[Science] 4th Grade: The Water Cycle (Where Rainwater Goes and How Water Circulates)

Hello! Today, let's explore one of the greatest journeys on Earth: the water cycle.
Have you ever wondered where rain goes after a storm, why puddles disappear on sunny days, or where rain comes from in the first place? Let's trace the journey of water step-by-step!

1. Where does rainwater go on the ground?

When rain falls onto the schoolyard, streets, or mountains, what happens to it?

Water flows from high places to low places

If you observe a sloping ground or hill during or after rain, you will notice something clear:
Water always flows from high ground toward low ground.

  • Rainwater gathers into small trickles and streams, flowing toward lower areas.
  • It collects in puddles, ditches, ponds, and rivers, eventually making its way toward the sea.

How water soaks into the ground (Soil particle size)

Does rainwater stay on the ground forever? No! Some water soaks right into the soil. However, how quickly it soaks in depends on the size of the soil particles.

  • Large particles (sand and gravel): Water drains and soaks through very quickly because there are plenty of gaps between the particles.
  • Small particles (fine soil and clay): Water soaks in slowly because the gaps between particles are very tiny. Puddles often form on grounds with fine soil.

【Section Summary】
・Rainwater flows and collects from high ground to low ground.
・How well rainwater soaks in depends on the size of soil particles (larger particles allow water to soak in faster).

2. Where do puddles go? (Evaporation)

A few hours after the rain stops and the sun comes out, puddles dry up and the ground becomes completely dry. Where did all that water vanish to?

Water evaporates into the air

The water on the ground turns into an invisible gas called water vapor and escapes into the surrounding air. This process is called evaporation.

  • Water does not need to be boiling hot to evaporate. Even at everyday temperatures, water from puddles, wet soil, leaves, and hanging laundry quietly turns into water vapor.
  • Water vapor is completely invisible. You cannot see it with your eyes, but it is mixed into the air all around us.

【Section Summary】
・Water from the ground and puddles turns into invisible water vapor and enters the air.
・This change of liquid water into airborne vapor is called evaporation.

3. What happens to water vapor in the sky? (Condensation)

Once water vapor is floating high up in the air, what happens next?

Cooling turns vapor back into liquid water

When water vapor rises higher into the sky, it cools down. When water vapor is cooled, it changes back into tiny droplets of liquid water. This is called condensation.

(Everyday Example) Think about what happens when you pour cold water into a glass on a warm day. Water droplets form on the outside of the glass! That is airborne water vapor being cooled and condensing into liquid water droplets.

Clouds and rain

  • High in the sky where the temperature is cold, airborne water vapor condenses into countless tiny water droplets (or tiny ice crystals). These gather together to form clouds (or fog near the ground).
  • When these tiny droplets combine and grow larger and heavier, they can no longer float in the air. They fall back down to Earth as rain (or snow when it is very cold)!

【Section Summary】
・When airborne water vapor cools down, it condenses back into liquid water droplets.
・Gathered water droplets in the sky form clouds, which eventually fall back to the ground as rain.

4. The endless journey: The Water Cycle

Now, let's connect all the pieces of the puzzle together! Water on Earth is constantly travelling in a giant loop called the water cycle:

  1. Rain falls: Water falls from clouds to the ground as rain.
  2. Water flows and soaks: Rainwater flows from high to low ground into rivers and oceans, or soaks into the soil.
  3. Water evaporates: Water from the ground, rivers, seas, and puddles evaporates into the air as invisible water vapor.
  4. Water condenses: Rising water vapor cools down, condenses into clouds, and falls again as rain!

【Key Takeaway】
The water you drink today, the rain falling from the sky, and the water in the oceans have been circulating around the Earth for millions of years. Next time you see a puddle drying up or watch rain clouds forming, remember this incredible, endless cycle!