The cycle of water

The water on earth is always on the move. Huge amounts of it are constantly moving – between sea, air and land – in an eternal cycle in which not a drop is lost.

The motor of the water cycle is the sun: it heats the water of the oceans, lakes and rivers so much that it evaporates. Plants also release water vapour into the atmosphere through tiny openings. The moist air rises upwards, tiny water droplets gather in the air and form clouds. As rain, hail or snow, the water falls back into the sea or onto the earth. If it falls to earth, it seeps into the ground, feeds plants or flows through the ground, via streams and rivers back into the sea. The eternal cycle of evaporation, precipitation and runoff starts all over again.

The water cycle has existed almost as long as the earth has. It ensures that living beings on our planet are supplied with fresh water. And not only that: without the water cycle, the weather as we know it would not even exist.

What water can do

No matter whether we drink tap water, jump into a lake or are surprised by a downpour – we constantly come into contact with water. And not only that: we ourselves are made of water, in fact about two-thirds of it. Without question, water is part of our everyday life. But what seems quite normal to us has all kinds of peculiarities. And water owes these above all to its structure.

Everything that exists on this earth is made up of tiny building blocks, the atoms. This is also the case with pure water: it is a compound of two hydrogen atoms and one oxygen atom. These combine to form a water molecule, or H2O for short. The individual water molecules are only loosely connected to each other.

This loose cohesion ensures that the bond between the molecules breaks down at high temperatures: the water evaporates. If, on the other hand, it cools down strongly, the molecules arrange themselves into a solid, regular lattice, the ice. The special thing about it: In solid form, water has a larger volume than in liquid form

The arrangement of the water molecules provides yet another property: the surface tension of the water. Because of this tension, water spiders and water striders can walk effortlessly on a pond. But water can do even more: it is able to dissolve substances. Small grains of salt or sugar dissolve completely in water. Sea water, for example, contains large amounts of salt that we can taste but not see.

The fact that lemons ripen on the island of Mainau on Lake Constance is thanks to another ability of water: it can store heat. Lakes or seas heat up in summer and retain the heat for a long time. That is why temperatures fluctuate less on the coast than inland. Far from the coast, the temperature differences between day and night and between summer and winter are much greater than near the sea.

The blue planet

Seen from space, the earth’s sphere appears a strong blue. This is because almost three quarters of the earth is covered with water. Although water is transparent in small quantities, from a certain depth it takes on an increasingly strong blue shimmer. Because we see the mighty oceans as blue, the Earth is also called “the blue planet”. The term is especially true south of the equator. This is because the southern hemisphere is almost completely covered by sea, because a large part of the continents have migrated northwards due to plate movement.

The vast oceans contain almost all the water on earth. A lot of salt is dissolved in seawater, which is why it is not suitable as drinking water. What little fresh water there is on earth is mainly frozen in glaciers and ice caps. Only a tiny fraction of freshwater is found in groundwater, lakes and rivers or in the air.

But the view from the outside is deceptive: the Earth’s surface is largely covered by water, but measured against the diameter of the Earth, the oceans are only a wafer-thin layer. Therefore, the water makes up only a fraction of the Earth’s mass. By comparison, if the Earth were the size of a basketball, all the Earth’s water would fit into a ping-pong ball. And the drinking water would be proportionally even smaller than a single popcorn.

Water as a quick-change artist

Water is known to be liquid. However, this is not always true. In nature, water occurs in three states: As liquid water, as gaseous water vapour or as solid ice. Depending on external conditions, it changes from one state to the other.

The state of water depends on its pressure and temperature. If liquid water exceeds the boiling point, it evaporates and floats in the air as gaseous water vapour. Water also changes into a gaseous state when it evaporates at room temperature. However, this happens more slowly than during evaporation. If, on the other hand, the temperature drops below 0° Celsius, the water freezes into ice. As soon as water changes its state between liquid, gaseous or frozen, it changes its properties.

The special thing about water is that it has its greatest density at 4° Celsius and takes up very little space. When it freezes into solid ice, it expands and increases its volume. At the same time, its density decreases. That is why ice is lighter than water with the same volume. This is why icebergs can float in the sea. For the same reason, a lake freezes over from above and not from below in winter. This is a good thing, because otherwise we would not be able to skate until the lake was completely frozen from the bottom to the surface.

So water expands when it freezes. If you prevent it from doing so, it exerts enormous pressure. Anyone who has ever left a bottle of water outside in the freezing cold knows the consequences: After some time, the bottle bursts and the ice spills out. Ice can also crack stone in this way. This happens when water flows into cracks in the rock, freezes there and pushes outwards due to the expansion. When this force causes pieces of the stone to break off, it is called frost blasting. Anyone who has ever driven into a pothole knows the consequences. Here, the constant alternation of wetness and frost has taken its toll on the asphalt.

How do clouds form?

How clouds are formed can be observed particularly well on cold winter days: When you exhale, steam comes out of your mouth – a whitish haze hangs in the air. It forms when the moist, warm air we breathe meets colder air. This is because warm air can store a lot of moisture – significantly more than cold air. When the warm air cools down, it can no longer absorb as much water. The excess water then collects to form small water droplets that float in the air and become visible as a white veil. It is quite similar with the “real” clouds.

The power of the sun heats the land and the water on the surface. The heat turns some of the liquid water into gaseous water: it evaporates. Because warm air is lighter than cold air, it rises. If the moist warm air cools down more and more towards the top, the excess water collects as droplets around tiny dust or soot particles. This is also called water condensation. The droplets are still so small and light that they float in the air. A cloud has formed.

Clouds therefore always form when warm air cools down. This can happen when the ground and the air above it warms up and rises. Also, when the wind drives the air up a mountain range, warmer air is forced upwards. At altitude, it cools down and clouds form. The same happens when a zone of warm air meets a zone of cold air. The cold air causes the lighter warm air to rise and clouds form again!

But it does not rain immediately from every cloud. Only when the water droplets combine into larger drops due to air movement and are heavy enough, do they fall back to earth as rain. If the temperature is below 0° Celsius, the drops freeze into ice crystals. The precipitation then falls as snow, or in the case of thunderclouds as small graupel or large hailstones.

There are also clouds that form directly above the earth’s surface. This often happens in autumn, when the air cools down more and more. The whole landscape then appears blurred white. If you can see less than a kilometre through this white haze, it is called fog.

Precipitation

No matter whether it rains, hails or snows – clouds are “to blame”. Because without clouds there would be no precipitation. However, it depends mainly on the temperature whether there is a downpour or wild snow.

Most precipitation on earth falls as rain. When small water droplets collide in a cloud, they join together to form increasingly larger and heavier drops. If they are too heavy to continue floating; if the temperature is above 0° Celsius, they fall to earth as rain.

At very low air temperatures, precipitation no longer falls as rain but as snow. The snowflakes grow from hexagonal ice crystals that stick together in very cold clouds due to water droplets. If the ice formations are large and heavy enough, they dance down from the sky as snowflakes.

If, on the other hand, strong updrafts move through a cloud that is piled high, hail can occur. Small drops from the lower part of the cloud are whirled upwards, where it is colder than below. There they freeze into small ice balls, about the size of pinheads. These ice pellets are called sleet. In a very high thundercloud, when the wind is strong and the pellets are tossed up and down in the cloud several times, more and more raindrops freeze to the pellets. The more the ice beads float around in the cloud, the larger and harder they become. From half a centimetre in diameter, these ice balls are called hail. Hailstones can become larger than tennis balls and have often caused a lot of damage.

In contrast to precipitation that falls from clouds, there is also precipitation that occurs close to the earth’s surface. When the temperature on the ground drops overnight, the air can absorb less moisture. Then the excess water settles on the ground, on plants or on objects: The moisture precipitates visibly as dew. If the temperature falls below 0° Celsius at night, the water freezes on the objects and forms a whitish layer. This is no longer referred to as dew, but as frost.

How is groundwater formed?

Like water in a sponge, groundwater collects in small and large cavities under the earth. It is formed when rainwater or melt water seeps into the ground or when water from streams, rivers or lakes flows through fissures into the subsoil.

Depending on whether the soil consists of loose sand or dense earth, the water travels downwards faster or slower. And it is only when the downward flowing water hits an impermeable layer of rock such as clay that the seepage is stopped. Then, above the impermeable layer, the groundwater collects in the cavities of the soil and is stored there. If the layer of “waterproof” rock tilts, then the groundwater itself flows downslope towards nearby streams and rivers. The places where the groundwater emerges again at the surface are called springs.

When it rains very heavily or a large amount of snow melts, more water accumulates on the earth’s surface in a short time than can seep away. The water then accumulates. If it cannot drain off quickly enough, flooding occurs.

As the groundwater flows through the different layers of the earth, it is filtered and purified. This is why drinking water can be obtained particularly well from groundwater.

Groundwater is part of the water cycle. It can remain inside the earth for less than a year, but also for several million years. Under the Sahara, for example, researchers have discovered groundwater that has been under the desert sand for many thousands of years.

From trickle to stream – flowing waters

Bubbling groundwater emerges from a spring and flows down the slope as a thin trickle or a small stream: a watercourse has been created. All flowing waters start out small. On their way towards their mouths, they merge with other watercourses and continue to grow until they have become a river or even a broad stream. At its lower end, the watercourse flows into another river, a lake or the sea.

Streams, rivers or creeks – terms that come to our lips fluidly are precisely distinguished from one another by scientists (geographers). They can be classified by their water volume, their length or their width: If the watercourse is less than half a metre wide, it is called a trickle; if it is more than 2 metres wide, it is called a stream. If the watercourse swells to a width of 10 metres, it is a river. And if it gets even wider, the river can be called a stream. The Amazon or the Nile, for example, are called rivers, but the Rhine and the Danube are also streams.

The amount of water in the flowing water increases from the source to the mouth. Nevertheless, it flows slower and slower downhill. This is because the slope down which it flows is steeper at the top than at the bottom. And because the water flows faster at the top and slower downstream, it can carry more sand and debris along the upper course than the lower. Thus, more sand and debris is removed from the upper course of a river, and more is deposited in the lower course.

Delta – watercourse between river and sea

Mighty and sluggish, the Nile pours into the Mediterranean. Like all large rivers flowing through a plain, the African stream slows down towards its mouth. From the slow flow, the cargo of eroded debris and sand sinks to the bottom and is deposited. With these deposits, the river builds its own obstacle that it has to flow around. The result is a finely branched branching of sandbanks, scree slopes and river arms that becomes wider and wider towards the mouth. From the air, this widely branched network looks like a triangle. Because of its shape, it is called a delta – after the Greek letter of the same name.

Over time, the river accumulates layer upon layer of sediment. The course of the river buries its own mouth and the delta protrudes further and further into the sea: the river lengthens. This process is particularly visible on the Nile. Its delta begins near Cairo and is now 160 kilometres long and 240 kilometres wide at the coast. And the Nile delta is getting bigger and bigger: shaped like a fan, it is constantly growing further into the Mediterranean.

For a delta to develop, other conditions must be met. The coast must be flat, the tides and the ocean current must be low, because only then will the sediments not be immediately transported away again by the moving seawater. The right conditions prevail, for example, in the lower reaches of the rivers Po or Danube. Both rivers flow into the shallow sea in a delta.