Chapter 8 — Solar Radiation, Heat Balance and Temperature

Chapter 8 of the Class 11 Geography NCERT textbook (Fundamentals of Physical Geography), "Solar Radiation, Heat Balance and Temperature", explains how the earth receives energy from the sun as insolation, how the atmosphere is heated through conduction, convection, advection and terrestrial radiation, and how the earth maintains a heat budget by returning exactly as much energy to space as it receives.

• Insolation and its variation — The earth's incoming short-wave solar energy, or insolation, is unevenly received because latitude, the angle of the sun's rays, day length, and atmospheric transparency all change how much reaches the surface.
• How the atmosphere is heated — The atmosphere warms not just directly but through conduction from heated ground, convection currents, horizontal advection, and the long-wave terrestrial radiation the earth emits after absorbing sunlight.
• The earth's heat budget — Incoming and outgoing energy stay in balance — the planet returns to space exactly the amount of heat it absorbs, which keeps its overall temperature stable despite constant energy exchange.
• Controls on temperature and inversion — Latitude, altitude, distance from the sea, and ocean currents govern how temperature is distributed. Under clear, still winter nights the normal lapse rate can reverse, producing a temperature inversion.

1. 01Insolation is the incoming solar radiation received by the earth in short wavelengths; the earth receives 1.94 calories per sq. cm per minute at the top of the atmosphere.
2. 02Earth is at aphelion (farthest, 152 million km) on 4th July and at perihelion (nearest, 147 million km) on 3rd January.
3. 03Factors affecting insolation received: rotation of the earth, angle of inclination of sun's rays, length of day, transparency of the atmosphere, and configuration of land.
4. 04The atmosphere is heated by conduction (contact with heated land), convection (vertical air currents in troposphere), advection (horizontal air movement), and terrestrial radiation (long-wave emission from the earth's surface).
5. 05Heat budget: of 100 units of insolation, 35 are reflected back to space (albedo), 51 units are absorbed by the earth's surface, and the remaining 14 by the atmosphere; the earth radiates 51 units back, of which 17 go directly to space and 34 are absorbed by the atmosphere; the atmosphere in turn radiates 48 units to space — total returned = 65 units, balancing the 65 units absorbed.
6. 06Surplus radiation balance exists between 40° N and 40° S; regions near the poles have a deficit; surplus heat is redistributed polewards by atmospheric and oceanic circulation.
7. 07Normal lapse rate is 6.5°C per 1,000 m; temperature generally decreases with increasing altitude.
8. 08Temperature inversion (the reversal of the normal lapse rate) is common on long winter nights with clear skies and still air; air drainage in hills causes cold air to collect in valley bottoms, protecting plants from frost.