What is the Water Cycle?
The water cycle (also called the hydrological cycle) describes the continuous movement of water through Earth's systems — from the oceans to the atmosphere to the land and back again. There is no "new" water on Earth — every drop has been recycled billions of times over the planet's 4.5-billion-year history. The water cycle is powered by the sun's energy and Earth's gravity.
The 4 Main Stages
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1. Evaporation
Water Becomes Vapor
- The sun heats water in oceans, lakes, rivers, and puddles
- Water molecules gain enough energy to break free and rise as invisible water vapor
- About 90% of atmospheric water vapor comes from ocean evaporation
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2. Condensation
Vapor Becomes Clouds
- As water vapor rises higher in the atmosphere, the air gets cooler
- Cool air can't hold as much moisture — water vapor condenses into tiny liquid droplets
- These droplets attach to tiny dust particles in the air, forming clouds and fog
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3. Precipitation
Water Falls Back Down
- Water droplets in clouds combine and grow heavier until they fall back to Earth
- Precipitation takes many forms depending on temperature: rain, snow, sleet, or hail
- About 75% of precipitation falls directly into the ocean
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4. Collection
Water Gathers Again
- Water that falls on land flows into rivers, lakes, and streams (runoff)
- Some soaks into the ground (infiltration) and becomes groundwater
- Eventually all water returns to the ocean — and the cycle begins again
Two More Important Processes
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Transpiration
Plants Release Water
- Plants absorb water through their roots and release it as vapor through tiny pores in their leaves (stomata)
- A single large tree can release up to 100 gallons of water per day!
- Forests are major contributors to rainfall — they "pump" water from the ground to the atmosphere
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Infiltration
Water Soaks into Soil
- When rain falls on permeable soil, water soaks downward through layers of rock and soil
- This creates groundwater stored in underground aquifers — the source of most well water
- Some aquifers hold water that fell as rain thousands of years ago!
How Clouds Form
Clouds are made of billions of tiny water droplets or ice crystals suspended in the atmosphere. They form when warm, moist air rises and cools — causing water vapor to condense. Different cloud types form at different altitudes and tell us different things about upcoming weather.
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Cumulus Clouds
Fair Weather
- Puffy, white, cotton-ball clouds that form on sunny days
- Form from warm air rising (convection) on clear afternoons
- Generally indicate fair weather — unless they grow very tall
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Cumulonimbus
Thunderstorm Cloud
- Towering storm clouds that can reach 40,000+ feet into the atmosphere
- Bring heavy rain, lightning, hail, and strong winds
- Sometimes called "the king of clouds" — the most powerful cloud type
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Stratus Clouds
Overcast Skies
- Flat, gray, layered clouds that cover the whole sky like a blanket
- Often bring drizzle or light rain
- When stratus clouds form at ground level, we call it fog
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Cirrus Clouds
High Altitude Ice
- Thin, wispy, feathery clouds made of ice crystals — very high in the atmosphere
- Form above 20,000 feet where temperatures are well below freezing
- Often indicate a weather change is coming in the next day or two
💧 Water Has Been Cycling for 4 Billion Years
The water cycle has been running since Earth formed its oceans about 4 billion years ago. Every drop of water on Earth is constantly being recycled — through the ocean, atmosphere, land, and living things. The water molecules in your glass of water may once have been part of an ancient ocean, a dinosaur's blood, a glacier, or a tropical raincloud. Water is endlessly reused — there is no "new" water!
Water Cycle Facts
3%
of Earth's water is fresh water — and most of that (2%) is locked up in glaciers and ice caps
37.5M mi³
of water is in Earth's atmosphere at any moment — enough to cover the entire land surface 1 inch deep
3,200 yrs
Average time a water molecule spends in the ocean before evaporating — water cycles very slowly
9 days
Average time a water molecule spends in the atmosphere before falling as precipitation
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