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The water cycle

In the water cycle, water continuously moves between Earth's surface and the atmosphere. This occurs through processes such as evaporation and transpiration, condensation, and precipitation. The water cycle is driven by solar energy and gravity. Created by Khan Academy.

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  • sneak peak yellow style avatar for user SecretCoder
    At , the narrator said the water rises. But how does it actually rise because we don’t see water in the oceans flying up to the clouds?
    (11 votes)
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    What is ligma
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  • blobby green style avatar for user Denis Pinsonnault
    The sentence appearing at the end of the video seems misleading to me. How does a student should understand the assertion appearing on the screen that “I’m the same water that dinosaurs drank!” It doesn’t take into account that water molecules are constantly destroyed and reconstructed. It is what didactic specialists in the natural sciences name a “naive conception.” Those specialists recognized those “naive conceptions” as major obstacles to learning scientifically sound knowledge. For example, during cellular respiration, water is a product. On the other hand, during photosynthesis, water is a reactant. How could the water contained in any glass be considered water previously drunk by dinosaurs if molecules are constantly destroyed and rebuilt?

    Thanks for your wonderful site. I’m a retired physics professor helping his grandsons doing their homework.
    (7 votes)
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  • blobby green style avatar for user jasleen
    the moon has something to do with the water?
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  • blobby green style avatar for user cscholtenrds22
    Because snow is also water, would it be in its solid or liquid state?
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    What are the major processes of Water Cycle?
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    • duskpin tree style avatar for user Alexa
      Evaporation: This is the process by which water changes from a liquid state to a vapor or gas state. It occurs when heat energy from the sun causes water from oceans, lakes, rivers, and other surfaces to turn into water vapor and rise into the atmosphere.

      Condensation: When water vapor in the atmosphere cools down, it undergoes condensation and changes back into liquid water. This process forms clouds, which are made up of tiny water droplets or ice crystals suspended in the air.

      Precipitation: Precipitation refers to the process in which condensed water vapor in the atmosphere falls back to the Earth's surface. It includes various forms of water reaching the ground, such as rain, snow, sleet, or hail.

      Runoff: When precipitation reaches the Earth's surface, it may flow over the land as runoff. Runoff collects in streams, rivers, and eventually makes its way to lakes, oceans, or groundwater reservoirs. It plays a crucial role in replenishing water bodies and maintaining the water cycle.

      Infiltration: Infiltration occurs when water from precipitation or runoff seeps into the ground and is absorbed by the soil. It replenishes groundwater reservoirs and becomes available for plants, animals, and human use.

      Transpiration: Transpiration is the process by which water vapor is released into the atmosphere by plants through tiny pores on their leaves called stomata. It is essentially the plant version of evaporation and contributes to the overall moisture content in the atmosphere.
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  • blobby green style avatar for user GiGi
    does the moon have something to do with the water ?
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  • duskpin sapling style avatar for user Ily
    Isn't the diagram at inaccurate? Can vapor really become ice in one step? I thought vapor had to become water before becoming ice.
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  • blobby green style avatar for user VictoriaF
    If we recycle water then how can we run out o f it?
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    ░░▐▒▒▒▄▄▒▒▒▒░░░▒▒▒▒▒▒▒▀▄▒▒▌░░ copy and paste him to take
    ░░▌░░▌█▀▒▒▒▒▒▄▀█▄▒▒▒▒▒▒▒█▒▐░░ khan academy
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Video transcript

- [Instructor] Did you know that the water you drink is actually the same water that dinosaurs drank over 65 million years ago? It might be hard to believe but your water is actually really, really old. In fact, water on Earth is much older than the dinosaurs. Scientists estimate that the water on Earth is at least 4.6 billion years old. And the amount of water on Earth today, in lakes, rivers, oceans, glaciers, even under the ground and up in the clouds, it's about the same as it was millions and millions of years ago. That's because water is recycled. It just gets used again and again. And that brings us to the water cycle, which is how water continuously moves from the ground to the atmosphere and back again. And as water moves through the cycle, it changes form. In fact, water is the only substance on Earth that naturally exists in three states, solid, liquid, and gas. Have you seen water in all of its different states? Maybe on a hot day, you'll add some ice, which is water in its solid state, to a glass of liquid water. Or maybe when you take out some food that you've heated in the microwave, you'll see steam coming off of the food, which is water in its gas state as water vapor. When you think of water, you might think of the wide open ocean. Over 95% of all the water on Earth is in the ocean, so this is a great place to start with the water cycle. Here, energy from the sun warms up water on the surface of the ocean enough to turn it into water vapor. This is called evaporation. This water vapor is less dense, meaning it's lighter, than liquid water, so it rises up and up into the atmosphere. However, as the water vapor rises, the temperature in the atmosphere cools. In turn, the water vapor condenses into tiny liquid water droplets, or, as we see them, clouds. This is called condensation. Air currents then move these clouds all around the Earth. As a cloud collects more and more liquid water droplets, the water may be released from the cloud, pulled down by gravity, and then return to the ocean or land as precipitation, like rain. If it's really cold, though, the water drops may crystallize and become snow. The snow will fall to the ground and eventually melt back into a liquid and run off into a lake or river, pulled down by gravity, which flows back into the ocean where the whole process starts over again. But that's just one path water can take through the water cycle. It's like a choose your own adventure. Instead of snow melting and running off into a river, the snow could become part of an icy cold glacier and stay there for a long, long time, for thousands of years. Or rain can seep into the ground and become groundwater, where it's then absorbed by plant roots. In turn, through transpiration, the water absorbed by the plants can transition to water vapor and leave directly through the leaves via tiny holes called stomata and return to the atmosphere. Or instead of being absorbed by plant roots, the groundwater can work its way to an underground aquifer or a lake, river, or even the ocean. There are many different paths for water and the water cycle can be very complicated. But it really comes down to something very simple. The amount of water on Earth stays pretty constant over time and moves from place to place, sometimes transitioning between phases, depending on things like weather, geography, solar energy, and gravity. Now, we know that water is essential to life on Earth and fresh water is an especially limited resource for a growing world population. Changes in the water cycle can impact everyone through the economy, energy production, health, recreation transportation, agriculture, and of course drinking water. And that's why understanding the water cycle is so important. That, and it's pretty cool to know that you drink the same water as dinosaurs did. Until next time.