Solar Radiation, Heat Balance and Temperature
How the Earth receives solar energy (insolation), redistributes and radiates it back to maintain a constant temperature, and what governs the resulting global distribution of temperature.
A perennial Prelims favourite: expect direct questions on heat-budget units, aphelion/perihelion, the modes of atmospheric heating (conduction/convection/advection), normal lapse rate and temperature inversion. In GS-I Physical Geography it is the foundation for understanding pressure belts, winds, ocean currents and the monsoon, and it feeds climate-change linkages such as the greenhouse effect and albedo.
Understand the chapter
Insolation and Why It Varies
Insolation is the incoming short-wave solar radiation intercepted by the Earth; because the Earth is a geoid, the sun's rays strike the top of the atmosphere obliquely and the planet captures only a tiny fraction — on average 1.94 calories per sq cm per minute. The Earth–Sun distance changes through the year (aphelion vs perihelion), so insolation at the top of the atmosphere varies slightly, but this effect is masked by land–sea distribution and atmospheric circulation. The decisive control on how much insolation a place actually gets is the angle of the sun's rays, which depends on latitude.
- Aphelion (4 July, 152 mn km, farthest) vs Perihelion (3 Jan, 147 mn km, nearest) — insolation is slightly higher on 3 January.
- Five factors of variation: rotation, angle of inclination of rays, length of day, atmospheric transparency, land configuration (last two minor).
- Earth's axis is inclined 66½° to the plane of its orbit.
- High latitude → slant rays → energy spread over a larger area plus a longer atmospheric path → less net energy per unit area.
Passage Through the Atmosphere and Spatial Distribution
The atmosphere is largely transparent to incoming short-wave radiation, so most of it reaches the surface; within the troposphere, water vapour, ozone and other gases absorb much of the near-infrared. Tiny suspended particles scatter the visible spectrum, giving the sky its blue colour and the rising and setting sun their red colour. At the surface, insolation is unevenly distributed — and counter-intuitively the equator is not the hottest belt.
- Surface insolation ranges from ~320 W/m² in the tropics to ~70 W/m² at the poles.
- Maximum over subtropical deserts (least cloud cover); the equator gets less than the tropics due to heavy cloudiness.
- At the same latitude, insolation is greater over continents than over oceans.
- Blue sky and red sunrise/sunset both result from the scattering of light.
Heating and Cooling of the Atmosphere
The Earth, heated by insolation, warms the air through three mechanisms. Conduction transfers heat by contact from the warm surface to the air and is important for heating the lowest layers; convection carries heat upward in rising currents and is confined to the troposphere; advection moves heat horizontally and dominates mid-latitude day-and-night weather. India's hot summer wind 'loo' is a classic product of advection.
- Conduction: heat flows from a warmer to a cooler body in contact until equalised — heats the lower atmosphere.
- Convection: vertical transfer by rising air currents — limited to the troposphere.
- Advection: horizontal transfer — causes most mid-latitude diurnal weather; 'loo' is an example.
Terrestrial Radiation and the Heat Budget
The surface absorbs short-wave insolation and re-emits it as long-wave terrestrial radiation, which is absorbed mainly by carbon dioxide and other greenhouse gases — so the atmosphere is heated from below, not directly by the sun. Treating incoming insolation as 100 units, the Earth–atmosphere system reflects 35 units (the albedo) and absorbs 65; the absorbed energy is ultimately returned to space, so the planet neither warms up nor cools down. This balance is the heat budget.
- 35 units reflected = albedo (27 from cloud tops, 2 from snow/ice, rest scattered).
- 65 absorbed = 14 by the atmosphere + 51 by the surface; the surface radiates 51 back (17 to space, 34 to the atmosphere).
- Atmosphere returns 48 units (14 + 34) plus 17 direct = 65 → balances the 65 received from the sun.
- The atmosphere is mainly heated by long-wave terrestrial radiation.
Net Radiation Balance and Factors Controlling Temperature
Although the planet as a whole is balanced, individual latitudes are not: there is a net radiation surplus between 40°N and 40°S and a deficit towards the poles. Winds and ocean currents transfer this surplus poleward, preventing the tropics from overheating and the poles from freezing further. The actual temperature of any place is then fixed by several controls.
- Surplus 40°N–40°S; deficit near the poles → poleward heat redistribution.
- Controls (LADAL): Latitude, Altitude, Distance from sea, Air-mass & ocean currents, Local aspects.
- Normal lapse rate = 6.5°C per 1,000 m of ascent.
- Land heats and cools faster than the sea → larger temperature range inland (continentality).
Distribution of Temperature and Isotherms
Global temperature is mapped using isotherms — lines joining places of equal temperature. Latitude's control shows up as isotherms running broadly parallel to the latitudes, but they bend where land, sea and currents intervene, with deviations far stronger in January and in the land-heavy Northern Hemisphere. Warm currents like the Gulf Stream and North Atlantic Drift push isotherms poleward, while cold continental interiors like Siberia pull them equatorward.
- Isotherm = line joining places of equal temperature; generally parallel to the latitudes.
- January deviations are sharper (NH); isotherms bend north over warm oceans and south over cold continents.
- Highest annual range (>60°C) over NE Eurasia (continentality); least (3°C) between 20°S and 15°N.
- Southern Hemisphere isotherms are more parallel and gradual due to the dominance of oceans.
Inversion of Temperature
Normally temperature falls with height, but when this is reversed — warm air overlying cold — it is a temperature inversion, usually short-lived yet common. It forms on long, clear, calm winter nights when the ground radiates away the day's heat and becomes colder than the air above; over polar regions it persists all year. Surface inversion traps smoke, dust and moisture, producing dense winter morning fogs.
- Ideal conditions: long winter night, clear sky, still air; permanent over the poles.
- Surface inversion → stability → trapped pollutants and dense morning fog.
- Air drainage: dense cold air slides down slopes into valley bottoms with warm air above — protects plants from frost.
Key terms
- Insolation
- Incoming short-wave solar radiation received by the Earth (avg 1.94 cal/cm²/min at the top of the atmosphere).
- Aphelion
- Position when the Earth is farthest from the Sun — 4 July, 152 million km.
- Perihelion
- Position when the Earth is nearest to the Sun — 3 January, 147 million km.
- Albedo
- The fraction of insolation reflected back to space (about 35 of every 100 units).
- Conduction
- Heat transfer by direct contact from a warmer to a cooler body; heats the lowest air layers.
- Convection
- Vertical transfer of heat by rising air currents; confined to the troposphere.
- Advection
- Horizontal transfer of heat by moving air; e.g., India's summer 'loo'.
- Terrestrial radiation
- Long-wave energy re-emitted by the heated Earth that warms the atmosphere from below.
- Isotherm
- A line on a map joining places having equal temperature.
- Normal lapse rate
- The normal fall of temperature with height — 6.5°C per 1,000 m.
Must-know facts exam-ready
- Earth receives on average 1.94 calories per sq cm per minute at the top of the atmosphere.
- Aphelion = 4 July (152 million km, farthest); Perihelion = 3 January (147 million km, nearest).
- Earth's axis is inclined at 66½° to the plane of its orbit.
- Surface insolation ranges from ~320 W/m² in the tropics to ~70 W/m² at the poles.
- Maximum insolation is over subtropical deserts (least cloud); the equator receives less than the tropics.
- Heat budget (of 100 units): 35 reflected (albedo) + 65 absorbed (14 atmosphere + 51 surface); 17 + 48 = 65 returned to space.
- The atmosphere is heated mainly by long-wave terrestrial radiation, absorbed by CO₂ and other greenhouse gases.
- Convection is confined to the troposphere; advection (horizontal) produces India's summer 'loo'.
- Normal lapse rate = 6.5°C per 1,000 m.
- Net radiation surplus lies between 40°N and 40°S; deficit near the poles, balanced by poleward heat transfer.
- Highest annual temperature range (>60°C) is over NE Eurasia (continentality); least (3°C) between 20°S and 15°N.
- On 21 June the sun is overhead at noon at 23.5°N (Tropic of Cancer); over polar areas inversion is normal year-round.
Memory tricks remember it for good
Traps to avoid
- Earth is NEAREST the Sun on 3 January (perihelion) yet the NH has winter — seasons are caused by axial tilt, not Earth–Sun distance; don't swap aphelion (4 July) and perihelion.
- The equator is NOT the zone of maximum insolation — subtropical deserts are (least cloud); the equator's heavy cloud cover lowers its surface insolation.
- The atmosphere is heated mainly by long-wave TERRESTRIAL radiation, not directly by incoming short-wave solar radiation.
- Convection = vertical (troposphere only); Advection = horizontal ('loo'); Conduction = contact (lower layers) — do not interchange them.
- The 66½° figure is the axis's angle with the orbital PLANE; the 23½° tilt is measured from the vertical/perpendicular — different references.
- Normal lapse rate (6.5°C/1000 m, temperature falls with height) is the opposite of inversion (temperature rises with height); inversion is normal year-round only over the poles.
Exam focus
🧠 Prelims angles
- Heat-budget numbers: matching reflected/absorbed/returned units (35, 65, 14, 51, 17, 48).
- Aphelion/perihelion dates and Earth–Sun distances.
- Definition matching — Insolation (incoming radiation), Albedo (reflected percentage), Isotherm (equal-temperature line), Annual range (warmest–coldest month difference).
- Modes of atmospheric heating — conduction vs convection vs advection; 'loo' as advection.
- Normal lapse rate value (6.5°C/1000 m), conditions for temperature inversion, and air drainage.
- Solstice facts — sun overhead at 23.5°N on 21 June; subtropics hotter than equator (less cloud); longest day at the highest latitude.
✍️ Mains angles GS-I
- How do latitude and the tilt of the Earth's axis determine the insolation received at different latitudes?Link the angle of incidence to area spread plus atmospheric path length; connect to the 40°N–40°S surplus and poleward heat transfer.
- Explain the Earth's heat budget and why global temperature remains constant despite huge heat transfers.Trace 100 units → 35 reflected + 65 absorbed → terrestrial radiation → 65 returned; stress that incoming equals outgoing.
- Discuss the factors controlling the spatial distribution of temperature, with reference to continentality.Use LADAL; contrast January vs July isotherms and the >60°C Siberian range against the 3°C minimum near the equator.
Last-minute revision tick as you recall
- Insolation = incoming short-wave solar radiation; avg 1.94 cal/cm²/min at top of atmosphere.
- Aphelion 4 July (152 mn km, far); Perihelion 3 Jan (147 mn km, near) — tilt, not distance, makes seasons.
- Axis 66½° to orbital plane; angle of sun's rays (latitude) is the key insolation control.
- Atmosphere transparent to short-wave in; heated from below by long-wave terrestrial radiation (CO₂/GHGs).
- Conduction (contact, lower layers), Convection (vertical, troposphere), Advection (horizontal, 'loo').
- Heat budget: 35 reflected (albedo) + 65 absorbed; returns 17 + 48 = 65 → balance.
- Surplus 40°N–40°S, deficit at poles → winds and currents move heat poleward.
- Insolation ~320 W/m² tropics to ~70 W/m² poles; max over subtropical deserts, not the equator.
- Normal lapse rate 6.5°C/1000 m; inversion = reversed (clear, calm winter nights; permanent over poles).
Distilled from NCERT Class 11 · Fundamentals of Physical Geography for UPSC. Always cross-check facts with the original NCERT.