How the Earth's Water Cycle Works
All living beings need water to live––it's an integral part of every culture worldwide, human or otherwise. Unfortunately, we know that some of nature's systems are breaking down due to human activity. Global warming, for example, is heating up the air that rain and snow would normally have cooled down. Could the water cycle be one of the breakdowns? Let's explore the water cycle and see how it works.
Water Cycle Diagram
Physical States of Water
Water alternates between gas, liquid, and solid. What makes the difference is the temperature. High temperatures cause water to evaporate into gas (water vapor), medium temperatures produce a liquid form, really low temperatures cause water to freeze.
All over the world and in the air water is constantly changing between these three forms. As it does, it also changes location, as shown by the blue arrows above.
When liquid is heated it changes into vapor that rises. When vapor is cooled it coalesces into rain, sleet, hail, or snow that falls. When ice and snow (solid water) is heated, it melts into liquid that flows to lower levels, where it's stored, until it heats again, evaporates, and rises again.
Thus the water cycle looks like this (from right to left on the diagram): Evaporation, condensation, precipitation, flow (runoff), storage, and repeat. Let's examine each of these stages in a little more detail, starting with storage, since that's the stage that humans have deemed most useful to civilization.
Gas, Liquid, Solid
Where Water is Stored
You will notice in the diagram (large arrows) that there are five main places of "storage" where water in one of its three stages collects and sits:
- As a solid––water is stored as ice and snow, always where temperatures are cold: The tops of mountains, the north and south poles and countries and oceans near them (icebergs), and often mid-country as well, near mountains and lakes in winter. Water is held in that form until temperatures rise and it melts, flowing down to join one of the other storage places.
These areas are where humans enjoy "winter sports" like skiing, ice skating, and snowboarding. This kind of storage has been breaking down rapidly in recent years, with fresh water snow and ice melting more and more readily, and merging into the salty ocean.
- As a gas––water that has evaporated and risen into the air stays there as vapor and clouds, until it has cooled down enough to condense into rain. "Humidity" is the term that measures the amount of water vapor being stored in the air. Water in the air helps keep skin moist and soft.
- As a liquid––water is stored in three main places: Surface water, groundwater, and oceans:
Surface water––includes the whole category of lakes and fake lakes (dams), rivers and streams. Lakes and dams are considered storage areas, since water sits there for some time, while slowly sinking into the earth, evaporating into the sky, or running out via a river or two. Water stays in a lake long enough to grow life forms, some of which we fish out.
Groundwater––water that has sunk into the earth all the way to its rock base (groundwater basin), if it has one. The earth is like a giant sponge. It holds water until needed to replenish the surface water. Meanwhile trees, plants, and humans draw from it for their own needs.
Oceans––hold the greatest quantity of water in storage. Because it is salty, humans don't like to drink it and can't use it for manufacturing, without rusting or encrusting their machines. But these vast bodies of water, filled with life of their own, are the greatest source of evaporation. Fresh water ultimately comes from the oceans in a form of water distillation––with salty water evaporating, condensing, and falling as fresh rainwater.
Water Cycle Process
In short, these are the stages the water cycle goes through, continually cycling around, with no real beginning and no end:
- Evaporation and repeat
It's not a straightforward process. Clouds can precipitate into rain, which starts to fall, only to evaporate again before it hits the ground. Or ice can start to melt, then freeze again before it ever flows anywhere. Before we go into detail about the process, therefore, let's look at the three physical states of water and what causes those.
How Water Evaporates
Water evaporates from any surface where there is water––the ocean, lakes, dams, rivers, streams, moist earth, snow, and ice. When it heats up with the sun or hot air or lava beneath the earth, water's molecules start spinning faster and further apart, and it gets lighter in weight. Up it goes, spinning into the air, sometimes as geysers, but higher and higher as it gets hotter and turns into water vapor (gas).
Moisture is also added to the air via sweat from humans and animals, and via transpiration (plant sweat), especially from trees. All of this moisture rises into the atmosphere, spiraling up until it reaches cooler air. This is evapotranspiration.
Eventually the water vapor reaches a stasis point in the atmosphere, where the air starts to cool and the vapor stays where it is, blown about by hot air and vapor still rising, which mixes and changes places with cooler air. This movement is called wind.
Condensation of Water Vapor
As the water molecules spin and others rise to join them in the cooler air above, they begin to slow down and coalesce. The more humid the air, the faster they coalesce. At 35,000 feet, even in the heat of summer, the air can be -70C (-94F). In cold air molecules spin more slowly and, being attracted to each other, gather to form clouds. This is condensation. Ground fog is a low level condensation.
Condensation is the opposite of evaporation. Where evaporation is the change of liquid into gas, condensation starts changing gas back into liquid. All it needs to complete that process is some kind of icy core around which rain, snow, or hail can form.
World Cloud Cover
"Rain is grace; rain is the sky condescending to the earth; without rain, there would be no life." - John Updike
Precipitation into Rain, Hail, or Snow
In nature, the precipitation core is provided mainly by a bacteria called Pseudomonas syringae. This bacteria has a nucleus that is like ice, that causes water vapor to condense around it, turning water vapor into raindrops. Cooling air speeds the process, converting nascent cloud cover into storm clouds. Bacteria and storm clouds multiply and spread, until they are thick and heavy enough that gravity can pull the raindrops down from the sky.
Unfortunately, P. syringae is the same bacteria that is well known for diseases it creates on cash crops. The bacteria freezes a plant's skin to soften it, so it can drink the juices beneath, then reproduces itself to form colonies. That process leaves black marks on fruit and leaves (see photo below). Growers have been trying to eradicate the bacteria for decades.
Whether the millions of bacteria needed for rain are blown up from the earth or grow into colonies in the atmosphere, is not yet known. What we do know is that a high percentage of rain, hail, or snow contains this bacteria––around 70% according to studies by the Louisiana State University. Volcanic dust and carbon dust from wildfires can also generate precipitation at higher, colder levels of the atmosphere.
The fact that rain, ice, and snow both cool and clean the air and the earth makes ice-nucleating bacteria a key component in counteracting global warming. Deliberately growing the bacteria, in locations where it's especially needed, could provide a way of more evenly distributing rain throughout the earth.
The Flow of Water - Runnels, Rivers, & Streams
The flow stage of the water cycle describes the movement of water after it hits the ground. Rainwater saturates an area, flowing across the ground's surface to lower elevations. It fills up rivers and streams that flow to lakes and dams, and ultimately to the lowest elevation of the sea - quickly in the case of young, straight rivers and slowly, in the case of meandering ones.
Rivers fall straighter where the elevation is steeper, pulled by gravity. Older, meandering rivers slow water down, which gives it time to be absorbed by the earth it passes over. The Mississippi River used to be an old, meandering river, saturating the ground for miles and miles on either side as it flowed south. There was once plenty of water in its aquifer from Canada down to the Caribbean Sea.
Unfortunately, humans prefer straight rivers, allowing for easier and faster transportation via boats, the production of electricity, and controlled diversion for agriculture. So humans dredge crooked rivers to make them deeper, and cut paths between meanders to make them flow straighter.
This prevents the ground from absorbing rainwater, lowering the storage level of the aquifer. With no water in the aquifer to replace water that evaporates or flows to the sea, rivers and streams start to run dry. Since the Mississippi River was first dredged, straightened, and dammed, many states through which it flows have experienced droughts.
As surface water flows from the mountains and lakes through ever-lower rivers and streams out into the ocean, gravity pulls groundwater slowly toward the lower levels of rivers and streams, replenishing what goes to the ocean, where it evaporates again. This keeps the rivers and streams flowing until all the groundwater is gone . . . or until it rains.
Until man started sucking out the groundwater for his own use, and blocking its replenishment by straightening rivers and building cities, most rivers and streams in the United States stayed full most of the year.
The oceans are forever being replenished and fed by fresh water flowing down from the mountains, and the richer, saltier groundwater flowing out from the land near the oceans. Groundwater cleans the earth, collecting loose salts (and man-made chemicals) as it passes through, carrying them along to its eventual destination in the ocean. Those salts then help feed coastal ocean life, while the chemicals help kill it.
Hydrologic Cycle Quizletview quiz statistics
How Humans Affect the Water Cycle
Straightening major river systems is not the only way humans have tampered with the natural water cycle. Many other ways have already been mentioned and there are still others. Here are some of them:
Straightening rivers, so water runs straight to the sea, instead of being absorbed by the aquifer.
Blocking the earth from absorbing rainfall by building cities, and laying concrete and asphalt across the surface of it.
Cutting down forests that provide moisture to the air and cool the earth, so rain can fall. (This map shows the extent of deforestation worldwide in red.)
Using pesticides to kill the bacteria that helps create rain. Also stripping the earth of native plants upon which the bacteria can grow.
- Drying and heating the air in city areas with car exhaust and airborne pollutants from manufacturers. The rising heat pushes clouds away and the chemicals disburse whatever rain starts to form.
- Growing cattle and other meat-producing animals en masse, so their gastric emissions (burps, farts, and feces) produce quantities of greenhouse gas emissions that heat the air. This 2015 report from Skeptical Science shows that 14-18% of the earth's human-induced greenhouse gas emissions come from livestock production.
A Water Sustainable Culture
To have a sustainable culture, to live in harmony with the environment, how can humans respect and wisely utilize the water where they live? How can we replicate nature's rain cycle in areas where it currently does not rain? How can we redirect rain from areas where it rains too much?
Learning more about the rain cycle is the first step to answering these questions. Figuring out how to apply what we know is the second: Conserving water at home and work, designing water-use products wisely, changing the manufacturing processes that use water, are some applications. What ideas do you have, based on what you know now?
Water Cycle Described in Sign Language
Questions & Answers
How did a water cycle form in the sky?
Who knows how it started? It could have started with clouds raining. It could have started with an ocean evaporating. No one knows, except through hearsay––none of us were here then.
What we do know is that it is a cycle, so it continually goes round and round. It rains. The fallen rain creates rivers, which flow to the sea. The sun evaporates the sea waters, which then rise to form clouds, which then rain, form rivers, etc. all over again.