Carolyn N. Kinder
Much of our planet Earth is covered with water. Almost eighty (80) percent of the Earth is covered with water. Most of the water on Earth, about ninety-seven (97) percent, is in the salty oceans and seas. Another two (2) percent is frozen in glaciers and ice around the North and South Poles. The remaining one (1) percent of the fresh water is found in rivers, lakes, streams and in aquifers under the ground.
Water is continuously recycling in a process called the hydrologic or water cycle.
Sometimes we call water by its chemical name, H2O, which means each water molecule is made up of two (2) atoms of hydrogen (H) and one atom of oxygen (O). Heat energy from the sun causes the liquid water to change into its gaseous form, water vapor, through evaporation happens constantly from lakes, rivers, oceans, etc. Water vapor enters the atmosphere in other ways. Water evaporates from the surface of leaves in a process called transpiration. The amount of water moving from the ground through the leaves of plants and into the air depends on factors such as: temperature, light, wind, and the amount of water in the soil.
When water vapor enters the atmosphere and rises, the water vapor cools because the air is cooler. Particles of dust and salt attract the water molecules. The water gathers together in larger and larger droplets and forms clouds. This change from vapor back into liquid form is called condensation. Condensation is the opposite of evaporation. Condensation will occur whenever warm air comes in contact with a cold surface.
When enough water droplets collect, the cloud turns grey. When conditions are right, water will fall from the clouds back to Earth is some form of precipitation such as rain, sleet, hail or snow.
Air currents carry air pollutants. Sulfurdioxide and nitrogen oxides cling to water molecules in the air producing sulfuric and nitric acids. The acids, mixed with water vapor, fall from the sky as acid rain. This type of pollution can kill fish and plants, erode buildings and may seriously affect drinking water. Scientist are working on finding a solution to this problem. The movement of water from the Earth to the atmosphere and back to the Earth is the hydrologic cycle. Evaporation, condensation, and precipitation are the three major parts of this cycle. See figure 1.