BACKGROUNDHow old is the water in your water bottle? It’s old as the earth itself! Chances are the water you are drinking was once part of an ocean, a glacier, or even a cloud. That’s because earth has a limited amount of water.
Water comes in three forms: water vapor (gas), rain (liquid), and snow (solid). Water is always moving between these forms through an endless circle called the
water or “hydrologic” cycle. What powers this cycle? The sun’s heat!
As liquid water on Earth warms, it can change form from into water vapor. This is called “evaporation.” Water evaporates from a river, lake, or ocean and enters the atmosphere. Even though we can’t see it, the sky can be full of water!
When atmospheric water vapor is cooled, it changes back into its liquid form as tiny droplets. This is called condensation. We see it as clouds or fog. When the atmosphere gets so full of water that it can’t hold it any longer, it falls back to earth surface as precipitation in either rain (liquid) or if temperatures are cold enough as snow or ice (solid). The cycle starts again as this water is evaporated back into the atmosphere.
Water also moves through Earth and is stored here. Some soaks deeply into the Earth and is stored as “groundwater.” Precipitation that falls in very cold regions can be stored as glacial ice. If there is too much precipitation at one time, water runs off the land can cause flooding and erosion.
We are part of the water cycle, too. Every time we breathe, we exhale water molecules into the atmosphere. Plants are “breathing” too. They pull in water through their roots and “exhale” water vapor through their leaves. This is called “transpiration.”
Water is always on the move above and through the Earth and all living things! What comes around, goes around when it comes to water.