# On what do clouds condense

### High and low - the air pressure

The earth has a thick packaging of air, the atmosphere. We only notice this atmosphere when it is moving. Then we feel a fine breeze or a strong wind. But although it seems weightless to us, this air has a lot of weight: a whole kilo of air presses on every single square centimeter of earth. If you calculate what this puts on our shoulders, the result is astonishing: It's several hundred kilograms! The fact that we are not compressed under this weight is due to the counter pressure that our body creates.

Due to its weight, the air exerts a pressure on the earth's surface: the air pressure. The further one moves away from the surface of the earth, the lower it becomes. This can be clearly felt in your ears when you are sitting in an airplane that is ascending or descending.

But not only the altitude, the temperature also has an effect on the air pressure. Because warm air expands, is lighter and rises: The air pressure on the ground drops. Cold air, on the other hand, is heavier and falls down: the air pressure near the ground rises. If the air masses are heated differently in different places on earth, areas with high and areas with low air pressure arise: the high and low pressure areas. In the high pressure areas, the air masses sink and warm up. Clouds dissolve, the sky is blue and the sun is shining. Low pressure areas, on the other hand, cause bad weather: When the warm, humid air rises, clouds form when it cools down and it can rain.

The high and low pressure areas are marked on weather maps with the letters H for high and T for low. Areas with the same air pressure are separated from one another on the maps by lines, the so-called isobars.

The wind compensates for the pressure differences between high and low: from the high pressure areas it always blows in the direction of the low. Because it is deflected by the Coriolis force, the air masses cannot flow directly from high to low. Instead of flowing straight as a bolt, they create 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 an anti-clockwise direction. In the southern hemisphere it is exactly the opposite.

### A shell made of gas

Seen from space, it appears like a fine bluish veil that surrounds the earth: the atmosphere. It is the envelope of air that surrounds our planet. Compared to the diameter of the earth, this shell is quite thin: if the earth were the size of an apple, the atmosphere would be about the thickness of its shell.

Without the atmosphere there would be no life on this planet, because plants, animals and humans need air to breathe. It protects us from the cold and from harmful radiation from space. It also lets meteorites burn up before they can hit the surface of the earth. This atmosphere is vital for us - but what is it actually made of?

The atmosphere is a mix of different gases. A large part of this gas mixture is nitrogen: At 78 percent, that's almost four fifths of the entire atmosphere. Only 21 percent consists of oxygen, which we need to breathe. The remaining one percent is made up of various trace gases - gases that only occur in traces in the atmosphere. These trace gases include methane, nitrogen oxides and, above all, carbon dioxide, or CO for short2 called. Although the CO2-Proportion is quite low, this trace gas has a tremendous impact on our earth's climate. This can be seen in the greenhouse effect, which is heating up our planet.

The fact that the earth has an atmosphere at all is due to gravity. It holds the gas molecules on earth and prevents them from simply flying out into space. In fact, the air becomes thinner and thinner with increasing altitude and thus decreasing gravity. Even at 2000 meters above sea level, this can become uncomfortable for people: He suffers from altitude sickness with shortness of breath, headaches and nausea. Extreme mountaineers who want to climb high peaks such as the 8000m high in the Himalayas therefore usually take artificial oxygen with them on their tour.

### How is wind created?

A fresh wind often blows on the coast. If it blows particularly hard, there is also talk of a stiff breeze. But not only by the sea - air is in motion all over the world. Only in a few places on earth does not the slightest breeze blow, as in the Kalmenzone at the equator - named after the French word for calm: "calme". This windless area was previously feared by seafarers, because the sailing ships stayed there for weeks. But why is it that sometimes there is calm and sometimes a violent storm sweeps across the country?

Wind is mainly created by the power of the sun. When the sun's rays heat up the ground, the air also warms up. The warm air expands and thus becomes thinner and lighter: the air mass rises upwards. This creates low pressure near the ground. Where it is cold, on the other hand, the air sinks and high pressure builds up on the ground. In order to equalize the pressure difference between neighboring air masses, colder air flows where warm air rises. The greater the temperature difference between the air layers, the faster this happens. This is how the air gets into action - a more or less strong wind is blowing.

The development of wind at the sea can be observed particularly well. During the day, the air warms up faster over land than over water. The warm air masses rise and suck in the cool and heavy air over the sea: The wind blows from the sea to the land. At night the wind changes direction. Because the water stores the heat longer than the land, the air above it is even warmer and rises. Then the wind blows from the land to the sea.

Where the wind blows from is always indicated with the direction of the compass. In our latitudes this is often from the west, we live in the so-called west wind zone. The hot trade winds, on the other hand, reliably blow from the east towards the equator. And the polar east winds transport icy air masses from the pole to the arctic circle.

### How do clouds form?

How clouds form can be observed particularly well on cold winter days: when you breathe out, the mouth steams - a whitish veil hangs in the air. It forms when the moist, warm air you breathe meets colder air. Because warm air can store a lot of moisture - significantly more than cold air. If the warm air cools down, it can no longer absorb as much water. The excess water then collects into small water droplets that float in the air and become visible as a white veil. It is very similar with the "real" clouds.

The power of the sun heats the land and the water on the surface. The heat turns part of the liquid water into gaseous water: it evaporates. Because warm air is lighter than cold air, it rises. If the warm, humid air cools further upwards, the excess water collects as droplets around tiny dust or soot particles. It is also said that the water condenses. The drops are still so small and light that they float in the air. A cloud has arisen.

So clouds always form when warm air cools down. This can happen when the ground and the air above it warms up and rises. Even if the wind drives the air up a mountain, warmer air is forced upwards. At altitude it cools down, clouds form. The same thing happens when a zone of warm air meets a zone of cold air. The cold air lets the lighter warm air rise and clouds form again!

But it doesn't rain immediately from every cloud. Only when the water droplets combine to form larger drops due to the movement of air and are heavy enough, they fall back on the earth as rain. If the temperature is below 0 ° Celsius, the drops freeze into ice crystals. Then the precipitation falls as snow, in thunderclouds also as small sleet or as large hailstones.

There are also clouds that form just above the surface of the earth. This often happens in autumn when the air continues to cool. The whole landscape then appears blurred whitish. If you can see less than a kilometer through this white haze, it is called fog.

### What is the Coriolis Force?

Planes flying from New York to Frankfurt have a lot of tailwind. The wind that drives them blows from west to east at a height of about 10 kilometers. Jetstream is the name of this strong air flow that can reach speeds of up to 500 km / h. Their direction is the result of the so-called Coriolis force.

It is named after the French scientist Gaspard Gustave de Coriolis, who was the first to examine it mathematically in 1835. The cause of the Coriolis force is the rotation of the earth around its own axis: At the equator, the earth rotates at 1670 kilometers per hour to the east; in the direction of the poles, the speed continues to decrease. When air masses flow from the equator to the North Pole, they take the momentum to the east and then move faster than the surface of the earth. Viewed from the surface of the earth, it looks as if they are diverted from their north course to the east - i.e. to the right. Conversely, air masses that flow from the pole to the equator are overtaken by the surface of the earth, so they are deflected on their southward course to the west - also to the right.

On the way to the South Pole, the directions are reversed: Air masses on the way to the Pole are diverted from their south course to the east, i.e. to the left - just like the air masses on the north course towards the equator, which are diverted to the west. So the Coriolis force leads to a right deflection in the northern hemisphere and a left deflection in the southern hemisphere, the stronger the closer you get to the poles.

In this way the Coriolis force influences the global wind system, the great air currents on earth. It therefore has a major influence on the weather: In our latitudes, for example, the air flows towards the North Pole and is therefore deflected to the east. With us, the wind mostly comes from the west, from the Atlantic, and therefore brings more humid air with moderate temperatures. The jet streams also owe their direction to the Coriolis force.

Even tropical cyclones several 100 kilometers in diameter are created with the help of the Coriolis force. Because through them, hot, humid air begins to rotate until it grows into a destructive vortex. The Coriolis force not only affects large air masses, it also deflects ocean currents. This explains why the warm Gulf Stream drifts to the right on its way north and heats large parts of Northern Europe.