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Class 11 Solar Radiation, Heat Balance and Temperature: cheat sheet
CBSE Social Science. How the earth is heated by the sun, how the atmosphere is heated and cooled, the heat budget, the lapse rate and the spread of temperature.
Key ideas
- Insolation is incoming short-wave solar energy; it varies with the earth’s rotation, the angle of the sun’s rays, the length of the day, the clearness of the air and the aspect of the land.
- Slanting rays at high latitudes spread over a larger area and pass through more air, so each unit area gets less energy.
- The atmosphere is heated mostly from below: by conduction from the warm ground, convection upward, advection sideways and long-wave terrestrial radiation.
- The heat budget balances: the earth loses as much heat as it receives, so it neither warms nor cools over time.
- Between 40° N and 40° S there is a surplus of energy and towards the poles a deficit; winds and ocean currents carry heat poleward.
- Temperature depends on latitude, altitude, distance from the sea, air masses and ocean currents.
- An inversion (temperature rising with height) happens on long, clear, calm winter nights; cold air drains into valleys and fog forms.
Remember
| Normal lapse rate | 6.5 °C fall per 1,000 m (1.3 °C per 200 m) |
| Temperature at a height | surface temperature − height in m × 6.5 ÷ 1,000 |
| Height from a temperature gap | difference in °C ÷ 6.5 × 1,000 m |
| Annual range | warmest month − coldest month (careful with minus signs) |
| Heat budget (of 100 units) | 35 reflected; 65 absorbed (14 by the air, 51 by the surface); 17 + 48 = 65 radiated to space |
| Perihelion and aphelion | nearest to the sun on 3rd January (147 million km), farthest on 4th July (152 million km) |
| Most insolation | over the subtropical deserts (least cloud), not at the equator |
Watch out
- Getting a smaller range with a negative month: 25 − (−10) = 35, not 15.
- Using 6.5 °C per 100 m instead of per 1,000 m.
- Mixing up advection (sideways, like the loo) and convection (upward).
Worked example
A climber starts at sea level, where it is 30 °C, and reaches 6,400 m. Using 6.5 °C per 1,000 m: (a) What is the temperature at 2,000 m? (b) At the top? (c) Why does temperature fall with height?
Working: 30 − 2,000 × 6.5 ÷ 1,000; 30 − 6,400 × 6.5 ÷ 1,000
Answer: (a) 17 °C (b) −11.6 °C (c) The air is heated from below by terrestrial radiation and is thinner higher up.
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