Chapter 8 – Solar Radiation, Heat Balance and Temperature | CBSE Notes

GEOGRAPHY | CLASS XI | NCERT

Book: Fundamentals of Physical Geography | Chapter 8

Solar Radiation, Heat Balance and Temperature

⭐ Topper Level 💬 Easy Language 📌 Point-Wise ☀️ Heat Budget Covered

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1. Learning Objectives

After reading these notes, you will be able to:

Understand what insolation is and the factors that cause variation in insolation received at Earth’s surface.

Explain the four processes of heating and cooling the atmosphere — conduction, convection, terrestrial radiation and advection.

Understand the Heat Budget of the Earth — how 100 units of solar energy are distributed and balanced.

Know the five factors controlling temperature distribution and understand isotherms.

Understand Temperature Inversion — its meaning, conditions, effects and examples.

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2. Solar Radiation (Insolation)

The energy received by the earth from the Sun is known as incoming solar radiation — in short, called Insolation. The earth’s surface receives most of its energy in short wavelengths. On average, the earth receives 1.94 calories per sq. cm per minute at the top of its atmosphere.

Aphelion

Earth farthest from Sun (152 million km) on 4th July — less insolation

Perihelion

Earth nearest to Sun (147 million km) on 3rd January — slightly more insolation

320 W/m²

Insolation received in the tropics (highest near subtropical deserts)

70 W/m²

Insolation received at the poles (lowest)

📋 Factors Causing Variation in Insolation

Rotation of Earth on its axis — determines day and night, affecting duration of solar energy received.
Angle of inclination of Sun’s rays — higher latitude = less angle = slant rays = more area covered = less energy per unit area. Slant rays also pass through greater depth of atmosphere → more absorption, scattering and diffusion.
Length of the day — longer the day, more the insolation received.
Transparency of the atmosphere — clouds, dust reduce insolation reaching the surface.
Configuration of land (aspect) — north or south-facing slopes affect insolation received. (Last two have less influence.)

📌 Earth’s Axis and Insolation

Earth’s axis makes an angle of 66½° with the plane of its orbit around the Sun — this has a greater influence on the amount of insolation received at different latitudes. Maximum insolation is received over subtropical deserts (cloudiness is least). Equator receives comparatively less insolation than the tropics. At same latitude, insolation is more over continent than ocean.

🌈 Passage of Solar Radiation through the Atmosphere

Atmosphere is largely transparent to short wave solar radiation.
Within the troposphere — water vapour, ozone and other gases absorb much of the near infrared radiation.
Very small suspended particles scatter visible spectrum both to space and towards Earth’s surface → adds colour to the sky.
Red colour of rising/setting Sun and blue colour of sky = result of scattering of light within the atmosphere.

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3. Heating and Cooling of the Atmosphere

🌡️ Mind Map — 4 Processes of Heating Atmosphere

Heating the Atmosphere

🪨 Conduction
Contact heating
Heats lower layers
Slow process

🌀 Convection
Vertical heating
Only in troposphere
Hot air rises

🌍 Terrestrial Radiation
Long-wave radiation
Earth heats atmosphere
from below

💨 Advection
Horizontal transfer
Most important in
middle latitudes

🪨 1. Conduction

Earth transmits heat to atmospheric layers near it in long wave form. Air in contact with land gets heated slowly; upper layers get heated by lower layers. Transfer of heat from warmer to cooler body by direct contact. Continues until both bodies attain same temperature or contact is broken. Important in heating lower layers of atmosphere.

🌀 2. Convection

Air in contact with Earth rises vertically on heating in the form of currents and transmits heat to the atmosphere. Process of vertical heating of atmosphere. Convective transfer of energy is confined only to the troposphere.

🌍 3. Terrestrial Radiation

Earth, after being heated, becomes a radiating body and radiates energy to atmosphere in long wave form. This heats up the atmosphere from below. Long wave radiation is absorbed by CO₂ and other greenhouse gases. The atmosphere in turn radiates heat to space. This is how the atmosphere is indirectly heated.

💨 4. Advection

Transfer of heat through horizontal movement of air. Horizontal movement is relatively more important than vertical. In middle latitudes, most of diurnal (day-night) variation in weather is caused by advection alone. In tropical regions — northern India’s summer local wind ‘Loo’ is the outcome of advection.

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4. Heat Budget of the Earth

The earth as a whole does not accumulate or lose heat. It maintains its temperature. This happens because the amount of heat received as insolation equals the amount lost by the earth through terrestrial radiation. This balance is called the Heat Budget or Heat Balance of the Earth.

📌 Albedo of the Earth

The reflected amount of radiation is called the albedo of the earth. Of the 100 units of insolation received at the top of atmosphere: 35 units are reflected back to space — 27 units by the top of clouds + 2 units from snow and ice-covered areas + 6 units scattered to space = 35. This is Earth’s average albedo.

📊 Heat Budget — 100 Units Distribution

Stage	Units	Details
Total Insolation received at top of atmosphere	100	Short wave solar radiation from the Sun
Reflected back to space (Albedo)	35	27 from clouds + 2 from snow/ice + 6 scattered to space = 35
Absorbed by atmosphere	14	Directly absorbed while passing through atmosphere
Absorbed by Earth’s surface	51	Heats the Earth’s surface (65 – 14 = 51)
Earth radiates back (terrestrial radiation)	51	17 directly to space + 34 absorbed by atmosphere
34 units absorbed by atmosphere from Earth:		6 by direct radiation + 9 through convection/turbulence + 19 through latent heat of condensation
Total absorbed by atmosphere	48	14 (from insolation) + 34 (from terrestrial radiation) = 48
Total radiated back to space	65	17 (directly from Earth) + 48 (from atmosphere) = 65 = BALANCED ✓

⭐ The Heat Balance in One Line
65 units received from Sun = 65 units returned to space (17 from Earth + 48 from atmosphere). This is why Earth neither warms up nor cools down over time.

📌 Latitudinal Variation in Heat Budget
Surplus of net radiation balance exists between 40°N and 40°S (tropics). Deficit near the poles. Surplus heat from tropics is redistributed polewards by winds — that is why tropics don’t keep heating up and poles don’t keep getting colder.

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5. Temperature

The interaction of insolation with the atmosphere and Earth’s surface creates heat — measured in terms of temperature. Heat = molecular movement of particles comprising a substance. Temperature = measurement in degrees of how hot or cold a place/thing is.

🔑 Factors Controlling Temperature Distribution (5 Factors)

🌐 1. Latitude

Temperature depends on insolation received. Insolation varies with latitude → temperature also varies. Higher latitude = less insolation = lower temperature.

⛰️ 2. Altitude

Atmosphere is indirectly heated by terrestrial radiation from below. So places near sea level record higher temperature than places at higher elevation. Temperature decreases with increasing height. Rate = Normal Lapse Rate = 6.5°C per 1,000 m.

🌊 3. Distance from the Sea

Sea heats slowly and cools slowly. Land heats quickly and cools quickly. Variation in temperature over sea is less compared to land. Places near sea come under moderating influence of sea and land breezes.

💨 4. Air Mass and Ocean Currents

Warm air masses → higher temperature. Cold air masses → lower temperature. Similarly, warm ocean currents along coast → higher temperature. Cold ocean currents → lower temperature.

🏙️ 5. Local Aspects

Local features like vegetation, urban heat islands, cloud cover also influence local temperature.

🗺️ Distribution of Temperature — Isotherms

📖 Isotherms
Isotherms = lines joining places having equal temperature. Generally parallel to latitude lines. Deviation more pronounced in January than July, especially in Northern Hemisphere (because of larger land area).

January: Isotherms deviate to north over oceans and to south over continents. Gulf Stream and North Atlantic Drift make North Atlantic warmer → isotherms bend north. Over Siberian plain, isotherms bend far south due to extreme cold.
January temperatures: Equatorial oceans >27°C; Tropics >24°C; Middle latitudes 2°C–0°C; Eurasian continental interior –18°C to –48°C.
July: Isotherms generally run parallel to latitudes. Equatorial oceans >27°C; subtropical Asia (30°N) >30°C.
Southern Hemisphere: Ocean influence is well-pronounced → isotherms more nearly parallel to latitudes → more gradual temperature variation.
Highest temperature range (>60°C) found over north-eastern part of Eurasian continent — due to continentality.
Least temperature range (~3°C) found between 20°S and 15°N — due to ocean influence.

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6. Inversion of Temperature

📖 Definition

Normally, temperature decreases with increase in elevation — this is the Normal Lapse Rate. When this situation is reversed and temperature increases with elevation instead of decreasing, it is called Inversion of Temperature.

🌙 Conditions Ideal for Temperature Inversion

A long winter night with clear skies and still air — ideal situation for inversion.
The heat of the day is radiated off during the night → by early morning, Earth is cooler than the air above.
Over polar areas, temperature inversion is normal throughout the year.
Inversion is usually of short duration but quite common nonetheless.
This inversion commonly lasts for a few hours until the Sun comes up and begins to warm the Earth.

⚠️ Effects of Temperature Inversion

Surface inversion promotes stability in the lower layers of the atmosphere.
Smoke and dust particles get collected beneath the inversion layer and spread horizontally.
Dense fogs in mornings — common occurrences especially during winter season.

🏔️ Temperature Inversion in Hills and Mountains

📌 Air Drainage

Cold air at hills and mountains, produced during night, flows under influence of gravity. Being heavy and dense, cold air acts like water and moves down the slope to pile up in pockets and valley bottoms with warm air above. This is called air drainage. It protects plants from frost damage.

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Summary — Quick Revision

Insolation = incoming solar radiation. Earth receives 1.94 cal/sq.cm/min at top of atmosphere in short waves. Aphelion = 4th July (152 million km far). Perihelion = 3rd January (147 million km near).

Factors causing variation in insolation: Earth’s rotation, angle of Sun’s rays, length of day, transparency of atmosphere, configuration of land. Earth’s axis = 66½° with orbit plane.

Max insolation = subtropical deserts (least cloudiness). Equator gets less than tropics. Insolation more over continents than oceans at same latitude.

4 processes of heating atmosphere: Conduction (contact, lower layers), Convection (vertical, troposphere only), Terrestrial radiation (long-wave, indirect), Advection (horizontal, most important in mid-latitudes).

Terrestrial radiation = Earth radiates energy in long-wave form → absorbed by CO₂ and greenhouse gases → atmosphere heated indirectly from below.

Heat Budget: 100 units received → 35 reflected (albedo) → 65 absorbed (14 by atmosphere + 51 by Earth) → 65 returned to space (17 from Earth + 48 from atmosphere). Earth stays in balance.

Latitudinal variation: Surplus between 40°N–40°S; Deficit near poles. Surplus redistributed polewards by winds → maintains balance.

5 factors controlling temperature: Latitude, Altitude (normal lapse rate = 6.5°C/1000m), Distance from sea, Air mass and ocean currents, Local aspects.

Isotherms = lines joining equal temperature places. Generally parallel to latitudes. Deviation more in January (Northern Hemisphere — larger land area). Highest range (>60°C) = NE Eurasian continent (continentality). Least range (3°C) = 20°S to 15°N.

Temperature Inversion = temperature increases with height (opposite of normal). Ideal condition: long winter night + clear sky + still air. Effects: dense fog, smoke trapped. In mountains: air drainage — cold air flows to valley bottoms — protects plants from frost.

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Important Terms to Remember

Insolation: The energy received by the earth from the Sun — incoming solar radiation. Earth’s surface receives it in short wavelengths. Average = 1.94 calories per sq. cm per minute at top of atmosphere.
Aphelion: Position of Earth when it is farthest from the Sun (152 million km) — occurs on 4th July. Slightly less insolation received.
Perihelion: Position of Earth when it is nearest to the Sun (147 million km) — occurs on 3rd January. Slightly more insolation received.
Angle of Incidence: The angle at which the Sun’s rays strike the Earth’s surface. Higher latitude = smaller angle = slant rays = less energy per unit area.
Scattering: Redirection of solar radiation in all directions by atmospheric particles. Causes blue colour of sky and red/orange colour of rising/setting Sun.
Conduction: Process of heat transfer by direct contact between two bodies of unequal temperature — from warmer to cooler body. Important in heating lower layers of atmosphere.
Convection: Process of vertical heating of the atmosphere. Heated air rises in the form of currents and transmits heat upward. Confined only to the troposphere.
Terrestrial Radiation: Long-wave radiation emitted by the Earth’s surface after being heated by insolation. Heats the atmosphere from below. Absorbed mainly by CO₂ and greenhouse gases.
Advection: Transfer of heat through horizontal movement of air. More important than vertical movement. In middle latitudes, most diurnal weather variations are caused by advection. ‘Loo’ winds in northern India during summer = advection process.
Albedo: The percentage of incoming solar radiation reflected back to space by clouds, ice, snow and other surfaces. Earth’s average albedo ≈ 35 units out of 100 received.
Heat Budget / Heat Balance: The balance between the total incoming solar radiation and the total outgoing radiation from Earth and atmosphere. Earth receives 65 units (after albedo) and returns 65 units (17 from Earth + 48 from atmosphere) — maintaining constant temperature.
Latent Heat of Condensation: Heat released when water vapour condenses into liquid water. Part of the 34 units (19 units) transferred from Earth’s surface to the atmosphere through this process.
Temperature: The measurement in degrees of how hot or cold a place or thing is. Different from heat — heat represents molecular movement of particles.
Normal Lapse Rate: The normal rate of decrease of temperature with increasing altitude in the atmosphere = 6.5°C per 1,000 metres.
Continentality: The effect of being located far from the sea — large temperature range (very hot in summer, very cold in winter). Highest temperature range (>60°C) found in north-eastern Eurasian continent due to continentality.
Isotherms: Lines on a map joining places having equal temperature. Generally parallel to latitude lines. Deviation more pronounced in January (especially Northern Hemisphere).
Thermal Equator: The belt of highest temperature on the globe — not the geographical equator but shifts north in July due to larger land mass of Northern Hemisphere.
Annual Range of Temperature: The difference between the mean temperature of the warmest month and the coldest month of a year.
Inversion of Temperature: When the normal lapse rate is inverted — temperature increases with increasing altitude instead of decreasing. Short duration but common. Ideal conditions: long winter night + clear sky + still air.
Air Drainage: The process in hills and mountains where cold dense air flows downslope under gravity to collect in valley bottoms and pockets, leaving warmer air above. Causes temperature inversion in valleys. Protects plants from frost damage.
Diurnal Range of Temperature: The difference between the highest temperature during the day and the lowest temperature at night at a given place.
Plank’s Law: States that the hotter a body, the more energy it will radiate and the shorter the wavelength of that radiation. Explains why the Sun (very hot) emits short-wave radiation and Earth (cooler) emits long-wave radiation.