Water, Air & Weather
Class 11–12 | Arts / Humanities
Subject: Environmental Studies / Science
“Understanding the three pillars of the atmosphere—water, air and weather—is essential to grasp how natural processes shape human life, culture and history.”
1. Simple Explanation
| Theme | What it is | Why it matters |
|---|---|---|
| Water | The liquid that covers ~71 % of Earth’s surface and cycles through evaporation, condensation, precipitation, infiltration and runoff. | Hydration, agriculture, transportation, cultural rituals. |
| Air | A mixture of gases (78 % N₂, 21 % O₂, trace gases). It carries energy (solar radiation, heat) and pollutants. | Breathing, weather, climate regulation, transport of gases. |
| Weather | The short‑term state of the atmosphere (temperature, humidity, wind, precipitation). | Daily life, safety, agriculture, cultural festivals. |
2. Key Points
2.1 Water
| Aspect | Details |
|---|---|
| Physical Properties | - Surface tension, cohesion & adhesion, high specific heat, universal solvent. |
| Water Cycle | Evaporation → Condensation → Precipitation → Infiltration/Runoff → Groundwater recharge. |
| Human Uses | Domestic, irrigation, industry, hydropower, recreation. |
| Water‑Related Issues | Over‑extraction, pollution (industrial, domestic, agricultural), salinisation, climate‑driven floods/droughts. |
| Cultural Significance | Sacred rivers, festivals (e.g., Ganga‑Dewali), myths of water deities. |
2.2 Air
| Aspect | Details |
|---|---|
| Composition | N₂ (78 %), O₂ (21 %), Ar (~1 %), CO₂ (0.04 %), trace gases (CH₄, N₂O, ozone). |
| Atmospheric Layers | Troposphere (weather), stratosphere (ozone layer), mesosphere, thermosphere, exosphere. |
| Pollutants | SO₂, NOₓ, PM₂.₅/PM₁₀, VOCs, CO, O₃ (ground‑level). |
| Air Quality Index (AQI) | Numerical scale (0–500) summarizing pollutant mix. |
| Health & Cultural Impact | Respiratory disorders, smog, visibility loss, effect on festivals, ancient wind‑powered architecture. |
2.3 Weather
| Concept | Explanation |
|---|---|
| Definition | The state of the atmosphere at a particular place and time. |
| Key Parameters | Temperature, humidity, wind speed/direction, atmospheric pressure, precipitation. |
| Weather Types | Dry (high pressure), humid (low pressure), stormy (cyclones, tornadoes). |
| Forecasting | Ground‑based observations, satellite imagery, numerical weather prediction models. |
| Climate vs Weather | Weather: short‑term (~hours–days). Climate: long‑term patterns (~30 yrs). |
| Historical & Cultural Effects | Monsoon cycles shaping agriculture, festivals, architecture (e.g., pavilions, courtyards). |
3. Worked Example
3.1 Calculating Daily Evaporation from a Lake (Simple Energy Balance)
Given
- Solar radiation (R_s = 800 \text{ W/m}^2)
- Surface albedo (α = 0.07) (typical for water)
- Net long‑wave radiation (R_{nl} = 220 \text{ W/m}^2) (outgoing)
- Latent heat of vaporization (L_v = 2.45 \times 10^6 \text{ J/kg})
- Surface area (A = 1 \text{ km}^2 = 1 \times 10^6 \text{ m}^2)
Step 1: Net Energy Input
[
E_{net} = (R_s \times (1-α)) - R_{nl}
]
[
E_{net} = (800 \times 0.93) - 220 = 744 - 220 = 524 \text{ W/m}^2
]
Step 2: Convert to Watts over the lake area
[
E_{total} = E_{net} \times A = 524 \times 1 \times 10^6 = 5.24 \times 10^8 \text{ W}
]
Step 3: Convert to Joules per day
[
\text{Seconds per day} = 86400
]
[
E_{day} = 5.24 \times 10^8 \times 86400 = 4.53 \times 10^{13} \text{ J}
]
Step 4: Mass of water evaporated
[
m = \frac{E_{day}}{L_v} = \frac{4.53 \times 10^{13}}{2.45 \times 10^6} \approx 1.85 \times 10^7 \text{ kg}
]
Step 5: Depth of water lost
[
\text{Volume} = 1.85 \times 10^7 \text{ m}^3 \quad (\text{since }1 \text{ kg of water} = 1 \text{ m}^3)
]
[
\text{Depth} = \frac{\text{Volume}}{A} = \frac{1.85 \times 10^7}{1 \times 10^6} = 18.5 \text{ m}
]
Result
Approximately 18 m of water evaporates from the lake surface in one day under the given conditions. (In reality, other flux