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Looking for NCERT Class 11 Geography Fundamentals of Physical Geography Chapter 8 Solar Radiation, Heat Balance and Temperature Notes? You’re in the right place! This blog provides simple and clear notes to help you understand the key concepts of this chapter. Whether you’re preparing for exams or need a quick revision, these notes will help you understand the essential ideas without going through the entire textbook. Let’s dive in!
Table of Contents
- 1 Introduction
- 2 Solar Radiation
- 3 Variability of Insolation at the Earth’s Surface
- 4 Spatial Distribution of Insolation
- 5 Heating and Cooling of the Atmosphere
- 6 Heat Budget of the Planet Earth
- 7 Variation in the Net Heat Budget at the Earth’s Surface
- 8 Factors Controlling Temperature Distribution
- 9 Distribution of Temperature
- 10 Inversion of Temperature
- 11 Important Definitions in NCERT Notes Class 11 Geography Fundamentals of Physical Geography Chapter 8: Solar Radiation, Heat Balance and Temperature Notes
- 12 FAQs
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Introduction
The Earth is surrounded by an atmosphere composed of numerous gases that support life. The Earth receives almost all its energy from the Sun in the form of solar radiation, known as insolation. The Earth radiates back an equal amount of energy to space, maintaining a stable temperature over time. Variations in insolation cause pressure differences in the atmosphere, leading to heat transfer through winds. This chapter explains how the atmosphere is heated and cooled and how temperature is distributed across the Earth’s surface.
Solar Radiation
The Earth’s surface receives most of its energy as shortwave solar radiation, termed insolation. The Earth, being a geoid, intercepts only a small portion of the Sun’s energy, receiving an average of 1.94 calories per sq. centimetre per minute at the top of its atmosphere. The solar output varies slightly due to the Earth’s distance from the Sun, being closest (perihelion, 147 million km) on January 3rd and farthest (aphelion, 152 million km) on July 4th. This variation has minimal impact on daily weather due to factors like land-sea distribution and atmospheric circulation.
Variability of Insolation at the Earth’s Surface
The amount and intensity of insolation vary daily, seasonally, and annually due to:
- Earth’s Rotation: Affects daily insolation patterns.
- Angle of Inclination of Sun’s Rays: At higher latitudes, slanting rays cover more area, reducing energy per unit area and passing through more atmosphere, leading to greater absorption, scattering, and diffusion.
- Length of the Day: Longer days increase insolation.
- Transparency of the Atmosphere: Water vapour, ozone, and other gases absorb near-infrared radiation, while suspended particles scatter visible light, affecting sky colours (e.g., red at sunrise/sunset, blue sky).
- Land Configuration (Aspect): Influences local insolation but has less impact.
The Earth’s axis, tilted at 66½° to its orbital plane, significantly affects insolation distribution across latitudes.
Spatial Distribution of Insolation
Insolation varies from 320 Watt/m² in the tropics to 70 Watt/m² at the poles. Maximum insolation occurs over subtropical deserts with minimal cloudiness. The equator receives less insolation than the tropics. At the same latitude, continents receive more insolation than oceans. In winter, middle and higher latitudes receive less radiation than in summer.
Heating and Cooling of the Atmosphere
The atmosphere is heated and cooled through several processes:
- Conduction: Heat transfers from the warmed Earth’s surface to the lower atmospheric layers in contact with it. This occurs when bodies of unequal temperature are in contact until they reach the same temperature or contact is broken.
- Convection: Heated air rises vertically as currents, transferring heat to higher atmospheric layers. This is confined to the troposphere.
- Advection: Horizontal movement of air transfers heat. It significantly affects daily weather variations in middle latitudes and causes local winds like the ‘loo’ in northern India during summer.
- Terrestrial Radiation: The Earth, heated by insolation, radiates energy back in longwave form, heating the atmosphere from below. Greenhouse gases like carbon dioxide absorb this radiation, indirectly warming the atmosphere. The atmosphere then radiates heat back to space, maintaining a constant temperature.
Also Read:
- NCERT Class 6 Geography: Chapter 3 Motions of the Earth
- NCERT Class 6 Geography: Chapter 5 Major Domains of the Earth
Heat Budget of the Planet Earth
The Earth maintains a heat balance, neither gaining nor losing heat overall. Of the 100 units of insolation received at the top of the atmosphere:
- 35 units are reflected back to space (albedo), with 27 units from clouds and 2 units from snow/ice.
- 65 units are absorbed: 14 units by the atmosphere and 51 units by the Earth’s surface.
- The Earth radiates back 51 units as terrestrial radiation: 17 units go directly to space, and 34 units are absorbed by the atmosphere (6 units directly, 9 units via convection/turbulence, 19 units via latent heat of condensation).
- The atmosphere radiates 48 units (14 from insolation + 34 from terrestrial radiation) back to space.
- Total radiation returned (17 + 48 = 65 units) balances the 65 units received, maintaining the Earth’s temperature.
Variation in the Net Heat Budget at the Earth’s Surface
There is a surplus of net radiation between 40°N and 40°S, while the poles have a deficit. Surplus heat from the tropics is redistributed to the poles via atmospheric and oceanic circulation, preventing excessive heating in the tropics or freezing at the poles.
Factors Controlling Temperature Distribution
The temperature at any place is influenced by many factors. Some of the factors are mentioned below:
- Latitude: Higher insolation at lower latitudes results in higher temperatures.
- Altitude: Temperature decreases with height at a normal lapse rate of 6.5°C per 1,000 m, as the atmosphere is heated from below by terrestrial radiation.
- Distance from the Sea: Land heats and cools faster than the sea, leading to greater temperature variation over land. Coastal areas experience moderated temperatures due to sea and land breezes.
- Air-Mass and Ocean Currents: Warm air masses and ocean currents (e.g., Gulf Stream) increase temperatures, while cold air masses and currents lower them.
- Local Aspects: Topography affects local temperature variations.
Also Read:
- NCERT Class 6 Geography: Chapter 2 Latitudes and Longitudes
- NCERT Class 7 Geography Chapter 3 ‘Our Changing Earth’: Notes and Solutions (Free PDF)
Distribution of Temperature
Temperature distribution is shown using isotherms (lines joining places of equal temperature).
In January:
- Isotherms deviate north over oceans and south over continents in the northern hemisphere due to larger landmasses.
- Warm ocean currents (e.g., Gulf Stream) make the North Atlantic warmer, bending isotherms north.
- Over land (e.g., Siberian plain), temperatures drop sharply (e.g., -20°C at 60°E, 80°N, and 50°N).
- Equatorial oceans have temperatures above 27°C, tropics above 24°C, middle latitudes 2°C to 0°C, and Eurasian interiors -18°C to -48°C.
- In the southern hemisphere, isotherms are more parallel to latitudes, with gradual temperature variation (e.g., 20°C at 35°S, 10°C at 45°S, 0°C at 60°S).
In July:
- Isotherms are generally parallel to latitudes.
- Equatorial oceans exceed 27°C, subtropical continents (e.g., Asia at 30°N) exceed 30°C, and 40°N and 40°S have 10°C.
- The temperature range is highest (>60°C) in northeastern Eurasia due to continentality and lowest (3°C) between 20°S and 15°N.
Inversion of Temperature
Normally, temperature decreases with altitude (normal lapse rate). In a temperature inversion, this is reversed, with warmer air above cooler air. This occurs:
- During long winter nights with clear skies and still air, the Earth radiates heat, cooling the surface faster than the air above.
- Over polar areas, where inversions are common year-round.
- In hills and mountains, due to air drainage, where cold, dense air flows downhill, pooling in valleys with warmer air above, protecting plants from frost. Inversions promote atmospheric stability, trapping smoke and dust, and causing dense fog in winter mornings. They typically last until the Sun warms the Earth.
Important Definitions in NCERT Notes Class 11 Geography Fundamentals of Physical Geography Chapter 8: Solar Radiation, Heat Balance and Temperature Notes
Here we have explained the key concepts and terms of this chapter to make it easy for you to understand.
- Insolation: Incoming solar radiation received by the Earth’s surface in shortwave form.
- Aphelion: The Earth’s position farthest from the Sun (152 million km) on July 4th.
- Perihelion: The Earth’s position closest to the Sun (147 million km) on January 3rd.
- Albedo: The portion of solar radiation reflected back to space by clouds, snow, and ice.
- Terrestrial Radiation: Longwave radiation emitted by the Earth, heating the atmosphere from below.
- Heat Budget: The balance between insolation received and radiation returned to space, maintaining Earth’s temperature.
- Conduction: Heat transfer from the Earth’s surface to the lower atmosphere when in contact.
- Convection: Vertical transfer of heat through rising air currents in the troposphere.
- Advection: Horizontal transfer of heat through air movement, affecting daily weather.
- Isotherms: Lines on a map joining places with equal temperatures.
- Normal Lapse Rate: The rate at which temperature decreases with altitude (6.5°C per 1,000 m).
- Temperature Inversion: A reversal of the normal lapse rate, where temperature increases with height, often occurring during winter nights or in valleys.
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FAQs
Insolation is the incoming solar radiation received by the Earth’s surface. It is crucial as it provides the energy that heats the Earth, drives atmospheric circulation, and supports life.
The Earth maintains its heat balance by absorbing insolation (65 units) and radiating an equal amount back to space through terrestrial radiation (17 units directly, 48 units via the atmosphere), ensuring no net gain or loss of heat.
Temperature inversion occurs when temperature increases with height, often during long winter nights with clear skies and still air, or due to air drainage in hills and valleys, trapping cooler air below warmer air.
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