This is a Clilstore unit. You can link all words to dictionaries.

the Water Cycle



What is the water cycle? I can easily answer that—it is "me" all over! The water cycle describes the existence and movement of water on, in, and above the Earth. Earth's water is always in movement and is always changing states, from liquid to vapor to ice and back again. The water cycle has been working for billions of years and all life on Earth depends on it continuing to work; the Earth would be a pretty stale place without it.

Where does all the Earth's water come from? Primordial Earth was an incandescent globe made of magma, but all magmas contain water. Water set free by magma began to cool down the Earth's atmosphere, and eventually the environment became cool enough so water could stay on the surface as a liquid. Volcanic activity kept and still keeps introducing water into the atmosphere, thus increasing the surface-water and groundwater volume of the Earth.

 Condensation is the process by which water vapor in the air is changed into liquid water. Condensation is crucial to the water cycle because it is responsible for the formation of clouds. These clouds may produce precipitation, which is the primary route for water to return to the Earth's surface within the water cycle. Condensation is the opposite of evaporation.

You don't have to look at something as far away as a cloud to notice condensation, though. Condensation is responsible for ground-level fog, for your glasses fogging up when you go from a cold room to the outdoors on a hot, humid day, for the water that drips off the outside of your glass of iced tea, and for the water on the inside of the windows in your home on a cold day.

The phase change that accompanies water as it moves between its vapor, liquid, and solid form is exhibited in the arrangement of water molecules. Water molecules in the vapor form are arranged more randomly than in liquid water. As condensation occurs and liquid water forms from the vapor, the water molecules become organized in a less random structure, which is less random than in vapor, and heat is released into the atmosphere as a result.

 The little cloud that could—but why? (Why does this cloud exist?)


Diagram of the water cycle.



Even though clouds are absent in a crystal clear blue sky, water is still present in the form of water vapor and droplets which are too small to be seen. Depending on weather conditions, water molecules will combine with tiny particles of dust, salt, and smoke in the air to form cloud droplets, which grow and develop into clouds, a form of water we can see. Cloud droplets can vary greatly in size, from 10 microns (millionths of a meter) to 1 millimeter (mm), and even as large as 5 mm. This process occurs higher in the sky where the air is cooler and more condensation occurs relative to evaporation. As water droplets combine (also known as coalescence) with each other, and grow in size, clouds not only develop, but precipitation may also occur. Precipitation is essentially water in its liquid or solid form falling from the base of a cloud. This seems to happen too often during picnics or when large groups of people gather at swimming pools.


Where Does the Water Cycle Begin?
If someone asked you where a bicycle wheel begins, you would know that was a trick question. So, the mention of the word "cycle" in "water cycle" is a hint that the water cycle has no beginning or ending. But if it did, where do YOU think it would begin? Make your choice from the list below and then see what others think.

Atmosphere Condensation Evaporation Evapotranspiration
Freshwater storage Groundwater flow Groundwater storage Ice and snow
Infiltration Oceans Precipitation Snowmelt
Springs Streamflow Surface runoff Sublimation


Water Properties True/False Quiz

Water is more than just plain old water -- it actually has some very unusual properties. It would be boring if I just told you that water is wet and clear. So, instead, here are some "maybe true/maybe false" water properties. See if you know the real water facts.

True   False   (1) Water contracts (gets smaller) when it freezes.
True   False   (2) Water has a high surface tension.
True   False   (3) Condensation is water coming out of the air.
True   False   (4) More things can be dissolved in sulfuric acid than in water.
True   False   (5) Rainwater is the purest form of water.
True   False   (6) To accurately read water height in a glass rain gauge, you read the center of the water column, not the edge (meniscus) where the water meets the glass.
True   False   (7) You can float a paper clip on top of water if you very carefully place it flat on the water surface because of the molecular skin created by the interaction of air molecules with the top layer of water molecules on the surface.



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