Millennium flood causes Dresden to sink

Heavy flooding and huge mudslides wreak havoc throughout Saxony. After the torrential rains of the past weeks, a huge flood wave rolls down the Elbe.

Tens of thousands of helpers, including Bundeswehr soldiers, try to stop the masses of water with sandbags. The water of the Elbe has risen from 2 metres to a destructive 9 metres. Despite tireless rescue efforts, numerous dams have already broken. The Saxon capital Dresden and many towns on the Elbe and its tributaries are completely cut off from the outside world by the floods. Dresden’s main railway station and large parts of the city centre are under water, bridges are closed and the power supply is interrupted. Thousands of people had to be evacuated.

We have the summer low-pressure area “Ilse” to “thank” for the devastating flood disaster. Its air masses moved from southern Europe over the Alps towards the north. When crossing the Alps, these very humid air masses cooled down strongly. This resulted in extremely heavy rainfall in a short time, which caused the millennium flood.

There’s the worm in it!

Earthworms can protect against floods. How? By the invertebrate burrowers perforating the soil! Wherever they burrow through the soil, a loose system of cavities is created. And in these cavities, vast amounts of rainwater can be stored – like in a sponge. This prevents too much water from running off at once and causing large-scale flooding. It’s just a pity that the earthworm is tilling less and less land in this country. Because preventive disaster control by the worm is much cheaper than technical means used against floods.

Precipitation

No matter whether it rains, hails or snows – clouds are “to blame”. Because without clouds there would be no precipitation. However, it depends above all 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 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 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 particles of dust or soot. 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 a whitish blur. If you can see less than a kilometre through this white haze, it is called fog.

High and low – the air pressure

The earth has a thick wrapping of air, the atmosphere. We only notice this envelope of air when it moves. Then we feel a fine breeze or a strong wind. But although it seems weightless to us, this air has quite a lot of weight: a whole kilo of air presses down on every single square centimetre of earth. If you calculate what this weighs on our shoulders, the result is astonishing: several hundred kilograms! The fact that we are not compressed under this weight is due to the counterpressure that our body generates.

Due to its weight, the air therefore exerts a pressure on the earth’s surface: the air pressure. The further you move away from the earth’s surface, the lower it becomes. This can be clearly felt in the ears when sitting in an aircraft that is rising or sinking.

But not only the altitude, also the temperature affects the air pressure. This is because warm air expands, is lighter and rises: The air pressure at ground level falls. Cold air, on the other hand, is heavier and falls: The air pressure near the ground rises. If the air masses are heated differently at different places on earth, areas with high and areas with low air pressure are formed: the high and low pressure areas. In the high-pressure areas, the air masses sink and warm up. Clouds dissipate, the sky is blue and the sun shines. Low-pressure areas, on the other hand, cause bad weather: as the warm, humid air rises, clouds form as it cools at altitude and it can rain.

On weather maps, the high and low pressure areas are marked with the letters H for high and T for low. Areas with the same air pressure are delimited from each other on the maps by lines, the so-called isobars.

The wind balances out the pressure differences between high and low: It always blows from the high-pressure areas in the direction of the low. Because it is deflected by the Coriolis force, the air masses cannot flow directly from the high to the low. Instead of flowing in a straight line, they make a serpentine line. In the northern hemisphere, they turn to the right and therefore circle the high in a clockwise direction and the low in a counterclockwise direction. In the southern hemisphere it is the other way round.

The cycle of the water

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, tiny water droplets gather at high altitudes 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.