The Water Cycle Explained

The water cycle, also called the hydrological cycle, is the continuous movement of water between the oceans, the atmosphere and the land. Water keeps changing state along the way: liquid in seas and rivers, gas (water vapour) in the air, and solid ice in glaciers and snow.

The cycle is powered by energy from the Sun, which heats water and lifts it into the air as vapour. Gravity does the rest, pulling rain and snow back down and making rivers run downhill. Across the whole planet, the amount of water that evaporates each year is roughly equal to the amount that falls as precipitation.

Water gets into the air in two main ways:

• Evaporation: sunlight warms oceans, lakes and rivers, and liquid water turns into invisible water vapour. Evaporation from oceans, seas and other water bodies supplies nearly 90 per cent of the moisture in the atmosphere.
• Transpiration: plants draw water up from the soil through their roots and release water vapour through tiny pores in their leaves called stomata. Plants supply most of the remaining 10 per cent.

You see evaporation every day when a wet school uniform dries on the balcony, or when a puddle disappears after a sunny afternoon.

As water vapour rises, the air around it gets colder. Cool air cannot hold as much vapour, so some of it condenses back into tiny liquid droplets or ice crystals. The droplets form around specks of dust, salt or pollution in the air, and billions of them together make a cloud.

You can watch condensation happen on the outside of a glass of cold water on a humid day. Water does not stay in the air for long: on average, a water molecule spends only about nine days in the atmosphere before it falls again.

Inside a cloud, droplets bump into each other and join. When they grow too heavy to stay up, they fall as precipitation: rain, snow or hail.

Once on the ground, water can take several paths:

1. Runoff: it flows over the surface into streams, rivers and lakes, and eventually back to the sea.
2. Infiltration: it soaks into the soil.
3. Percolation: water that has entered the soil keeps moving down into groundwater, which feeds wells and springs.

Most of Earth's water, about 96.5 per cent, is salty ocean water. Only about 2.5 per cent is fresh, and most of that is locked up in ice or stored underground.

India's south-west monsoon shows every stage of the cycle at once. In summer, the land heats up much faster than the sea, creating an area of low pressure over the subcontinent. Moist winds are drawn in from the Indian Ocean and the Arabian Sea, carrying water that evaporated from the warm sea surface.

These winds reach India's west coast in June. There they meet the Western Ghats, a mountain barrier roughly 1,000 km long. The air is forced upwards, cools, and its vapour condenses into thick clouds that drop very heavy rain on the coast. Over the June to September season, this rain fills rivers and reservoirs and recharges the groundwater that farms and cities depend on.

The water cycle constantly refreshes our supply of fresh water for drinking, farming and industry. It also moves heat around the planet and shapes weather, from daily showers to floods and droughts.