) Concrete absorbs heat during the day and re-radiates it slowly at night (1 mark).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What is the approximate value of the solar constant?Show answer
About 1.4 kW m⁻².
2.What is albedo?Show answer
The fraction of sunlight reflected by a surface.
3.Which layer of the atmosphere contains the ozone layer?Show answer
The stratosphere.
4.By how much does temperature fall per km in the troposphere?Show answer
About 6.5 °C.
5.Name two greenhouse gases.Show answer
Carbon dioxide and methane (also water vapour).
6.Which agreement cut the CFCs that thinned the ozone layer?Show answer
The Montreal Protocol.

Solar Energy, Albedo and the Atmosphere: frequently asked questions

Why do cities feel hotter than villages at night?

Concrete, steel and asphalt store heat during the day and re-radiate it at night, and cities have less greenery to cool the air.

Is ozone good or bad?

Stratospheric ozone is good because it absorbs UV. Ground-level ozone formed in smog is harmful to breathe.

Why do the poles stay cold even in summer?

Sunlight strikes at a slant and spreads over a large area, and snow and ice reflect much of it because of high albedo.

Do EM waves need a medium?

No. They travel through vacuum at 3 × 10⁸ m s⁻¹, unlike sound.

Is the greenhouse effect always harmful?

No. It keeps the Earth warm enough for life. The problem is the extra warming from the added CO₂.