NCERT Class 11 Physical Geography Chapter 9 – Solar Radiation, Heat Balance and Temperature

NCERT Class 11 Physical Geography Chapter 9 explains how energy from the Sun reaches the Earth and controls temperature patterns. Students should refer to the official NCERT website at for authentic textbooks and syllabus updates. In NCERT Class 11 Physical Geography Chapter 9, students study insolation (incoming solar radiation), the Earth’s heat budget and factors that influence temperature distribution.

NCERT Class 11 Physical Geography Chapter 9 is extremely important for CBSE board exams and competitive exams like UPSC and BPSC because questions related to heat balance, greenhouse effect and temperature variation are frequently asked. A strong understanding of NCERT Class 11 Physical Geography Chapter 9 builds the base for atmospheric circulation and monsoon concepts.

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1. Solar radiation

• The Earth receives almost all of its energy from the Sun, and the Earth in turn radiates this energy back into space, due to which the planet neither continuously heats up nor cools down.
• The energy received by the Earth in short wavelengths is called Incoming Solar Radiation (Insolation).
• As the Earth is a geoid resembling a sphere, the Sun’s rays fall obliquely at the top of the atmosphere, and the Earth intercepts only a very small portion of the Sun’s total energy.
• On an average, the Earth receives 1.94 calories per sq. cm per minute at the top of its atmosphere.
• The solar output received at the top of the atmosphere varies slightly during the year because of changes in the distance between the Earth and the Sun.
• On 4th July, the Earth is farthest from the Sun at about 152 million km, and this position is called Aphelion.
• On 3rd January, the Earth is nearest to the Sun at about 147 million km, and this position is called Perihelion.
• Although insolation is slightly higher during Perihelion (3rd January) than during Aphelion (4th July), this variation has little effect on daily weather because it is masked by other factors like the distribution of land and sea and atmospheric circulation.

2. Variability of Insolation at the Surface of the Earth

• The amount and intensity of insolation vary during a day, season and year, which leads to spatial and temporal differences in heating over the Earth’s surface.
• The main factors causing variation in insolation are: rotation of the Earth on its axis, angle of inclination of the Sun’s rays, length of the day, transparency of the atmosphere, and configuration (aspect) of land, though the last two have comparatively less influence.
• The Earth’s axis is inclined at 66½° to the plane of its orbit, and this inclination greatly influences the amount of insolation received at different latitudes.
• The angle of inclination of the Sun’s rays depends on latitude; at higher latitudes the rays are more slanting, cover a larger area and pass through a greater thickness of the atmosphere, resulting in more absorption, scattering and diffusion, and hence lower energy per unit area.
• The atmosphere is largely transparent to shortwave solar radiation, but within the troposphere, water vapour, ozone and other gases absorb much of the near-infrared radiation.
• Very small suspended particles in the troposphere scatter the visible spectrum both towards space and towards the Earth’s surface, causing the blue colour of the sky and the red colour of sunrise and sunset.
• The insolation received at the Earth’s surface varies from about 320 Watt/m² in the tropics to about 70 Watt/m² in the polar regions.
• Maximum insolation is received over subtropical deserts due to minimum cloudiness; the equator receives comparatively less insolation than the tropics, and at the same latitude insolation is generally more over continents than over oceans, while middle and higher latitudes receive less radiation in winter than in summer.

3. Heating and cooling of atmosphere

• The atmosphere is heated indirectly because the Earth first absorbs insolation and then radiates heat in the form of long wave radiation, which warms the air from below.
• Conduction is the process by which heat is transferred between two bodies in direct contact; the air in contact with the heated land surface gets warmed and then heats the upper layers, making conduction important in warming the lower layers of the atmosphere.
• Conduction continues until both bodies attain the same temperature or until contact is broken, but it is effective only in a very limited vertical extent near the Earth’s surface.
• Convection is the vertical transfer of heat, in which heated air rises in the form of currents and transmits heat upward; this process is confined mainly to the troposphere.
• Advection refers to the horizontal transfer of heat through the movement of air, and it plays a more significant role than vertical motion in influencing daily weather changes.
• In the middle latitudes, most of the diurnal (day–night) variations in weather are caused by advection, and in the tropical regions, especially in northern India during summer, the hot local wind called ‘Loo’ is an example of advection.

3.1 Terrestrial Radiation

• The insolation received by the Earth is in short wave form, which heats the Earth’s surface, and after being heated, the Earth itself becomes a radiating body.
• The Earth radiates energy back to the atmosphere in the form of long wave radiation, and this outgoing energy is known as terrestrial radiation.
• The atmosphere is mainly heated from below because it absorbs this long wave terrestrial radiation, rather than being heated directly by solar radiation.
• The long wave radiation is absorbed by atmospheric gases, particularly carbon dioxide (CO₂) and other greenhouse gases, which helps in warming the atmosphere.
• After absorbing heat, the atmosphere also radiates and transmits heat into space, thereby playing a role in maintaining the overall thermal balance of the Earth-atmosphere system.

3.2 Heat Budget of the Planet Earth

• The Earth as a whole neither accumulates nor loses heat, because the amount of heat received in the form of insolation is equal to the amount lost through terrestrial radiation, maintaining a constant average temperature.
• If the insolation received at the top of the atmosphere is considered as 100 units, about 35 units are reflected back to space before reaching the Earth’s surface.
• Out of these 35 units, 27 units are reflected from the top of clouds and 2 units from snow and ice-covered areas, and this reflected radiation is called the albedo of the Earth.
• The remaining 65 units are absorbed, out of which 14 units are absorbed by the atmosphere and 51 units by the Earth’s surface.
• The Earth radiates back 51 units as terrestrial radiation; of these, 17 units escape directly to space, while 34 units are absorbed by the atmosphere (6 units directly, 9 units through convection and turbulence, and 19 units through latent heat of condensation).
• The atmosphere, having absorbed 48 units (14 from insolation + 34 from terrestrial radiation), radiates these 48 units back into space, so that the total outgoing radiation becomes 17 + 48 = 65 units, balancing the 65 units received.
• This balance between incoming and outgoing radiation is called the heat budget or heat balance of the Earth, explaining why the Earth does not continuously heat up or cool down.

3.3 Temperature

• Temperature is the measurement in degrees of how hot or cold a place or object is, while heat refers to the molecular movement of particles within a substance.
• Temperature results from the interaction of insolation with the atmosphere and the Earth’s surface, which creates heat that is measured in degrees Celsius (°C).
• The temperature of air at any place is influenced by several factors: latitude, altitude, distance from the sea, air-mass circulation, presence of warm and cold ocean currents, and local aspects.
• Latitude controls temperature because insolation varies with latitude; places near the equator receive more direct rays and are generally warmer than higher latitudes.
• Altitude affects temperature since the atmosphere is heated from below by terrestrial radiation; temperature generally decreases with height at the normal lapse rate of 6.5°C per 1,000 m.
• Distance from the sea influences temperature as land heats and cools faster than water; coastal areas experience moderated temperatures due to land and sea breezes, while continental interiors show greater variation.
• The movement of air-masses and the presence of warm and cold ocean currents also modify temperature; warm currents raise coastal temperatures, while cold currents lower them.
• The global distribution of temperature is shown by isotherms, which are lines joining places having equal temperature, and the distribution is best understood by comparing January and July temperature maps.

3.4 Distribution of Temperature

• The global distribution of temperature is best understood by studying the January and July temperature maps, where temperature is shown by Isotherms, which are lines joining places having equal temperature.
• In general, the effect of latitude is clearly visible as isotherms run roughly parallel to latitudes, but this pattern is more disturbed in January, especially in the Northern Hemisphere, due to greater landmass.
• In January, isotherms bend northward over oceans and southward over continents; over the North Atlantic Ocean, the warm currents like the Gulf Stream and North Atlantic Drift cause higher temperatures and northward bending of isotherms.
• In contrast, over the continental interiors like the Siberian Plain, temperatures fall sharply; along 60°E longitude, the mean January temperature is about –20°C at both 80°N and 50°N, showing strong continental influence.
• In January, mean monthly temperatures are over 27°C in equatorial oceans, over 24°C in the tropics, about 2°C to 0°C in middle latitudes, and between –18°C to –48°C in the Eurasian continental interior.
• In the Southern Hemisphere, isotherms are more parallel to latitudes and temperature variation is more gradual due to the dominance of oceans; the 20°C, 10°C and 0°C isotherms run roughly along 35°S, 45°S and 60°S latitudes respectively.
• In July, isotherms generally run parallel to latitudes; equatorial oceans record temperatures above 27°C, and over land more than 30°C is recorded along 30°N latitude in the subtropical continental region of Asia.
• The annual range of temperature is highest (more than 60°C) over the north-eastern part of the Eurasian continent due to continentality, while the least range (about 3°C) is found between 20°S and 15°N.

4. Inversion of temperature

• Normally, temperature decreases with height (normal lapse rate), but when temperature increases with elevation, it is called Inversion of Temperature.
• It usually occurs during long winter nights with clear skies and still air, when the Earth’s surface loses heat rapidly and becomes cooler than the air above.
• In polar regions, inversion is common throughout the year.
• Surface inversion creates atmospheric stability, leading to accumulation of smoke and dust particles below the inversion layer and formation of dense winter fogs.
• In hilly and mountainous areas, inversion occurs due to air drainage, where cold, dense air flows downslope and collects in valleys, with warmer air above.
• This process protects plants from frost damage, as cold air settles in lower areas rather than on upper slopes.

NCERT Class 11 Physical Geography Chapter 9 provides a scientific understanding of how solar energy maintains the Earth’s heat balance. Mastering NCERT Class 11 Physical Geography Chapter 9 helps students understand temperature differences between equator and poles and the concept of global warming.

A detailed study of NCERT Class 11 Physical Geography Chapter 9 strengthens preparation for atmospheric circulation, monsoon mechanism and climate change topics.

Continue reading NCERT Class 11 Physical Geography Chapter 10 – Atmospheric Circulation and Weather Systems to understand wind systems and global pressure belts in a structured and exam-oriented manner.

Frequently Asked Questions (FAQs)

Q1. What is NCERT Class 11 Physical Geography Chapter 9 about?
NCERT Class 11 Physical Geography Chapter 9 explains solar radiation, heat balance and temperature distribution on Earth.

Q2. Why is NCERT Class 11 Physical Geography Chapter 9 important for exams?
NCERT Class 11 Physical Geography Chapter 9 is important because heat budget and temperature variation are frequently asked in CBSE and UPSC examinations.

Q3. What is insolation in NCERT Class 11 Physical Geography Chapter 9?
In NCERT Class 11 Physical Geography Chapter 9, insolation means incoming solar radiation received by the Earth.

Q4. How does NCERT Class 11 Physical Geography Chapter 9 help in UPSC preparation?
NCERT Class 11 Physical Geography Chapter 9 strengthens conceptual clarity about heat balance and climate processes, which are important for Geography and Environment sections.

Q5. Is NCERT Class 11 Physical Geography Chapter 9 linked with later chapters?
Yes, NCERT Class 11 Physical Geography Chapter 9 forms the base for understanding atmospheric circulation, weather systems and monsoon discussed in later chapters.

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