Water in the Atmosphere (Humidity, Clouds, Precipitation) — Practice

Question 1

At 25°C, air can hold a maximum of 20 g of water vapour per cubic metre. An air sample at 25°C actually contains 14 g/m³ of water vapour. (a) What is the absolute humidity? (b) What is the relative humidity? (c) If this air is cooled to the temperature where maximum capacity is 14 g/m³, what happens?

Accepted Answer

(a) **Absolute humidity** = actual water vapour content = **14 g/m³**. (This does NOT change when temperature changes; it only changes if water is added or removed.)

(b) **Relative humidity** = (Actual / Maximum at same temperature) × 100 = (14/20) × 100 = **70%**. The air is 70% saturated — still 30% capacity for more vapour.

(c) When the air is cooled to the temperature where maximum capacity = 14 g/m³, **RH reaches 100%** — the air is now SATURATED. This temperature is the **dew point**. Any further cooling causes condensation — water vapour converts to liquid droplets. If the surface temperature is above 0°C, dew forms on surfaces; if droplets form in the air itself, fog or clouds form.

Question 2

Explain why Mumbai receives ~220 cm of annual rainfall while Pune, only 150 km east, receives only ~72 cm. Include: (a) the mechanism of orographic rainfall, (b) why rain is heavy on the windward side, and (c) why the leeward side is drier.

Accepted Answer

(a) **Orographic rainfall mechanism**: Moist air from the Arabian Sea (loaded with water vapour after the long ocean crossing) flows northeastward during the SW Monsoon. The Western Ghats — a continuous mountain barrier 1,000–1,500m high — force this air to rise (the process is called orographic or relief rainfall).

(b) **Heavy rain on windward side (Mumbai/Western Ghats)**:
As air rises over the Western Ghats: (i) Temperature decreases at the lapse rate (~6.5°C/1000m). (ii) Cooling air can hold less moisture → RH increases. (iii) When RH reaches 100%, condensation begins → clouds form. (iv) Condensation releases latent heat, causing further convective uplift. (v) Heavy rainfall on the western slopes. Cherrapunji (Meghalaya), also on a windward slope, receives the world's highest annual rainfall (~12,000 mm) by the same mechanism.

(c) **Dry leeward side (rain shadow — Pune, Deccan)**:
After crossing the Ghats, air descends on the eastern side. Descending air: (i) Compresses and WARMS (adiabatic warming). (ii) Warm air can hold MORE moisture → RH decreases. (iii) No condensation occurs. (iv) Any clouds that formed on the western side dissipate. Result: Pune gets ~72 cm vs Mumbai's ~220 cm — even though they are in the same region and season. The Deccan Plateau behind the Ghats is a rain shadow zone.

Question 3

Explain the hydrological (water) cycle. Why is it called a 'cycle'? What would happen if any one stage of the cycle were disrupted — for example, if global evaporation dropped by 30% due to ocean cooling?

Accepted Answer

**The Hydrological (Water) Cycle**:

The water cycle describes the continuous movement of water between the Earth's surface and atmosphere through the processes of evaporation, condensation, precipitation, and runoff/infiltration.

**Stages**:
1. **Evaporation**: Solar energy converts liquid water (oceans, lakes, rivers — 71% of Earth's surface) to water vapour. Oceans account for ~86% of global evaporation. Transpiration from plants (evapotranspiration) contributes ~10%.

2. **Condensation**: Rising vapour cools (convection, orographic uplift, frontal systems) → RH reaches 100% → water vapour condenses around condensation nuclei (dust, smoke particles) → cloud droplets form.

3. **Precipitation**: Droplets coalesce into larger drops → fall as rain, snow, sleet, or hail when heavy enough. Distribution: 78% of precipitation falls over oceans; only 22% over land.

4. **Runoff and Infiltration**: Precipitation on land: part runs off into rivers (surface runoff → river → ocean), part infiltrates soil (becomes groundwater, feeds springs and wells over years to centuries).

5. **Return to ocean**: Rivers carry surface runoff to oceans; submarine springs carry groundwater. The cycle is complete.

**Why it's called a 'cycle'**: Because water is neither created nor destroyed — it changes FORM (liquid → vapour → liquid/ice) and LOCATION (ocean → atmosphere → land → ocean). The total amount of water on Earth is fixed (~1.386 billion km³) — the cycle redistributes it continuously.

**If evaporation dropped 30%**:
- Atmospheric water vapour would decrease → less condensation → 30% less precipitation globally
- River flows and groundwater recharge would drop → drought conditions on land
- Agricultural production collapse → food crisis
- But the reduced moisture would mean less cloud cover → more solar radiation reaches Earth → surface warms → this would eventually RESTORE evaporation (negative feedback loop). The cycle would find a new equilibrium at lower moisture levels — but the transition period (decades) would be catastrophic for human civilisation
- This theoretical disruption models what happens at regional scales during El Niño events (warm Pacific Ocean = altered evaporation patterns = changed monsoon = drought in India, floods in Peru)

Practice — Water in the Atmosphere (Humidity, Clouds, Precipitation)

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