Hydrologic Cycle — Class 8 Social Science (Samacheer Kalvi)

TN State Board (Samacheer Kalvi) Class 8 Social Science, Geography — Chapter 3. How water moves endlessly between the oceans, the air and the land.

1. About this lesson

This lesson explains the distribution of water on Earth, the stages of the hydrologic (water) cycle, the forms of condensation and precipitation, and the movements of ocean water.

2. Distribution of water

• About 97% of Earth's water is salt water in the oceans; only about 2.8% is fresh water, and most of that is locked in glaciers and ice caps.

3. Stages of the hydrologic cycle

The hydrologic cycle is a sun-driven global process in which water moves from ocean → atmosphere → land → ocean.

1. Evaporation — the sun changes water into vapour.
2. Transpiration — water vapour escapes from plant leaves; evaporation + transpiration together = evapotranspiration.
3. Condensation — vapour cools and changes back to tiny droplets, forming clouds, dew and fog.
4. Precipitation — water falls as rain, snow, sleet or hail.
5. Runoff — water flows over the land into streams and rivers back to the sea (some seeps in as groundwater).

4. Condensation, precipitation and ocean movements

• Forms of condensation: dew, fog, mist and clouds. Dew forms when a surface cools below the dew point.
• Movements of ocean water:
  • Waves — caused by wind on the surface.
  • Tides — the regular rise and fall of the sea, caused by the gravitational pull of the moon and the sun.
  • Ocean currents — large streams of water (warm or cold) flowing through the oceans.

5. Worked examples

Example 1. How much of Earth's water is fresh water? About 2.8%.

Example 2. What is transpiration? The escape of water vapour from plant leaves.

Example 3. What causes tides? The gravitational pull of the moon and the sun.

6. Book-back questions (Samacheer Kalvi)

I. Choose the correct answer

1. The percentage of fresh water on Earth is about — (a) 2.8% / (b) 50%. Ans: (a) 2.8%.
2. The change of water vapour to liquid is called — (a) evaporation / (b) condensation. Ans: (b) condensation.
3. The escape of water vapour from plant leaves is — (a) transpiration / (b) runoff. Ans: (a) transpiration.
4. Tides are caused by the gravitational pull of the — (a) moon and sun / (b) wind. Ans: (a) moon and sun.
5. The hydrologic cycle is driven by the — (a) sun / (b) wind only. Ans: (a) sun.

II. Fill in the blanks 6. Water flowing over the land into streams is called runoff. 7. Dew forms when a surface cools below the dew point. 8. Waves are caused by the wind.

III. Answer briefly 9. Name the stages of the hydrologic cycle in order. 10. Differentiate waves and tides.

7. Common mistakes

• Mistake: Confusing evaporation and transpiration. Fix: Evaporation is from water bodies; transpiration is from plants (together = evapotranspiration).
• Mistake: Saying tides are caused by wind. Fix: Waves are caused by wind; tides by the moon and sun's gravity.
• Mistake: Thinking most of Earth's water is fresh. Fix: Only about 2.8% is fresh; the rest is salt water.

8. Quick revision

• Geography Ch 3 · the water cycle.
• Only ~2.8% of Earth's water is fresh (most in ice).
• Cycle (sun-driven): evaporation → transpiration → condensation → precipitation → runoff.
• Condensation forms: dew, fog, mist, clouds; dew below dew point.
• Ocean movements: waves (wind), tides (moon + sun gravity), currents (warm/cold streams).