Solar Radiation, Heat Balance and Temperature

Why is the equator hotter than the poles? Why is summer in the Northern Hemisphere warmer than winter even though Earth is actually slightly farther from the Sun in June? How does the Earth maintain a stable temperature despite continuously receiving solar radiation? This chapter answers these questions through the concepts of insolation, heat balance, albedo, and temperature distribution — the quantitative framework underlying all of climatology.

UPSC Prelims tests specific facts: factors affecting insolation, albedo values, the concept of heat budget, and temperature inversion. Mains questions on climate, agriculture, and disaster management often require explaining the spatial distribution of temperature and its causes.

PART 1 — Quick Reference Tables

Table 1: Factors Affecting Insolation (Incoming Solar Radiation)

Table 2: Earth's Heat Budget (Energy Balance)

Table 3: Albedo Values of Different Surfaces

(Albedo = proportion of incoming radiation reflected back; higher albedo = cooler surface)

Table 4: Factors Controlling Temperature Distribution

Table 5: Temperature Inversion — Types

PART 2 — Detailed Notes

Solar Constant and Insolation

The solar constant is the amount of solar energy received per unit area per unit time at the top of the atmosphere on a surface perpendicular to the Sun's rays. Its value is approximately 1,361 W/m² (watts per square metre).

Insolation (INcoming SOLar radiATION) is the actual solar energy received at the Earth's surface. It is less than the solar constant because of:

• Reflection by clouds (~30% of total incoming radiation is reflected back)
• Scattering and absorption by atmospheric gases and aerosols

The Sun radiates shortwave radiation (visible light and UV). The Earth absorbs this and re-radiates it as longwave radiation (infrared/heat).

The Earth's Heat Budget

Earth's temperature remains relatively stable over time — neither continuously heating nor cooling. This is because the Earth's system is in heat balance: the amount of energy received equals the amount re-radiated to space.

If we take 100 units of incoming solar radiation:

• ~30 units reflected back to space (by clouds, surface, atmosphere) — albedo effect
• ~70 units absorbed by Earth and atmosphere
• These 70 units are eventually re-radiated as longwave radiation back to space

Without the greenhouse effect, all 70 units would escape, and the planet would be ~33°C colder. The greenhouse gases intercept some outgoing longwave radiation, re-emitting it downward and warming the lower atmosphere.

Albedo and Its Significance

Albedo (Latin for "whiteness") is the fraction of incoming solar radiation reflected by a surface. High albedo = high reflectivity = less warming.

Key implications:

• Ice–albedo feedback: Ice has high albedo (~80–90%). When ice melts due to warming, it reveals dark ocean or soil (low albedo ~5–15%), which absorbs more heat → further warming → more melting. This positive feedback amplifies warming in polar regions.
• Clouds: Cloud albedo moderates surface temperature. More clouds → more reflection → cooler surface. But clouds also trap outgoing radiation (warming effect). Net effect depends on cloud type and altitude.
• Urban heat island: Cities replace vegetation and soil (moderate albedo) with concrete, asphalt, and buildings (low albedo, high heat absorption) → urban areas are warmer than rural surroundings by 1–3°C.
• Forest vs desert: Tropical forests have low albedo but also high evapotranspiration (cooling). Deserts have higher albedo but no evapotranspiration — complex net effect.

Factors Controlling Temperature Distribution

Latitude: The primary control. Moving from equator to poles, the sun's rays strike at an increasingly oblique angle, spreading the same amount of energy over a larger area. Also, the atmosphere's path length is greater at higher latitudes, allowing more scattering and absorption.

Altitude: For every 1,000 m gain in altitude, temperature drops by ~6.5°C (Normal Lapse Rate). This is why mountain stations (Shimla, Ooty, Nainital) are cooler than nearby plains. High-altitude plateaus like Tibet experience extreme cold despite low latitude.

Land and water (continentality): Water heats up and cools down slowly (high specific heat capacity); land heats rapidly and cools rapidly. Coastal areas have moderate temperature (small annual range); continental interiors have extreme temperature (large annual range). Mawsynram (Meghalaya — coastal influence) vs Rajasthan interior.

Ocean currents: Warm currents (Gulf Stream, North Atlantic Drift) warm adjacent coasts; cold currents (Labrador, Benguela) cool adjacent coasts. The UK (~50°N) has much milder winters than Labrador (~50°N) because of the warm North Atlantic Drift.

💡 Explainer: Temperature Inversion

Normally, temperature decreases with altitude in the troposphere — warm air is below, cold air above. Temperature inversion reverses this: a layer of warm air overlies cooler air near the surface.

Radiation (surface) inversion: On calm, clear nights, the ground loses heat rapidly by longwave radiation (no clouds to trap it). The air immediately above the surface cools fastest, creating a layer of cold air near the surface beneath warmer air above. This:

• Prevents convection (cold air is stable, denser)
• Traps pollutants, smoke, and fog near the surface
• Explains Delhi's severe winter pollution — cold air traps vehicle and industry emissions

In valleys, cold air (denser, heavier) drains down from surrounding slopes and pools in the valley floor — valley inversion. Frost damage to crops is common in such locations.

Upper air (subsidence) inversion: In subtropical high-pressure cells, air descends and warms adiabatically (compression warming). This creates a permanent inversion layer that suppresses convection, preventing cloud formation and rainfall — a key factor in desert formation at 20–30°N and S latitudes (Sahara, Arabian Desert, Thar, Namib, Atacama).

🎯 UPSC Connect: Temperature Anomaly and Isotherms

Isotherms are lines connecting points of equal temperature on a map. They show the distribution of temperature globally.

Key patterns:

• Isotherms generally run east–west (parallel to latitude) but are deflected where ocean currents, mountains, and land–sea contrasts intervene
• January isotherms: Northern continents are much colder than the adjacent oceans at the same latitude; isotherms bend sharply northward over oceans (warm currents) and southward over continents (continental cooling)
• The temperature difference between land and ocean at the same latitude is the temperature anomaly

India's temperature context:

• Highest temperatures (summer): Rajasthan, south Punjab, Sindh — 45–50°C
• Winter inversions: Indo-Gangetic Plain — cold dense air gets trapped; fog disrupts transportation
• Temperature in mountains: Leh (3,500 m) — winter minimum –30°C, summer maximum 30°C (extreme range due to altitude + continentality)

PART 3 — Frameworks & Analysis

Heat Balance at Different Latitudes

Without this meridional heat transfer by winds and ocean currents, the tropics would be unbearably hot and the poles impossibly cold.

Temperature Controls: Summary Comparison

Exam Strategy

Prelims Traps:

• Earth is closest to the Sun in January (perihelion), not July — but Northern Hemisphere summer is in June–July because it is tilted toward the Sun (axial tilt effect overwhelms distance effect).
• Higher albedo = cooler (reflects more energy). Snow > desert > forest > ocean.
• Temperature inversion inverts the normal lapse rate — warm air above, cold air below. It TRAPS pollutants near the surface.
• The heat budget gives the Earth–atmosphere system equilibrium; the surface receives more than it emits (greenhouse gases warm the surface more than the atmosphere alone).
• Normal lapse rate: ~6.5°C per 1,000 m. Environment lapse rate varies; when it is less than 6.5°C/1,000 m, the atmosphere is stable.

Mains Frameworks:

• Delhi winter pollution: temperature inversion + lack of wind + emissions → smog. Solution: reduce emissions, improve wind ventilation.
• Climate change: altered heat budget due to enhanced greenhouse gases → rising surface temperatures → ice–albedo feedback → amplified polar warming.
• Agriculture: temperature and insolation distribution explains crop zoning (wheat in north, rice in wet tropics, coffee in Western Ghats highlands).

Previous Year Questions

1. UPSC Prelims 2020: What is temperature inversion? In which season does it most commonly affect the Indo-Gangetic plain? (Winter — radiation inversion)
2. UPSC Prelims 2016: If the Earth's axis of rotation were to become vertical (no tilt), what would be the consequence? (No seasons, no variation in day length)
3. UPSC Mains GS1 2015: Explain the mechanism and factors responsible for the unequal distribution of temperature on the Earth's surface.
4. UPSC Mains GS3 2019: Global surface temperature has been rising consistently since the industrial revolution. Analyse the causes and consequences of this trend.