Solar Radiation, Heat Balance and Temperature 11 Geography Chapter 8 CBSE Notes 2025-26

Air surrounds us everywhere, forming the Earth’s atmosphere, a vital blanket of various gases that support all life. The sun is the prime source of all energy our planet receives, and this energy is distributed unevenly in different regions, leading to temperature differences and causing atmospheric movements and weather patterns. Understanding how this solar energy is received, balanced, and spread is crucial to grasping the working of weather and climate on Earth.

Solar Radiation and Insolation ‘Insolation’ refers to the incoming solar radiation that the Earth receives in the form of short waves. On average, at the top of the atmosphere, about 1.94 calories per square centimeter per minute are received. The amount of solar energy that reaches different locations is not constant and can be affected by several factors. For example, the Earth's distance from the sun changes slightly during the year (farthest—aphelion—on 4th July, nearest—perihelion—on 3rd January), but this has a small effect on actual insolation compared to other controls.

Factors Affecting Insolation The intensity and amount of insolation received vary daily, seasonally, and yearly. The main factors that influence this variability include:

• Earth’s rotation on its axis
• The angle at which the sun’s rays strike the surface (related to latitude)
• The length of the day
• Atmospheric transparency
• The nature and aspect of the land (though these play a smaller role)

The higher the latitude, the more slanting the sun’s rays and the less energy is concentrated per unit area, as slant rays must also travel through more of the atmosphere, increasing absorption and scattering.

Transmission Through the Atmosphere The atmosphere is mostly transparent to short-wave solar radiation. As sunlight passes through, some energy is absorbed by gases like ozone and water vapor. Dust and small particles in the air scatter light, producing the blue color of the sky during the day and red at sunrise or sunset.

Spatial Variation of Insolation Insolation at the Earth’s surface is highest—about 320 Watt/m²—in the tropics and lowest—around 70 Watt/m²—at the poles. Subtropical deserts receive maximum insolation due to clear skies, while the equator actually gets less due to greater cloud cover. Continents usually record more insolation than oceans at the same latitude.

Heating and Cooling Mechanisms Heat from the sun is first absorbed by the ground. This heat moves up into the air by:

• Conduction: Direct heat transfer when molecules in contact warm up, important near the surface.
• Convection: Vertical movement as warm air rises in currents, helping heat reach upper layers.
• Advection: Horizontal movement, where wind carries heat sideways. For example, the hot loo winds in India come due to advection.

The Earth itself radiates heat back into the air in the form of long waves, a process called terrestrial radiation. Important greenhouse gases like carbon dioxide absorb some of this heat and slow its escape to space, keeping the lower atmosphere warmer.

Earth’s Heat Budget The planet maintains a balance between the solar energy it receives and the terrestrial energy it sends back to space. If the insolation coming in equals the outgoing radiation, Earth’s temperature stays roughly constant. Suppose 100 units of energy arrive at the top of the atmosphere:

• 35 units are reflected back—mainly by clouds (27 units) and ice/snow surfaces (2 units). This reflectivity is called Earth’s albedo.
• 14 units are absorbed by the atmosphere; 51 units reach and are absorbed by the Earth’s surface.
• The Earth emits 51 units as heat; some go directly to space, and some are reabsorbed by the atmosphere and then returned to space. Ultimately, 65 units are balanced out—that is, returned by the atmosphere and Earth together, matching the 65 units absorbed.

In tropical zones (about 40° north and south of the equator), a surplus of net radiation is observed. Near the poles, there’s a deficit. Heat is constantly redistributed by winds and ocean currents, preventing the tropics from growing ever hotter or the poles from perpetually freezing.

Measurement and Factors Affecting Temperature Heat that interacts with the air and ground is measured as temperature. A place’s temperature depends mainly on:

• Latitude (how far from the equator, affecting insolation received)
• Altitude (height above sea level; temperature drops about 6.5° C per 1,000 m, known as the normal lapse rate)
• Distance from the sea (sea moderates temperature as it heats/cools slowly compared to land)
• Air-mass movements and ocean currents (warm/cold masses modify local temperatures)
• Local physical features, such as valleys or mountain slopes

Distribution of Temperature and Isotherms Temperature patterns across the world are mapped using lines called isotherms, which join places having the same temperature. The general pattern shows isotherms running parallel to latitude, but deviations occur due to land-ocean distribution, elevation, and currents. In January, isotherms shift northward over oceans (warmer due to currents like the Gulf Stream) and southward over continents (cooler, especially over Siberia). The southern hemisphere shows more regular isotherms, since there is more ocean.

In July, the northern hemisphere’s landmasses, especially interior Asia, are much warmer—more than 30°C along 30°N. The highest temperature ranges (over 60°C between the coldest and warmest months) are found in the north-eastern parts of Eurasia, due to how quickly land heats and cools. Tropical regions see the lowest temperature variation, sometimes as little as 3°C.

Inversion of Temperature (Temperature Inversion) Normally, temperature falls as elevation increases. That’s the ‘normal lapse rate.’ Sometimes, the air near the ground gets cooler than the air above it—this is temperature inversion. This can happen on long, clear winter nights when Earth loses heat rapidly. Polar areas often experience inversion, and in valleys or hills, cold air flows down slopes to collect in low areas, trapping warm air above. This air drainage helps protect plants from frost. Inversions typically only last for a few hours but can cause fogs and trap pollution near the ground.

Key Terms

• Plank’s law: Hotter objects emit more energy at shorter wavelengths.
• Specific heat: The energy needed to raise the temperature of one gram of a substance by one degree Celsius.

Sample Climatological Data and Calculations A meteorological observatory, such as New Delhi (Safdarjung) at 28°35' N and 216 m elevation, records maximum daily mean temperature in May (39.6°C), minimum in January (7.3°C), and calculates the daily mean temperature by averaging these. The annual range of temperature is the difference between mean temperatures in May (32.75°C) and January (14.2°C): 18.55°C.

Practice and Exercises Typical questions range from multiple choice and matching terms (like insolation, albedo, isotherms, annual range), to short and long-answer questions on the causes and effects of uneven heat distribution, temperature variation, and the influence of local and global factors.

Project work can involve analyzing observatory data to better understand real-life patterns of temperature change, the influence of geography, and how to calculate annual and monthly means, giving practical context to theoretical understanding.

Class 11 Geography Chapter 8 Notes – Solar Radiation, Heat Balance and Temperature: Revision Essentials

Make your Geography preparation easier with these Class 11 Chapter 8 Solar Radiation, Heat Balance and Temperature notes. All critical points, data, and concepts are explained clearly—ideal for understanding how the sun’s energy affects weather and climate. These revision notes simplify complex ideas for last-minute revision.

Use these concise notes to strengthen your grasp on topics such as insolation, heat transfer, temperature distribution, and inversion. Suitable for CBSE board exam prep, each section summarises only what you need—making revision fast, efficient, and less stressful for students.