4. Earth's Water & the Water Cycle

About 71% of Earth's surface is covered by water, but most of it is salty ocean. Only a small fraction is the fresh water that living things need, and much of that is frozen in ice caps and glaciers.

Earth's water is constantly recycled through the water cycle, powered by energy from the Sun:

• Evaporation — the Sun heats surface water (oceans, lakes) and turns it into invisible water vapor that rises. (Plants release vapor too, called transpiration.)
• Condensation — high up, the cooler air turns the vapor back into tiny droplets that form clouds.
• Precipitation — droplets join, grow heavy, and fall as rain, snow, or hail.
• Collection — water gathers in oceans, lakes, and rivers, or soaks into the ground, and the cycle repeats.

⚠️ Common misconception: the water cycle does not create new water. The same water is recycled again and again — the water you drink today is the same water dinosaurs once drank.

💡 Tip: follow the loop with the Sun as the engine: evaporate up, condense into clouds, precipitate down, collect, repeat.