Insolation, Temperature & the Heat Budget
How the Earth receives and balances solar energy — insolation, the heat budget, factors affecting temperature distribution, and temperature inversion.
Key Takeaways
- Insolation is the incoming solar radiation received by the Earth.
- The Earth's heat budget stays balanced — incoming solar energy equals outgoing terrestrial radiation.
- Temperature distribution depends on latitude, altitude, distance from the sea, and ocean currents.
Core concept
Insolation (INcoming SOLar radiATION) is the energy the Earth receives from the Sun — the ultimate driver of weather, winds and life. Although the amount received and lost varies from place to place, the Earth as a whole neither heats up nor cools down over time, thanks to its balanced heat budget.
Static foundation — factors affecting insolation
- The angle of the Sun's rays (highest at the equator, slanting at the poles).
- The duration of daylight.
- The transparency of the atmosphere (clouds, dust).
- Altitude and aspect.
The heat budget
Of the insolation reaching the top of the atmosphere, a portion is reflected (albedo), some is absorbed by the atmosphere, and the rest reaches the surface. The Earth re-radiates this as long-wave terrestrial radiation. Because incoming = outgoing over the year, the planet's temperature stays in equilibrium — the greenhouse gases trap some outgoing heat and keep the Earth warm enough for life.
Factors Controlling Temperature Distribution
| Factor | Effect |
|---|---|
| Latitude | Temperature falls from the equator toward the poles (slanting rays, less insolation) |
| Altitude | Temperature falls with height (~6.5°C per km) |
| Distance from the sea | Coastal areas are milder (maritime); interiors have extremes (continentality) |
| Ocean currents | Warm currents raise, and cold currents lower, coastal temperatures |
| Prevailing winds | Onshore winds moderate temperature; offshore winds bring extremes |
Temperature inversion
Normally temperature falls with height. In a temperature inversion, a layer of warm air sits above cooler air near the surface — common on clear, calm winter nights and in valleys. It traps pollutants and fog (a cause of smog) and can cause frost that damages crops.
Relevance & current linkage
The heat budget underlies global warming — rising greenhouse gases trap more outgoing radiation, upsetting the balance. Temperature inversion explains winter smog episodes in cities like Delhi. (Add the latest data on warming or an air-quality event.)
Prelims trap zones
- Insolation is INCOMING solar radiation; terrestrial radiation is OUTGOING from the Earth.
- Albedo = reflectivity — snow/ice have high albedo, oceans/forests low.
- In a temperature inversion, temperature rises with height — the opposite of normal.
Prelims Pointers
- Insolation is greatest at the equator (near-vertical sun) and least at the poles (slanting rays).
- Albedo is the proportion of insolation reflected back to space.
- Lines joining places of equal temperature are called isotherms.
- Temperature inversion is when temperature increases with height (the reverse of normal).
Mains Angle
- 'The heat budget maintains the Earth's thermal equilibrium.' Explain.
- Discuss the factors that control the horizontal distribution of temperature.
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