The hydrologic (or water) cycle has been around since the beginning of time. This cycle repeats over and over. The hydrologic cycle has three main processes: evaporation, condensation, and precipitation.
Heat from the sun starts the hydrologic cycle causing evaporation. During evaporation, water is heated by the sun's energy and changes into water vapor that is held in the air of the atmosphere. Water is evaporated from lakes, streams, oceans, and plants. In addition, water is released by animals' breathing and perspiration. This water is also evaporated into the atmosphere.
Plants take in large amounts of water from the Earth. The water moves through plants' stems and branches. This water is evaporated through tiny openings in the leaves of the plant. This process is called transpiration. Most of the water that evaporates from Earth's land surface does so through transpiration. A big oak tree gives off about 150,000 liters of water a year.
Condensation is another step in the hydrologic cycle. This step is the opposite of evaporation. As the air holding water vapors cools, it cannot hold as much water. This causes the water vapor to condense (change) into liquid water. Condensation can form fog, dew, and clouds.
Fog forms when air near the surface is cold and nearly saturated with water. Now when water from the ground evaporates, it condenses immediately forming tiny water droplets that create a low-lying cloud we call fog.
Dew forms at night when air becomes saturated with water vapor. When this saturated air comes in contact with plants or other objects it condenses, leaving tiny water droplets behind on the object.
When the air containing water vapor is heated by the sun, it rises into the atmosphere by convection. The water vapor in the air is then cooled by the colder air higher in the atmosphere causing the relative humidity to increase. As the relative humidity increases, the air eventually becomes saturated. The water vapor then condenses into tiny water droplets around particles of dust or salt in the air. These tiny water droplets make up clouds.
At times, cloud droplets combine with other water droplets in the atmosphere. When this happens, the drops become too large and heavy to remain suspended in the atmosphere. They then fall to the ground as precipitation. Precipitation brings new fresh water to the Earth.
On an average day in the United Sates of America.....
Some precipitation seeps into the groundwater and is stored in layers of rock below the surface of the Earth. This water stays there for varying amounts of time. Some water may evaporate into the hydrologic cycle within days, while other water will stay in the ground for centuries or more. This process of precipitation seeping into the groundwater is called infiltration.
When the water droplets in the clouds grow large, they can no longer be suspended in the atmosphere, and fall. If the air is warm, the precipitation will be rain. Raindrops range is size from 0.5 millimeters to 6 millimeters. This shown in the left side of the picture above.
Snowflakes form when frozen water droplets called ice crystals freeze onto dirt particles in clouds. The ice crystals then grow large, and are no longer able to be suspended in the atmosphere. They then fall to the ground as snow. This is shown in the middle left side of the picture above. However, if the snowflake hits warm air as it falls, it will melt into rain.
Hailstones are 5 to 75 millimeters pellets composed of many layers of ice. Hailstones form when raindrops freeze, and then are kept in the air by strong wind currents in storm clouds. The hailstones are moved through warm and cold layers of air, collecting more water droplets which freeze and add layers of ice to the stone. The more ice layers the stone collects, the larger it is. This process is shown in the right middle side of the picture above.
Sleet forms when rain from a warmer layer of air falls through cold air, freezing the rain into small ice pellets called sleet. This process is shown in the far right side of the picture. In cold weather, rain may first freeze when it reaches cold air very close to the ground. This precipitation is called freezing rain.