The Water Cycle

The last time you took a shower, did you think about where the water came from? Sure, it came out of the shower-head, but what about before that? The water you used to wash could have spent time in the South China Sea. Or, maybe it was part of an ancient glacier at the South Pole. The water cycle has no beginning or end. Water moves continuously from one stage to the next.

The water in your shower could have come from anywhere in the world, because all of Earth’s water
is recycled in a process called the water cycle. So just how does water from a glacier halfway around the world find its way to your bathroom?

What drives the water cycle?
Water is the only substance on Earth that exists in all three states of matter naturally. Water can be solid ice, a flowing liquid, or a gaseous vapor. When water moves through the water cycle, it changes among these states of matter over and over again. The Sun’s energy and Earth’s gravity move water among land, ocean, and atmosphere by driving different processes in the water cycle. The Sun’s energy drives melting and evaporation, and Earth’s gravity drives precipitation, groundwater penetration, and downhill flow.

What are the different components of the water cycle?
A cycle is like a circle—it has no beginning and no end. However, we’ll start by looking at the water cycle in the ocean. About 70% of Earth’s surface is covered by ocean water. When water at the ocean’s surface is heated by the Sun it gains energy. With enough energy, the molecules of liquid water change into water vapor and move into the atmosphere. This process is called evaporation. Ocean water is saltwater, a mixture of salt and water. When evaporation occurs, only the water evaporates and the salt is left behind.

Wherever water is heated by the Sun, evaporation can occur. Water evaporates from lakes, rivers, puddles, soil, and even your body. When sweat dries on your skin, it is because the water in your sweat has evaporated into the air. You might have noticed that when sweat evaporates off you, your skin feels and tastes salty. Similar to the ocean, sweat is comprised of saltwater. The water evaporates and the salt is left behind on your dry skin.

Reflect

After water evaporates from the surface of the land, it rises in the atmosphere. The higher it rises, the more energy it loses, because of the lower temperatures. As water vapor loses energy and cools, the molecules change state from gas to liquid and form small droplets of water. This process is known as condensation. Condensation is the opposite of evaporation.

Look Out

Fluffy, white clouds in the sky look like they are giant puffs of vapor floating in the air. Because they float high above the ground, many people think clouds are made of gas. Clouds are actually small droplets of liquid water that have condensed on bits of dust floating in the atmosphere.

Reflect

If the, now, liquid water molecules continue to lose energy and cool further, they will eventually change into a solid via crystallization. Crystallization is the process by which the liquid molecules form a highly ordered solid. Snow, hail, and sleet all form from this process.

As water droplets or ice in the atmosphere become larger, they get heavier. When they become too heavy, gravity pulls them from the cloud. They fall back to Earth as precipitation. There are many types of precipitation, including rain, snow, hail, and sleet.

Water that falls onto Earth’s surface can take many pathways. Much of the water returns to the ocean and the water cycle begins again. Some water may infiltrate the ground to help replenish groundwater and aquifers.

When this happens, there is a good chance that a living organism will use it. For example, a plant might absorb water from the soil or an animal might drink from a stream. Living things take in water and can release it as well. Plants release water into the atmosphere through their leaves, which is a process known as transpiration. Animals release it through sweat and urination. In both cases, the Sun’s energy causes the water to evaporate, and the water cycle continues.