Daily Static Quiz Prelims Practice 2027
Earth's heat budget is the balance between incoming shortwave solar radiation (insolation) and outgoing longwave terrestrial radiation; over time and globally, the energy received equals the energy lost, keeping Earth's average temperature at roughly 15°C. Option (a) is wrong since Earth does not absorb all incoming radiation — about 30% is reflected back to space through the albedo effect of clouds, ice, and land. Option (b) is wrong because only around 30%, not all, of incoming radiation is reflected, with the remaining ~70% absorbed by the atmosphere and surface. Option (d) is wrong as the heat budget is a global atmospheric concept, not one confined to the oceans alone.
- Of the total incoming solar radiation (taken as 100 units), approximately 35 units are reflected back to space by clouds, dust, and the Earth's surface — this is called Earth's albedo.
- The atmosphere directly absorbs about 14 units of incoming solar radiation, mainly through water vapour, dust, and ozone.
- The Earth's surface absorbs approximately 51 units of incoming solar radiation.
- Longwave radiation emitted by the Earth's surface is entirely lost to outer space without being absorbed by the atmosphere.
Statement 1 is correct — Earth's average albedo is around 30–35%, with clouds reflecting about 27 units, dust and gases about 6 units, and the surface about 2 units, totalling roughly 35 units reflected to space. Statement 2 is correct, as the atmosphere directly absorbs about 14 units of incoming shortwave radiation through water vapour, dust, and the ozone layer's absorption of UV rays. Statement 3 is correct since the Earth's surface absorbs about 51 units of incoming insolation, the largest single share of the heat budget. Statement 4 is incorrect because longwave radiation emitted by the surface is largely absorbed by atmospheric gases, especially water vapour and CO₂, and re-radiated — the greenhouse effect — with only a small fraction escaping directly to space.
Terrestrial radiation refers to the longwave infrared radiation emitted by the Earth's surface after it has been heated by absorbed solar radiation, and this outgoing radiation is the primary way Earth loses heat to balance incoming insolation. Option (a) instead describes insolation — the incoming shortwave solar radiation that reaches the surface. Option (c) describes reflected radiation or albedo, a separate component of the heat budget, while option (d) describes the absorption of UV rays by ozone, which relates to stratospheric heating rather than terrestrial radiation.
Reason (R): The angle of incidence of solar radiation is more oblique at higher latitudes, causing solar energy to be spread over a larger surface area and to pass through a thicker layer of atmosphere, reducing intensity.
Assertion A is correct — the Equatorial zone experiences a perpetual heat surplus, receiving more energy than it loses, while polar regions run a heat deficit, and this imbalance drives global atmospheric and oceanic circulation. Reason R is also correct and directly explains A: at higher latitudes, solar rays strike the surface at a low, oblique angle, spreading the same energy over a larger area and reducing intensity per unit area, while also passing through a greater thickness of atmosphere, increasing absorption, scattering, and reflection along the way. Since R precisely accounts for the mechanism behind A, option (a) is the correct choice.
The standard heat budget breakdown gives reflection to space (albedo) at about 35 units, atmospheric absorption at about 14 units, and surface absorption at about 51 units, together totalling 100 units. The 51 units absorbed by the surface act as the primary driver of terrestrial radiation and the water cycle. Options (a), (c), and (d) each misassign these figures — (a) swaps surface absorption and albedo, (c) greatly overstates atmospheric absorption, and (d) wrongly removes atmospheric absorption altogether.
