The earth receives 100 units of solar energy and sends back exactly 65 from the surface and 48 from the atmosphere. That balance is why the planet never runs away hot or cold.
This article covers insolation, how the atmosphere heats and cools, the heat budget, and what controls temperature.
🏛️ Must Know
Insolation: What Controls It
- The BasicsIncoming solar radiation, called insolation, averages 1.94 calories per sq. cm per minute at the top of the atmosphere.(NCERT Class 11)
- Five VariablesInsolation varies with the earth’s rotation, the angle of the sun’s rays, day length, atmospheric transparency, and land configuration.(NCERT Class 11)
- The Axial TiltThe earth’s axis tilts 66.5 degrees from its orbital plane, strongly shaping how much insolation different latitudes receive.(NCERT Class 11)
Aphelion and Perihelion
- AphelionOn 4 July, the earth sits farthest from the sun, at 152 million km.(NCERT Class 11)
- PerihelionOn 3 January, the earth is nearest the sun, at 147 million km, so it receives slightly more insolation than on 4 July.(NCERT Class 11)
- A Masked EffectLand-sea distribution and atmospheric circulation mask this small variation, so it barely affects daily weather.(NCERT Class 11)
Conduction, Convection and Advection
- ConductionHeat flows from a warmer body to a cooler one in direct contact, warming the atmosphere’s lowest layers.(NCERT Class 11)
- ConvectionHeated air rises vertically as currents, but this transfer is confined only to the troposphere.(NCERT Class 11)
- AdvectionHorizontal air movement drives most day-to-day weather change in the middle latitudes. India’s hot summer “loo” wind is a real example.(NCERT Class 11)
The Heat Budget: 100 Units In, 65 Out
- AlbedoOf 100 units of insolation, 35 are reflected straight back to space before ever reaching the surface. This reflected share is called the earth’s albedo.(NCERT Class 11)
- AbsorptionThe remaining 65 units are absorbed: 14 within the atmosphere and 51 by the earth’s surface itself.(NCERT Class 11)
- Balancing the BooksThe earth radiates its 51 units back (17 straight to space, 34 absorbed by the atmosphere). Combined with the atmosphere’s own 14 units, a total of 48 units return to space from the atmosphere.(NCERT Class 11)
- Why It Matters17 units from earth plus 48 from the atmosphere equals the original 65 absorbed, which is why the earth neither steadily warms nor cools.(NCERT Class 11)
📘 Good to Know
Five Factors Controlling Temperature
- Latitude and AltitudeTemperature depends on insolation received by latitude, and decreases with height at the normal lapse rate of 6.5°C per 1,000 m.(NCERT Class 11)
- Distance from the SeaBecause the sea heats and cools more slowly than land, coastal places see less temperature variation than inland places.(NCERT Class 11)
- Air Masses and CurrentsWarm air masses and warm ocean currents raise coastal temperatures; cold ones lower them.(NCERT Class 11)
Isotherms and Seasonal Patterns
- What They ShowIsotherms are lines joining places of equal temperature, generally running parallel to latitude.(NCERT Class 11)
- January’s Bigger DeviationIsotherms bend far more in January than July, especially in the Northern Hemisphere, where a much larger landmass sharpens land-ocean temperature contrasts.(NCERT Class 11)
- Gulf Stream’s EffectWarm currents like the Gulf Stream bend isotherms northward over the ocean, while temperatures drop faster over nearby land.(NCERT Class 11)
🌟 Great to Know
Temperature Inversion
- The ReversalTemperature inversion reverses the normal lapse rate, usually briefly, most often on clear winter nights with still air.(NCERT Class 11)
- Polar RegionsOver the poles, temperature inversion is normal all year round, not just an occasional event.(NCERT Class 11)
- Air DrainageCold, dense air flows downhill at night into valley bottoms under gravity, a process called air drainage that protects hillside plants from frost.(NCERT Class 11)
Net Radiation Balance by Latitude
- Surplus and DeficitRegions between 40°N and 40°S run a radiation surplus, while polar regions run a deficit.(NCERT Class 11)
- Why the Tropics Don’t OverheatWinds redistribute this surplus heat poleward, which is why the tropics don’t keep heating up and the poles don’t freeze permanently solid.(NCERT Class 11)
Exam Point of View
Exam Wisdom: Lapse Rate and Isotherm Traps
- CAPF (ACs) 2022Asked which statement correctly describes the normal lapse rate. The answer confirms temperature is highest at ground level and decreases with height. See Q63 on the full CAPF (ACs) 2022 paper →
- NDA & NA (I) 2026Asked test-takers to evaluate statements about temperature inversion. See Q107 on the full NDA & NA (I) 2026 paper →
- CDS (I) 2022Asked which land surface has the highest albedo. See Q88 on the full CDS (I) 2022 paper →
- UPSC CDS (II) 2020Asked which isotherm line divides India roughly North-South in January. See Q9 on the full UPSC CDS (II) 2020 paper →
- NDA & NA (II) 2025Asked which factors cause variation in the amount of insolation the earth receives. See Q120 on the full NDA & NA (II) 2025 paper →
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