In this lesson, students explore phenomena in Earth’s systems and the connection between the amount of thermal energy present and the concentration and state of water on Earth as it moves through a phenomena called the water cycle.
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The Water Cycle
This is what happens to a cup of salt water on a sunny day. The sun produces energy through exothermic chemical reactions. That solar energy is then transferred to Earth through radiation (heat transfer that occurs without contact between the heat source and the object heated).
The transfer of energy causes Earth’s atmosphere to heat up as solar energy transforms to thermal energy that is absorbed by air molecules. Thermal energy is then transferred to the cup and then to the water molecules through conduction (heat transfer that occurs when molecules collide). When enough thermal energy is transferred to the water in the cup, those water molecules speed up so much that the water becomes a gas and evaporates. Remember heat transfer—those water molecules are now carrying thermal energy as they move into the atmosphere.
Now think about that cup of salt water representing the global ocean. Earth is constantly being warmed by the sun. Through radiation, energy is transferred from the sun to Earth’s atmosphere and landforms, as well as its oceans, rivers, lakes, and other sources of water on Earth’s surface. Land areas and the atmosphere absorb some sunlight, but the majority of the sun’s radiation is absorbed by the ocean. When enough thermal energy is transferred to the ocean water, liquid water will evaporate into the atmosphere, carrying thermal energy and leaving behind the salt molecules.
Once water evaporates from Earth’s surface, it moves into the atmosphere. There is always water in the atmosphere. About 90 percent comes from evaporation from bodies of water. The other 10 percent comes from plant transpiration—the process of water moving through plants from their roots to their leaves, where it is released back to the atmosphere as water vapor.
Water vapor in the atmosphere transfers some of its thermal energy to the cooler atmosphere. This causes it to cool off and condense, turning back into liquid water. Drops of water join together and attach to particles such as dust or pollen, forming clouds. When the cloud gets heavy enough, precipitation will occur. Precipitation is water falling back to Earth’s surface in the form of rain, snow, sleet, or hail. Some of that water is absorbed into the ground as groundwater. Some is absorbed by the biosphere (the Earth system made up of all living things on Earth), including plants through their roots and animals when they drink. The rest of the water collects on Earth’s surface in the ocean, lakes, and rivers, where it moves downhill, pulled by the force of Earth’s gravity.
When enough thermal energy from the sun warms water on Earth’s surface, it evaporates back into the atmosphere. When living things die and their bodies decompose, the water they’ve stored in their bodies returns to Earth’s surface. This circulation of water through the hydrosphere from Earth’s surface to the atmosphere and back is called the water cycle.
The water cycle is hugely influenced by interactions between the atmosphere and the ocean. Oceans are constantly exchanging energy and matter with the atmosphere: 86 percent of all water evaporation and 78 percent of all precipitation occurs over the ocean. This in turn plays a major role in weather and climate. Evaporation increases the temperature and humidity of the surrounding air. This then causes rain and storms that are carried by winds around the planet.
The water cycle is complex. It varies in different parts of the world and at different times of year depending on the amount of thermal energy available. For example, most of the fresh water on Earth is stored in glaciers and ice caps, where it can remain frozen for thousands of years. The process of water freezing into solid ice is called crystallization. In contrast, in hot climates and seasons, precipitation sometimes evaporates just seconds after it falls to Earth’s surface.
Earth’s varied surfaces also result in uneven heating. For example, the surface of blacktop on a newly paved road gets very hot during the summer. Its dark color makes it a strong light absorber, so little of the sun’s energy is reflected away. Instead, that energy is absorbed as thermal energy. This causes the road to become hotter.
Earth's ability to reflect the sun's light is called albedo [al-bee-doh]. Albedo is influenced by the color, type, and texture of various surfaces. Snow-covered land, forests, oceans, polar ice caps, and cities all reflect energy from the sun differently. This is because dark-colored surfaces, such as oceans and forests, reflect very little solar energy, similar to blacktop roads. Light-colored surfaces of the planet, such as snow and ice, reflect almost all solar energy. This is why the edges of snow next to an exposed patch of earth or concrete melt faster. The white snow reflects the sun's radiation, while the darker earth absorbs it, heating up as a result.
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