The Water Cycle
One of the oldest systems on earth is the water cycle. It is the process that involves water found on earth being taken up into the atmosphere and coming back down onto the earth. When the water is back on earth, it gets taken up into the atmosphere again, and the cycle continues. This cycle is billions of years old. It started long ago when the first water formed on earth and was evaporated by the sun and into the earth's atmosphere. From there, it has never stopped. If the water cycle ever stopped, life on earth would not be possible.
There are five stages in the water cycle. The first stage is evaporation. This is what happens when the sun's heat causes water to turn into vapor or steam. Evaporated water can come from all the bodies of water on earth like lakes, rivers, seas, oceans, ponds, etc. It can also come from plants! Plants can lose their water out of their leaves due to the heat of the sun, this is called transpiration.
The first thing the evaporated water does is add to moisture in the air. Animals and plants need a certain amount of moisture in the air to remain comfortable. The amount of moisture in the air is measured by humidity. Sometimes it is very humid (in the summer) or it is very dry (in the winter.) As the sun gets hotter, the water vapor's molecules begin to rise higher. This is because hot air floats upwards and above cold air.
The water vapor or steam makes its way up into the earth's atmosphere where it is cooler and it cools down in a process called condensation. In the process of condensation, water vapor turns into water again! If you've ever observed water form on the outside of your cold soda can, this is because the water vapor or moisture in the air turned back to water when it began touching your cold soda can. This is the same thing that happens in condensation.
The process of condensation creates clouds. These clouds are filled with ice crystals. When the clouds fill with ice crystals or water, they get bigger and bigger. They also become darker. When clouds are dark, this usually means they are heavy because there is so much condensed water in them. This is called saturation. When the clouds are filled with water to the point that they can't hold any more inside, they release all the water in a process called precipitation.
We see precipitation all the time when we see rain and snow. This is the effect of the water coming down from the clouds. When the weather on earth is very cool (like it is in winter) the ice crystals come down from the clouds and remain ice crystals so we see it coming down as snow. When the weather on earth is warm (like it is in summer) the ice crystals melt into droplets on its way down to earth and we see it coming down as rain.
As the water comes down from the clouds, it begins to go through a process known as run off, wherein the water collects in certain areas. The water can water plants and trees and be absorbed by these organisms. It can go over the ocean, into rivers, lakes, and seas, and it goes on land too, getting absorbed by the soil. After this, the sun shines and the cycle starts again. Many times the water in clouds comes down in a place far from where it started from! This means that the rain in your city could have traveled hundreds of miles from its source!
Next Article: Water Cycle Demonstration
