Q1.
Condensation plays a significant role in the process of bringing evaporated water back to the Earth's surface. How does this happen?
Answer
It happens in five stages — and the whole of it is just evaporation followed by condensation, on the scale of the sky.
- Water goes up. Water evaporates from oceans, rivers, lakes, wet soil, plants and wet clothes, and rises with the air as invisible water vapour.
- The air cools as it rises. As the book says, when air moves higher above the Earth's surface, it becomes cooler and cooler.
- Condensation. At a certain height the air gets so cool that the vapour in it turns into tiny droplets. These droplets generally form around dust particles floating in the air.
- Clouds. These small droplets are light enough to float, and countless numbers of them together are what we see as a cloud.
- Rain. Many droplets join together to form bigger drops. Some drops become so heavy that the air can no longer hold them up and they start falling — and these falling water drops are what we call rain. Under special conditions they fall as hail or snow instead.
Evaporation → rising, cooling air → condensation on dust particles → droplets float as cloud → droplets merge → too heavy → rain, hail or snow
Why dust particles matter: vapour finds it hard to start forming a droplet in perfectly clean air. A speck of dust, salt or smoke gives it a surface to gather on. Activity 8.10 proves this beautifully — the bottle stays clear until a pinch of smoke from burnt newspaper is added, and only then does a haze appear.
Do you know? Why does air containing water vapour go up in the atmosphere at all? Gas balloons containing lighter gases go up in the air. Similarly, water vapour is lighter than air, causing it to rise. Warm, moist air is lighter than the cool dry air around it, so it floats upward — and the atmosphere is the thin layer of air that surrounds the Earth.