Insolation ICSE Class 9: Terrestrial Radiation and Factors Affecting Temperature

What Insolation is about

Weightage: CISCE publishes no chapter weightage and no question-paper layout for ICSE Class 9 Geography. Class IX is examined by your school: one 80-mark paper of 2 hours plus 20 marks of internal assessment (a record file and one project). Insolation is topic 4(b), Atmosphere, of the 2028 syllabus.Source: CISCE ICSE 2028 Geography syllabus, Class IX

Key concepts

Insolation and terrestrial radiation

Insolation (incoming solar radiation) is the energy received by the earth from the sun. It travels as short waves, which pass through the atmosphere easily. The earth intercepts only a tiny fraction of the sun's total energy.

Of the insolation reaching the top of the atmosphere (taken as 100 units), roughly 35 units are reflected back to space by clouds, dust and ice (this reflectivity is the earth's albedo), about 14 units are absorbed by the atmosphere, and about 51 units are absorbed by the earth's surface.

Terrestrial radiation is the heat given out by the earth's surface after it has been warmed. It travels as long waves, which are readily absorbed by carbon dioxide and water vapour in the lower atmosphere.

So the atmosphere is heated mainly from below, by the earth, not directly by the sun. This is why temperature usually falls with height in the troposphere.

Heat balance: over a year the earth gives back as much energy as it receives, so its average temperature stays nearly constant (apart from human-caused warming).

Worked example

(2 marks) Distinguish between insolation and terrestrial radiation.

Try it first, then show the solutionSolution

1. Insolation: incoming energy from the sun, in short waves - 1 mark.
2. Terrestrial radiation: heat given out by the earth's surface, in long waves, which heats the atmosphere - 1 mark.

Answer: Insolation is the solar energy received by the earth, travelling as short waves that pass easily through the air. Terrestrial radiation is the heat given out by the warmed earth as long waves, which the lower atmosphere absorbs; it is the main way the air is heated.

How the atmosphere is heated

The air is heated in four ways, all starting from the warmed ground:
1. Radiation: the earth gives out long-wave terrestrial radiation that is absorbed by the air above.
2. Conduction: air in contact with the warm ground is heated directly; this affects only the lowest layer of air.
3. Convection: warmed air expands, becomes lighter and rises, while cooler air sinks to take its place; this carries heat upward (strongest in the tropics).
4. Advection: horizontal movement of air (winds) carries heat from one place to another. In northern India the hot Loo of summer afternoons is an example.

Because the air is heated from below, the lowest layers are warmest and temperature falls with height at the normal lapse rate of about 6.5 °C per 1,000 m.

Worked example

(2 marks) Why is the atmosphere said to be heated from below?

Try it first, then show the solutionSolution

1. Short-wave insolation passes through the air and heats the ground first - 1 mark.
2. The ground then heats the air above by terrestrial radiation, conduction and convection - 1 mark.

Answer: Short-wave insolation passes through the air without heating it much and warms the earth's surface. The warm ground then heats the air above it by long-wave terrestrial radiation, conduction and convection, so the atmosphere is heated from below.

Latitude

Temperature generally decreases from the equator towards the poles.

• Angle of the sun's rays: near the equator the midday sun is high and its rays are nearly vertical, so the energy falls on a small area and heats it intensely. Towards the poles the rays are slanting (oblique), so the same energy spreads over a larger area and heats it less.
• Thickness of atmosphere crossed: slanting rays pass through a greater thickness of air, so more energy is absorbed, scattered and reflected before reaching the ground.
• Length of day also varies with latitude and season.

This gives the heat zones: torrid (between the tropics), temperate (between the tropics and polar circles) and frigid (beyond the polar circles).

Example: Thiruvananthapuram (about 8°N) is warm all year, while Srinagar (about 34°N) has cold winters with snow.

Worked example

(3 marks) Explain why places near the equator are hotter than places near the poles.

Try it first, then show the solutionSolution

1. Rays near the equator are nearly vertical; near the poles they are slanting - 1 mark.
2. Vertical rays concentrate energy on a smaller area; slanting rays spread it over a larger area - 1 mark.
3. Slanting rays pass through more atmosphere and lose more energy - 1 mark.

Answer: Near the equator the sun's rays fall almost vertically, concentrating their heat on a small area and passing through less atmosphere. Near the poles the rays are slanting, so the same energy is spread over a larger area and more of it is lost in the thicker layer of air crossed. Hence the equator is hotter than the poles.

Altitude

Temperature decreases with height above sea level, at an average rate of about 6.5 °C per 1,000 m (the normal lapse rate).
Reasons:

• The air is heated from below, so the farther from the ground, the less heat it receives.
• Air at higher levels is thinner and holds less water vapour, dust and carbon dioxide, so it absorbs less terrestrial radiation and loses heat faster.

Examples: hill stations such as Shimla, Darjeeling and Ooty are cool in summer though the plains below are very hot; Quito, the capital of Ecuador, lies almost on the equator but at a height of about 2,850 m, so it has a mild, spring-like climate. High mountains even near the equator, such as Kilimanjaro, have snow on their peaks.

Calculation: temperature at height = temperature at sea level − (height in m ÷ 1,000 × 6.5).
E.g. sea level 30 °C; at 2,000 m: 30 − 2 × 6.5 = 17 °C.

Worked example

(2 marks) The temperature at sea level is 25 °C. Find the temperature at the top of a hill 3,000 m high, using the normal lapse rate.

Try it first, then show the solutionSolution

1. Fall = 3,000 ÷ 1,000 × 6.5 = 19.5 °C - 1 mark.
2. Temperature = 25 − 19.5 = 5.5 °C - 1 mark.

Answer: 5.5 °C.

Distance from the sea

Land heats up and cools down much faster than water.

• Water is transparent, so heat penetrates deeper; it moves and mixes; and it needs more heat to raise its temperature (higher specific heat). Land heats only a thin surface layer.
• So the sea is cooler than the land in summer and warmer in winter.

Coastal places have a maritime or equable climate: the sea keeps summers cooler and winters milder, giving a small annual range of temperature. Sea breezes also moderate daytime heat.

Places far inland have a continental or extreme climate: very hot summers, very cold winters and a large range (continentality).

Example: Mumbai, on the coast, has a small annual range of temperature, while Delhi, far inland (and about 10° farther north), has very hot summers and cold winters; distance from the sea explains most of the difference. In the same way Kolkata is more equable than Patna.

Worked example

(3 marks) Why do coastal places have an equable climate while interior places have an extreme climate?

Try it first, then show the solutionSolution

1. Land heats and cools faster than water - 1 mark.
2. The sea, cooler in summer and warmer in winter, moderates coastal temperatures; sea breezes help - 1 mark.
3. Interior places lack this influence, so summers are very hot and winters very cold (e.g. Mumbai vs Delhi) - 1 mark.

Answer: Land heats up and cools down faster than water. Near the coast the sea, which is cooler in summer and warmer in winter, moderates temperatures, giving an equable climate like Mumbai's. Interior places such as Delhi lack this influence, so they have hot summers and cold winters: an extreme climate.

Slope of land, winds and ocean currents

Slope of land (aspect): slopes facing the sun receive more direct rays and are warmer. In the northern hemisphere, south-facing slopes are sunnier and warmer than north-facing slopes, which stay shaded. On the Alps and in the Himalayas settlements, orchards and farms are found mostly on the sunny south-facing slopes, while north-facing slopes are forested or snow-covered.

Winds: winds carry the temperature of the region they come from (advection).

• Warm winds raise temperature: the hot, dry Loo raises summer temperatures in northern India; the warm Chinook melts snow on the eastern slopes of the Rockies.
• Cold winds lower it: icy winds from the interior of Siberia make winters very cold in northern China; the cold Mistral blows into southern France.
• Onshore winds from the sea moderate coastal temperatures.

Ocean currents: warm currents raise the temperature of the coasts they wash, cold currents lower it.

• The warm North Atlantic Drift keeps the coasts of north-western Europe mild in winter and its ports ice-free.
• The cold Labrador Current keeps the coast of Labrador cold, with ports frozen in winter.
• The warm Kuro Shio warms southern Japan; the cold Oya Shio cools its north-east.

Worked example

(3 marks) How do slope of land and ocean currents affect temperature? Give an example of each.

Try it first, then show the solutionSolution

1. Slope facing the sun gets more direct rays and is warmer - 1 mark.
2. Example: south-facing slopes in the Alps/Himalayas are warmer and settled - 1 mark.
3. Warm currents raise and cold currents lower coastal temperatures, e.g. North Atlantic Drift keeps Norway mild - 1 mark.

Answer: Slopes facing the sun receive more direct rays and are warmer; in the northern hemisphere south-facing slopes of the Alps and Himalayas are warmer and more densely settled. Ocean currents warm or cool nearby coasts: the warm North Atlantic Drift keeps Norway's coast mild, while the cold Labrador Current keeps Labrador cold.

Insolation in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Insolation and the Factors Affecting Temperature

Insolation: key facts

Normal lapse rate

$T(height)=T(sealevel)-(h÷1,000)\times 6.5°C$

h in metres; average value.

Height from temperatures

$h=(T_1-T_2)÷6.5\times 1,000m$

T₁ lower place, T₂ higher place.

Insolation budget (100 units)

≈ 35 reflected (albedo); ≈ 14 absorbed by the atmosphere; ≈ 51 absorbed by the surface

Surface re-radiates as terrestrial radiation.

Wave types

$Insolation=shortwaves;terrestrialradiation=longwaves$

Air absorbs long waves better.

Six factors

Latitude, altitude, distance from the sea, slope of land, winds, ocean currents

Each with an example.

How the school exam asks Insolation

The question patterns to expect, and the method that works for each.

Tricks to remember Insolation

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

• LADS-WC

  Latitude, Altitude, Distance from sea, Slope, Winds, Currents.

  Use it when: The six factors in syllabus order.

• Sun in short, earth out long

  Insolation comes in as Short waves; terrestrial radiation goes out as Long waves.

  Use it when: Wave types.

• 6.5 per 1,000

  Six and a half degrees colder for every kilometre up.

  Use it when: Lapse rate numericals.

Insolation: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 7-mark question split into 2 + 2 + 3 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Insolation

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

• ✗Mistake: Saying mountain tops are colder because they are farther from the sun

  The real reason is not understood.

  Wrong working

  Hills are cold because the sun is farther away.

  Right working

  The air is heated from below; thinner air at height absorbs and holds less heat.

  ✓Do this: Link altitude to terrestrial radiation and thin air.

  Usually costs: 1 to 2 marks.

• ✗Mistake: Using 6.5 °C per 100 m

  Units slip.

  Wrong working

  At 2,000 m: 30 − 20 × 6.5 = −100 °C.

  Right working

  At 2,000 m: 30 − 2 × 6.5 = 17 °C.

  ✓Do this: Divide the height by 1,000 first.

  Usually costs: Full marks of the numerical.

• ✗Mistake: Mixing up insolation and terrestrial radiation

  Both are 'radiation'.

  ✓Do this: In from the sun = insolation (short); out from the earth = terrestrial (long).

  Usually costs: 1 mark.

• ✗Mistake: Explaining a factor without an example

  Process written, place forgotten.

  ✓Do this: Always add a named place, wind or current.

  Usually costs: 1 mark per factor.

Practice questions on Insolation

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

1. Q1 · 1 mark · MCQExam level

   Temperature in the troposphere falls by about:
   (a) 1 °C per 1,000 m
   (b) 6.5 °C per 1,000 m
   (c) 10 °C per 100 m
   (d) 6.5 °C per 100 m

   Hint

   Normal lapse rate.

   Answer

   (b) 6.5 °C per 1,000 m.

2. Q2 · 1 mark · MCQExam level

   Which of these winds raises the temperature of northern India in summer?
   (a) Mistral
   (b) Loo
   (c) Chinook
   (d) Polar easterlies

   Hint

   Hot, dry, May-June afternoons.

   Answer

   (b) Loo.

3. Q3 · 2 marks · Very short answerHarder exam level

   What is meant by albedo? Why does fresh snow keep a place cold?

   Hint

   Reflection.

   Answer

   Albedo is the proportion of insolation reflected back by a surface. Fresh snow has a very high albedo, reflecting most sunlight, so little energy is absorbed and the place stays cold.

4. Q4 · 2 marks · Very short answerExam level

   The temperature at sea level is 28 °C. What will it be at 1,500 m?

   Hint

   1.5 × 6.5.

   Answer

   Fall = 1.5 × 6.5 = 9.75 °C; temperature = 28 − 9.75 = 18.25 °C.

5. Q5 · 2 marks · Very short answerHarder exam level

   Why do slanting rays give less heat than vertical rays? Give two reasons.

   Hint

   Area and atmosphere.

   Answer

   (1) Slanting rays spread the same energy over a larger area. (2) They travel through a greater thickness of atmosphere, losing more energy by absorption, scattering and reflection.

6. Q6 · 2 marks · Very short answerHarder exam level

   Explain how a warm ocean current affects the temperature of the coast near it, with an example.

   Hint

   North Atlantic Drift.

   Answer

   Winds blowing over the warm current carry its warmth to the land, raising winter temperatures. The North Atlantic Drift keeps the coast of Norway mild and its ports ice-free in winter.

7. Q7 · 2 marks · Very short answerHarder exam level

   Why is the earth's surface warmer than the air a few kilometres above it on a sunny day?

   Hint

   Short waves pass, long waves absorbed.

   Answer

   Short-wave insolation passes through the air and is absorbed by the ground. The air is heated only afterwards, mainly near the surface, by the earth's long-wave radiation, conduction and convection, so it is warmest near the ground.

8. Q8 · 2 marks · Very short answerHarder exam level

   A hill town is at 1,800 m. If its temperature is 14.3 °C, what would you expect at sea level nearby at the same time?

   Hint

   Add the fall back.

   Answer

   Fall over 1,800 m = 1.8 × 6.5 = 11.7 °C; sea-level temperature ≈ 14.3 + 11.7 = 26 °C.

9. Q9 · 3 marks · Short answerHOTS / competency

   Two cities lie on the same latitude. One has mild, wet winters; the other has freezing winters with frozen ports. Suggest two factors that could explain this.

   Hint

   Currents and sea.

   Answer

   (1) Ocean currents: the mild city may be washed by a warm current (like Norway's coast by the North Atlantic Drift), the cold one by a cold current (like Labrador). (2) Distance from the sea or winds: the mild city may receive moist onshore westerlies, while the cold one lies inland or receives icy winds from a continental interior.

10. Q10 · 2 marks · Very short answerExam level

    Why does temperature fall with altitude? Give two reasons.

    Hint

    Heated from below; thin air.

    Answer

    (1) The air is heated from below by the earth, so it gets less heat farther from the ground. (2) Higher air is thinner, with less water vapour and carbon dioxide, so it absorbs less heat and loses it faster.

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. 1.Full form of insolation?Show answer

   Incoming solar radiation.

2. 2.Wave type of terrestrial radiation?Show answer

   Long waves.

3. 3.Normal lapse rate?Show answer

   About 6.5 °C per 1,000 m.

4. 4.Climate of coastal places?Show answer

   Equable (maritime).

5. 5.Warmer slope in the northern hemisphere?Show answer

   South-facing slope.

6. 6.Share of insolation reflected back to space?Show answer

   About 35 units out of 100 (albedo).

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Insolation: incoming short-wave solar energy; terrestrial radiation: outgoing long-wave heat from the earth.
• ☐Air is heated from below; ≈ 35 of 100 units reflected, 14 absorbed by air, 51 by the surface.
• ☐Latitude: vertical rays at the equator, slanting at poles.
• ☐Altitude: −6.5 °C per 1,000 m; hill stations, Quito.
• ☐Distance from sea: equable coasts (Mumbai), extreme interiors (Delhi).
• ☐Slope: sunny south-facing slopes warmer (N hemisphere). Winds: Loo, Chinook warm; Siberian winds cold. Currents: North Atlantic Drift warm, Labrador cold.

Insolation: frequently asked questions