Atmospheric Temperature: Insolation, Heat Budget, Distribution of Temperature and Isotherms, ISC Class 11

What Atmospheric Temperature is about

Weightage: CISCE gives no Class XI unit weightage for ISC Geography. Atmospheric Temperature is topic 13 of the Class XI syllabus (Atmosphere) and can be asked anywhere in the 70-mark Paper I (Theory, 3 hours). Class XI is examined by the school, so your school sets the paper and decides how many marks each topic carries.Source: CISCE ISC Geography (853) syllabus, examination year 2028, Class XI

Key concepts

Heating and cooling of the atmosphere

Solar radiation is short-wave; most of it passes through the air without heating it much. The earth's surface absorbs it, warms up and then heats the air above.

1. Radiation: the warmed earth gives out long-wave (terrestrial) radiation, which water vapour, carbon dioxide and other greenhouse gases absorb, heating the atmosphere. This is the main way the air is heated, which is why the air is warmest near the ground. At night, the earth loses heat by radiation and cools, cooling the air above it.
2. Conduction: air in contact with the warm ground is heated directly; because air is a poor conductor, this affects only the lowest layers.
3. Convection: heated air expands, becomes lighter and rises, while cooler air sinks to take its place, carrying heat upward through the troposphere. Convection is strongest in the tropics on hot afternoons.
4. Advection: horizontal movement of air carries heat from one region to another, for example warm winds such as the loo in northern India, or the land and sea breezes on coasts. In middle latitudes, much of the day-to-day change in weather is due to advection.

Cooling occurs by terrestrial radiation (especially on clear nights), by contact with cold surfaces, by mixing with cold air and by the expansion of rising air (adiabatic cooling).

Worked example

[4] 'The atmosphere is heated from below and not directly by the sun.' Explain with reference to the processes of heating. (Understanding)

Try it first, then show the solutionSolution

1. Short-wave insolation passes through air and heats the ground - 1 mark.
2. Ground emits long-wave radiation absorbed by greenhouse gases - 1 mark.
3. Conduction heats the air in contact with the ground - 1 mark.
4. Convection carries heat upward; advection moves it sideways - 1 mark.

Answer: Short-wave solar radiation passes through the air with little absorption and heats the earth's surface. The warm ground then gives out long-wave radiation, which water vapour and carbon dioxide absorb; it also heats the air in contact with it by conduction. Warm air rises by convection, carrying heat upward, while advection moves heat horizontally. So the air is heated from below and is warmest near the ground.

Insolation and factors influencing it

Insolation (incoming solar radiation) is the energy received by the earth from the sun. On average the outer atmosphere receives about 1.94 calories per square centimetre per minute (roughly 1,360 watts per square metre, the solar constant); the earth intercepts only a tiny fraction of the sun's output.

Factors
1. Angle of the sun's rays: vertical rays concentrate energy on a small area and pass through less atmosphere; slanting rays spread the same energy over a larger area and pass through more air, where more is absorbed and scattered. So insolation is highest in the tropics and least near the poles, and highest at noon.
2. Duration of the day: longer days give more insolation. At the equator days are about 12 hours throughout the year; in summer, higher latitudes have very long days, and the poles have six months of daylight (but at a low angle).
3. Transparency of the atmosphere: clouds, water vapour, dust and smoke reflect, scatter and absorb insolation. Cloudy equatorial regions receive less insolation at the surface than the clear subtropical deserts, which receive the most.

Other factors: distance between the earth and the sun (the earth is nearest the sun at perihelion, about 3 January, and farthest at aphelion, about 4 July, but the difference has little effect compared with the others), and the configuration of land (slope and aspect: south-facing slopes in the northern hemisphere receive more sun).

Worked example

[4] Explain how the angle of the sun's rays and the transparency of the atmosphere influence insolation. (Understanding)

Try it first, then show the solutionSolution

1. Vertical rays: energy concentrated on a small area - 1 mark.
2. Vertical rays pass through a shorter path of air, less absorption - 1 mark.
3. Clouds, dust and vapour reflect and absorb insolation - 1 mark.
4. Clear subtropical deserts receive most; cloudy equator less - 1 mark.

Answer: Vertical rays concentrate the sun's energy on a small area and pass through a shorter path of air, so less is lost; slanting rays spread energy over a larger area and lose more in the atmosphere. Transparency matters because clouds, water vapour and dust reflect, scatter and absorb insolation, so clear subtropical deserts receive more insolation at the surface than the cloudy equatorial belt.

Heat budget and latitudinal heat balance

The earth as a whole neither warms nor cools over long periods: the heat it receives from the sun equals the heat it loses to space. This balance is the heat budget.

Taking insolation at the top of the atmosphere as 100 units:
1. About 35 units are reflected back to space without heating anything (27 by clouds, 2 by snow and ice, 6 scattered by the atmosphere). This reflected share is the earth's albedo.
2. The remaining 65 units are absorbed: 14 by the atmosphere and 51 by the earth's surface.
3. The earth gives off 51 units as terrestrial radiation: 17 units go straight to space; 34 units are absorbed by the atmosphere (6 by direct absorption, 9 by convection and turbulence, 19 as latent heat of condensation).
4. The atmosphere thus absorbs 14 + 34 = 48 units and radiates them to space.
5. Total lost to space = 17 + 48 = 65 units, which balances the 65 units absorbed.

Latitudinal heat balance: between about 40 degrees N and 40 degrees S, more heat is received than lost (heat surplus); towards the poles, more is lost than received (heat deficit). Heat is transferred from the tropics to the poles by winds (including cyclones and storms) and by ocean currents. Without this transfer the tropics would grow ever hotter and the poles ever colder.

Worked example

[4] Explain the heat budget of the earth, taking insolation as 100 units. (Understanding)

Try it first, then show the solutionSolution

1. 35 units reflected (albedo); 65 absorbed - 1 mark.
2. 14 absorbed by atmosphere, 51 by the surface - 1 mark.
3. Surface returns 51: 17 to space, 34 to atmosphere - 1 mark.
4. Atmosphere radiates 48; 17 + 48 = 65, balancing intake - 1 mark.

Answer: Of 100 units of insolation, 35 are reflected to space as albedo and 65 are absorbed: 14 by the atmosphere and 51 by the surface. The surface returns these 51 units: 17 go directly to space and 34 are absorbed by the atmosphere through radiation, convection and latent heat. The atmosphere radiates its 48 units (14 + 34) to space, so 17 + 48 = 65 units leave, balancing the 65 absorbed.

Distribution of temperature: factors, anomalies and inversion

Factors controlling horizontal distribution
1. Latitude: insolation, and so temperature, generally decreases from the equator towards the poles.
2. Altitude: temperature falls with height at about 6.5 degrees C per 1,000 m (normal lapse rate), so hill stations such as Shimla are cooler than the plains.
3. Distance from the sea: land heats and cools faster than water, so interior places (Delhi) have a large annual range (continentality) while coastal places (Mumbai) have a small range (maritime influence).
4. Ocean currents and air masses: warm currents (North Atlantic Drift) warm coasts; cold currents (Labrador, Peru) cool them; warm or cold winds have the same effect.
5. Relief and aspect: mountains block cold winds (the Himalayas shield India from Central Asian cold air); sunny slopes are warmer than shaded ones.
6. Cloud cover, vegetation and nature of the surface: clouds reduce the daily range; snow reflects insolation; dark soils absorb more.

Temperature anomaly: the difference between the mean temperature of a place and the mean temperature of its latitude (parallel). Positive anomalies occur over the warm oceans of high northern latitudes in winter (North Atlantic) and over continents in summer; negative anomalies over continents in winter (Siberia).

Vertical distribution and inversion: normally temperature falls with height. In a temperature inversion it rises with height in a layer near the ground. Conditions: long winter nights, clear skies, calm air and dry air, so the ground loses heat quickly and the air near it becomes colder than the air above. Inversions are common in valleys (cold air drains downslope at night, so valley bottoms are colder: frost pockets), cause fog and trap smog over cities such as Delhi in winter.

Worked example

[4] What is temperature inversion? State the conditions favourable for it. (Understanding)

Try it first, then show the solutionSolution

1. Inversion: temperature increases with height near the ground - 1 mark.
2. Long nights let the ground lose much heat by radiation - 1 mark.
3. Clear skies and dry air allow heat to escape - 1 mark.
4. Calm air prevents mixing; valleys, winter; fog and smog - 1 mark.

Answer: Temperature inversion is a reversal of the normal lapse rate, so that temperature increases with height in a layer near the ground. It develops on long winter nights with clear skies, dry air and calm conditions, when the ground loses heat rapidly by radiation and cools the air in contact with it, especially in valleys where cold air drains down; it causes fog and traps smog.

Isotherms and practical work on temperature

Isotherms are lines joining places having the same temperature, usually reduced to sea level to remove the effect of altitude.

Characteristics
1. They run roughly east-west, parallel to the lines of latitude, because temperature depends mainly on latitude.
2. They bend sharply where land meets sea. In January (northern winter), isotherms in the northern hemisphere bend towards the equator over the cold continents and towards the poles over the warmer oceans; in July the pattern is reversed.
3. They are more regular in the southern hemisphere, which is mostly ocean, and more irregular in the northern hemisphere, which has large landmasses.
4. Closely spaced isotherms show a rapid change of temperature (steep temperature gradient); widely spaced isotherms show gradual change.
5. The thermal equator (line of highest mean temperature) lies north of the geographical equator for most of the year and shifts with the sun.

Practical work
1. Instruments: a maximum thermometer (mercury, with a constriction) records the highest temperature of the day; a minimum thermometer (alcohol, with an index) records the lowest; Six's maximum and minimum thermometer combines both. Thermometers are kept in a Stevenson screen, a white, louvred wooden box on legs about 1.2 m above the ground, so they measure air temperature in the shade.
2. Calculations: daily mean = (maximum + minimum) / 2; daily range = maximum - minimum; monthly mean = average of daily means; annual range = warmest month mean - coldest month mean.
3. Graph: a line graph of monthly mean temperatures (months on the x-axis, temperature in degrees C on the y-axis) shows seasonal variation; comparing two cities shows the effect of continentality or altitude.

Worked example

[4] A Six's thermometer records a maximum of 34 degrees C and a minimum of 22 degrees C. Find the daily mean and daily range. Why is the thermometer kept in a Stevenson screen? (Application)

Try it first, then show the solutionSolution

1. Daily mean = (34 + 22) / 2 = 28 degrees C - 1 mark.
2. Daily range = 34 - 22 = 12 degrees C - 1 mark.
3. Screen shades the thermometer from direct sun and rain - 1 mark.
4. Louvres let air circulate; raised above ground radiation - 1 mark.

Answer: Daily mean = (34 + 22) / 2 = 28 degrees C; daily range = 34 - 22 = 12 degrees C. The thermometer is kept in a Stevenson screen, a white, louvred box raised above the ground, so that it measures the true air temperature in the shade, protected from direct sunlight, rain and heat radiated by the ground, while air circulates freely.

Atmospheric Temperature 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

• Heating of the Atmosphere, Insolation and the Heat Budget
• Temperature Distribution, Inversion, Isotherms and Practical Work

Atmospheric Temperature: key facts

Heat budget (100 units)

$Reflected35(27clouds+2snow/ice+6scattered);absorbed65=14atmosphere+51surface;out17+48=65$

Balance between insolation and terrestrial radiation.

Daily mean and range

$Mean=(max+min)/2;range=max-min$

Six's thermometer.

Annual range

Warmest month mean - coldest month mean

Large inland, small on coasts.

Lapse rate

$Fall=6.5degreesCper1,000m$

Normal lapse rate.

Temperature anomaly

$Anomaly=meantemperatureofplace-meantemperatureofitslatitude$

Positive or negative.

Latitudinal balance

Surplus 40 degrees N-40 degrees S; deficit poleward

Winds and currents transfer heat.

How the school exam asks Atmospheric Temperature

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

Tricks to remember Atmospheric Temperature

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

• Heating: 'RCCA'

  Radiation, Conduction, Convection (up), Advection (across).

  Use it when: Four processes.

• Insolation: 'ADT'

  Angle, Day length, Transparency.

  Use it when: Factors in the syllabus.

• Budget: '35-14-51; 17-34-48'

  Reflected 35; absorbed by air 14 and ground 51; ground sends 17 out and 34 to the air; air sends 48 out.

  Use it when: Heat budget numbers.

Atmospheric Temperature: 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 D style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 8-mark question split into 3 + 2 + 2 + 1 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 Atmospheric Temperature

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

• ✗Mistake: Saying the sun heats the air directly

  Ignoring that air is nearly transparent to short-wave radiation.

  Wrong working

  The atmosphere is heated mainly by direct sunlight.

  Right working

  The ground absorbs insolation and heats the air from below by terrestrial radiation, conduction and convection.

  ✓Do this: Sun -> ground -> air.

  Usually costs: 1-2 marks.

• ✗Mistake: Wrong heat-budget arithmetic

  Mixing up 14, 34 and 48.

  ✓Do this: Atmosphere absorbs 14 (insolation) + 34 (from earth) = 48; out: 17 + 48 = 65.

  Usually costs: 1-2 marks.

• ✗Mistake: Confusing advection and convection

  Both move heat.

  ✓Do this: Convection is vertical; advection is horizontal.

  Usually costs: 1 mark.

• ✗Mistake: Annual range from daily values

  Daily and annual range mixed.

  ✓Do this: Annual range uses the warmest and coldest monthly means.

  Usually costs: 1 mark.

• ✗Mistake: Reversing the January bend of isotherms

  Not thinking about which surface is colder.

  ✓Do this: Northern winter: land colder, so isotherms bend equatorward over land.

  Usually costs: 1 mark.

Practice questions on Atmospheric Temperature

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 · Very short answerExam level

   What is insolation?

   Hint

   Incoming.

   Answer

   Incoming solar radiation received by the earth.

2. Q2 · 1 mark · Very short answerExam level

   What is an isotherm?

   Hint

   Equal temperature.

   Answer

   A line on a map joining places with the same temperature (usually reduced to sea level).

3. Q3 · 1 mark · MCQExam level

   Out of 100 units of insolation, the units reflected back to space are about:
   (a) 14
   (b) 35
   (c) 51
   (d) 65

   Hint

   Albedo.

   Answer

   (b) 35.

4. Q4 · 1 mark · Very short answerHarder exam level

   Define temperature anomaly.

   Hint

   Compare with latitude.

   Answer

   The difference between the mean temperature of a place and the mean temperature of its latitude.

5. Q5 · 4 marks · Short answerHarder exam level

   [4] Why do the poles receive less insolation than the equator even though they have six months of continuous daylight?

   Hint

   Angle; path; snow.

   Answer

   (1) The sun's rays strike the poles at a very low angle, spreading energy over a large area. (2) Slanting rays pass through a longer path of atmosphere, losing more energy. (3) Snow and ice reflect much of what arrives (high albedo). (4) So despite long days, polar regions remain cold; the equator gets near-vertical rays all year.

6. Q6 · 4 marks · Short answerHarder exam level

   [4] State four characteristics of isotherms.

   Hint

   Direction, bends, hemispheres, spacing.

   Answer

   (1) They run roughly east-west, parallel to latitudes. (2) They bend sharply at land-sea boundaries, in opposite directions in January and July. (3) They are more regular in the ocean-dominated southern hemisphere. (4) Close spacing shows a steep temperature gradient; wide spacing a gentle one.

7. Q7 · 4 marks · Short answerHarder exam level

   [4] Explain how ocean currents and relief affect temperature, with one example each.

   Hint

   Warm and cold currents; barriers.

   Answer

   (1) Warm currents raise coastal temperatures: the North Atlantic Drift keeps north-west Europe's ports ice-free in winter. (2) Cold currents lower them: the Labrador current chills north-eastern North America. (3) Mountains act as barriers: the Himalayas block cold winds from Central Asia, keeping northern India warmer in winter. (4) Aspect: sun-facing slopes are warmer than shaded slopes.

8. Q8 · 4 marks · Short answerHOTS / competency

   [4] A Six's thermometer shows readings for three days: (36, 24), (38, 26), (35, 23) as (maximum, minimum) in degrees C. Find each day's mean, the mean for the three days and the greatest daily range.

   Hint

   Mean = (max + min) / 2.

   Answer

   (1) Day means: (36 + 24)/2 = 30; (38 + 26)/2 = 32; (35 + 23)/2 = 29 degrees C. (2) Three-day mean = (30 + 32 + 29)/3 = 91/3 = about 30.3 degrees C. (3) Ranges: 12, 12 and 12 degrees C. (4) The greatest daily range is 12 degrees C (all three days are equal).

9. Q9 · 4 marks · Short answerHOTS / competency

   [4] 'Without winds and ocean currents, the tropics would get hotter and the poles colder every year.' Evaluate this statement using the idea of latitudinal heat balance.

   Hint

   Surplus and deficit.

   Answer

   (1) Between about 40 degrees N and S the earth receives more energy than it loses (surplus). (2) Poleward it loses more than it receives (deficit). (3) Winds, storms and ocean currents carry the surplus heat poleward. (4) So the statement is correct: without this transfer, the tropics would keep warming and the poles cooling, but in reality average temperatures remain balanced.

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.What is the albedo of the earth?Show answer

   The proportion of insolation reflected back to space, about 35 per cent.

2. 2.When is the earth at perihelion?Show answer

   Around 3 January.

3. 3.Between which latitudes is there a heat surplus?Show answer

   About 40 degrees N and 40 degrees S.

4. 4.Which thermometer records both maximum and minimum temperature?Show answer

   Six's maximum and minimum thermometer.

5. 5.What is a frost pocket?Show answer

   A valley floor where cold air collects at night, making it colder than the slopes.

6. 6.What is the thermal equator?Show answer

   The line joining places with the highest mean temperature; it shifts with the sun.

Quick revision checklist

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

• ☐Air heated from below: radiation, conduction, convection (vertical), advection (horizontal).
• ☐Insolation factors: angle of rays, day length, transparency; also distance, slope.
• ☐Budget: 35 reflected; 14 + 51 absorbed; earth 17 + 34; atmosphere 48; 17 + 48 = 65.
• ☐Surplus 40 N-40 S, deficit poleward; winds and currents transfer heat.
• ☐Factors: latitude, altitude (6.5 degrees C/km), distance from sea, currents, relief, clouds.
• ☐Anomaly = place mean - latitude mean; inversion on long, clear, calm winter nights.
• ☐Isotherms: east-west; bend at land-sea; January equatorward over land (NH); regular in SH.
• ☐Daily mean = (max + min)/2; range = max - min; Stevenson screen; Six's thermometer.

Atmospheric Temperature: frequently asked questions