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.

Accepted Answer

A

Question 2

Explain why Mumbai receives ~220 cm of annual rainfall while Pune, only 150 km east, receives only ~72 cm. Include

Accepted Answer

A

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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