Solar Radiation, Heat Balance and Temperature — Practice

Question 1

Out of 100 units of incoming solar radiation: 27 units are reflected by clouds, 8 units by Earth's surface, 14 units are absorbed by the atmosphere, and 51 units are absorbed by Earth's surface. (a) What is Earth's average albedo? (b) What happens to the 51 units absorbed by Earth's surface? (c) Why is Earth's average temperature stable (not continuously rising)?

Accepted Answer

(a) **Earth's albedo** = total reflected / total incoming × 100 = (27 + 8) / 100 × 100 = **35%**. Of every 100 units, 35 are bounced back to space without heating Earth.

(b) **The 51 units absorbed by Earth's surface**: Earth re-radiates this energy as long-wave (infrared) radiation upward. The atmosphere absorbs most of this (greenhouse effect: water vapour and CO₂ absorb infrared) and re-radiates some back to Earth (counter-radiation) and some to space.

(c) **Thermal equilibrium**: Earth is in a steady state where incoming solar radiation = outgoing long-wave radiation. The 51 units absorbed at the surface are ultimately re-emitted to space. This is Earth's heat budget — like a bank account where income (solar) equals expenditure (re-radiation). Increasing greenhouse gases (CO₂, CH₄) trap more outgoing radiation → budget deficit → Earth warms until a new equilibrium forms at a higher temperature. This IS global warming.

Question 2

The city of Mumbai (19°N latitude, coastal) has an annual temperature range of only 7°C (January avg 24°C, May avg 31°C). The city of Delhi (28°N, inland) has an annual range of 22°C (January avg 14°C, May avg 36°C). Explain why their temperature ranges are so different, using at least THREE factors.

Accepted Answer

**Factor 1 — Distance from Sea (Continentality)**:
Mumbai is coastal — surrounded by the Arabian Sea on three sides. Water has HIGH specific heat capacity, so it heats and cools slowly. The sea moderates Mumbai's temperature: cools it in summer (sea breeze) and warms it in winter (sea retains heat). Delhi is landlocked (continental) — land heats rapidly in summer and cools rapidly in winter, producing extreme temperature swings.

**Factor 2 — Latitude**:
Mumbai (19°N) is closer to the equator than Delhi (28°N). The Sun's angle is higher at Mumbai throughout the year, so seasonal variation in solar angle (and therefore insolation) is smaller. At Delhi's higher latitude, the Sun is much lower in the winter sky → much less solar energy → cold winters. This amplifies Delhi's seasonal contrast.

**Factor 3 — Sea Breeze/Land Breeze effect**:
In coastal Mumbai, sea breezes blow onshore during the day (cooler sea air moderates temperature) and land breezes blow offshore at night. This diurnal moderating effect also reduces the seasonal range. Delhi has no such moderating sea influence — temperatures can reach 45°C in May and drop to 5°C in January.

**Factor 4 — Ocean Current (bonus)**:
The Arabian Sea current is relatively warm, which slightly raises Mumbai's winter temperatures compared to a similar latitude facing a cold ocean current.

Question 3

Every winter, Delhi's air quality drops to 'severe' (AQI >400) during November–January. Explain the role of temperature inversion in causing this pollution episode. What are the meteorological conditions that create inversion over Delhi in winter?

Accepted Answer

**Normal atmosphere vs inversion**:
Normally, temperature DECREASES with altitude (lapse rate ~6.5°C/1000m). This creates instability — warm surface air rises (convection), carrying pollutants upward and dispersing them. The atmosphere self-cleans through vertical mixing.

**Temperature inversion**: A layer where temperature INCREASES with altitude. The warm air above acts as a 'lid' — cold, dense, polluted air near the surface CANNOT rise through the warmer layer above. Vertical mixing stops. Pollutants accumulate near the surface.

**Causes of Delhi winter inversion**:

1. **Radiative cooling of ground**: In clear winter nights, Delhi's ground loses heat rapidly through radiation (no cloud cover to trap it). The ground becomes very cold, cooling the air just above it. Meanwhile, the air higher up retains heat absorbed during the day. This creates a classic **radiation inversion**: cold at bottom, warm just above.

2. **Calm winds**: Winter brings anticyclonic (high pressure) conditions over northwest India — light, calm winds or no winds. No wind = no horizontal mixing to dilute pollutants.

3. **Low mixing height**: The height to which pollutants can disperse (mixing layer height) drops from ~2,000m in summer to 200–400m in winter Delhi. The same amount of pollution is compressed into 1/5th the volume.

4. **Fog formation**: Cold, polluted air at the surface is also HUMID (Delhi winters have higher relative humidity). Water vapour condenses on pollution particles → fog forms. Fog blocks sunlight → ground doesn't warm → inversion persists all day (not just nighttime).

**The pollution trap**:
Pollution sources — crop stubble burning (Punjab, Haryana), vehicle emissions, construction dust, Diwali firecrackers — produce particulate matter (PM 2.5). The inversion traps these particles in a shallow layer. AQI 400+ = emergency level, causing respiratory damage. The meteorological explanation is: inversion + calm winds + high humidity + multiple emission sources. Solutions require both emission control (stubble management, EV adoption) AND understanding inversion dynamics for emergency response planning (odd-even vehicle rationing on worst days).

Practice — Solar Radiation, Heat Balance and Temperature

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