The space flows like air currents

The global wind system

The air masses of the atmosphere flow around the globe: They rise and fall, meet and mix. However, this does not happen wildly, but the winds follow a very specific pattern. This global wind system (also called planetary circulation) is influenced primarily by radiation from the sun and by the Coriolis force.

The tireless cycle of air begins at the equator, where warm air rises constantly. A whole chain of low pressure areas, the so-called equatorial low pressure trough, forms on the ground. The ascended air moves at a great height towards the poles. Because it cools down on the way, it sinks again in the subtropics at about 30 ° north and south latitudes and flows back on the ground as a trade wind towards the equator. The entire wind cycle around the equator was described by the English scientist George Hadley as early as 1753 and is therefore called the "Hadley cell". (Meteorologists call a "cell" a circular flow of air.)

Air masses also circulate around the poles and form the two “polar cells”: Because cold air sinks to the ground at the pole, a high pressure area is created at this point. From here, cold air flows on the ground towards the equator. As soon as this air mass has warmed up sufficiently, it rises again: A whole series of lows arise around the 60th parallel, the subpolar low pressure trough. The air that rises here flows back up to the pole.

Between the polar cell and the Hadley cell, roughly between the 30th and 60th degrees of latitude, the air masses of the polar regions and the Passat Zone meet: this is where the third large wind cell has spread. It is also called the "Ferrel cell" after its discoverer, the American William Ferrel. Because cold and warm air masses meet in this region, the weather here is often changeable and rainy, which we know well in Central Europe. The wind comes predominantly from the west. That is why the region between the 40th and 60th parallel is called the west wind zone in Europe. The wind also comes from the west at high altitudes: At the border to the polar cell, strong high-altitude winds flow that are turned by the Coriolis force and directed to the east - the so-called jet streams.

So three major wind cycles have built up on each hemisphere: the Hadley cell, the Ferrel cell and the polar cell. Why there are just three is related to the speed of the earth's rotation. What would happen if the earth rotated much more slowly can be simulated with the computer: Then the warm air would simply rise at the equator, cool down at the pole and flow back on the ground. There would only be one large wind cell in each hemisphere. However, the faster the earth is rotated in the computer model, the more wind cells split off. When simulating the actual rotational speed of the earth, the computer also comes to the conclusion that there are exactly three large wind cells in each hemisphere.

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, like 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. In contrast, where it is cold, 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 formation 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 land to 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 easterly winds transport icy air masses from the pole to the arctic circle.

What is the Coriolis Force?

Airplanes 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 current 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 earth's surface. 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.

The effect of sunlight

Inside the sun it is unimaginably hot: a total of 15 million degrees prevail here. After all, it is still 5,600 degrees Celsius on the surface of the sun. This means that the sun is incandescent and appears to our eyes as a white ball.

Without the sun there would be no life on this planet, at least not as we know it today. The sun is a gigantic source of energy that radiates light and warmth into space. Some of their radiation also reaches the earth. This energy warms our atmosphere, the earth and the oceans.

The sun heats up the area around the equator the most, because there its rays hit a relatively small area perpendicularly. The poles, on the other hand, reach the sun's rays at a flatter angle. Here the solar energy is therefore distributed over a larger area; and in these regions it stays cooler. The different levels of solar radiation ensure different climate zones. Seasons and weather are also the result of different levels of solar radiation.

If the earth were to store all of the solar energy, it would be unbearably hot here in no time. This can already be felt on a hot summer's day when the temperature climbs to 30 degrees Celsius in a very short time after sunrise. In order for the climate to remain stable for centuries, the earth has to get rid of about the same amount of solar energy.

This happens through the radiation of the earth into space. About a third of the solar energy is immediately reflected back from the atmosphere, land area, bodies of water and ice masses. The earth initially absorbs the rest of the energy in the form of heat. It then slowly releases this heat back into space in all directions.

Trade winds

There are areas on earth where the wind always blows from the same direction. In the tropics, for example - the region around the equator - trade winds blow from the east. Seafarers used this fact in the past: They set the routes of their sailing ships according to the direction of the wind. With the support of the east wind, a safe crossing from Europe across the Atlantic to North America was possible. From this crossing - in Italian "passata" - the reliable winds got their name: trade winds. Because they transport hot, dry air, they dry out the soil. In the area of ​​the trade winds there are large deserts such as the Sahara in northern Africa and the Kalahari in southern Africa, the Australian deserts or the Atacama in South America.

The trade winds have their origin at the equator. There the rays of the sun hit the earth vertically and heat the air very strongly. The air masses expand and rise. At the top they spread out in the direction of the tropics. Because the air cools down on this journey, it sinks back down after a while and creates high pressure on the ground. A whole series of high pressure areas are formed at about 30 ° north and south latitude: the subtropical high pressure belt. This subtropical high pressure belt includes, for example, the Azores high, which has a strong impact on the weather in Europe.

At the equator itself, the rising air masses have created areas with low air pressure. Due to this negative pressure, air masses are sucked in from the subtropical high pressure belt, the trade winds. However, these do not blow directly from high to low, but are deflected by the Coriolis force. That is why the Passat always blows from the northeast in the northern hemisphere and from the southeast in the southern hemisphere. These trade winds meet at the equator. Due to the strong sunlight, the air rises again so that there is almost no wind. This is where the cycle of trade winds, which are part of a global wind pattern, closes.

Because the position of the sun changes over the course of a year, the location of the strongest solar radiation also shifts. This shifts the entire Passat circulation by a few degrees of latitude between north and south.

What are climate zones?

“In the morning it is changing to very cloudy with showers. In the afternoon the sun shows up at temperatures between 16 and 22 degrees ”, this is perhaps the weather report for southern Germany. The forecast is interesting for us because the weather is constantly changing. The situation is different with the climate, because that remains. Climate is the average weather in a region over a longer period of time. For example, the climate at the equator is hot and humid all year round. At the North Pole, on the other hand, the temperatures are icy and there is little precipitation. Between the equator and the poles there are again areas where, like us, things can be very changeable. But why is it that the climate on earth is so different?

The sun's radiation is not equally strong all over the world. How intensely it warms the earth depends on the angle of the sun's rays and thus on the latitude. Because the sun near the equator is almost vertical all year round, the earth is very heated here. In the direction of the poles, the rays of the sun strike at an increasingly flat angle: the same solar energy is distributed over an ever larger area. Therefore, the greater the distance from the equator, the cooler it becomes. This creates regions with different climates, the climatic zones.

According to the strength of the solar radiation, four different climate zones can be divided on the mainland of the earth: The tropics around the equator, the subtropics (from the Latin word “sub” for “under”) between the 23rd and 40th parallel, the temperate Zone of our latitudes and the polar regions around the north and south poles. Like belts, they draw these climatic zones around the earth in an east-west direction.

The climate does not only depend on the latitude, other influences also play a role. There is snow on Kilimanjaro, even though it is in the tropics. The fact that its summit is icy is due to the fact that the temperature drops with increasing altitude. The mountain climate is therefore always cooler than lower lying areas.

The distance to the sea also has an impact on the climate: water can store solar heat longer than the mainland. It is also warming up more slowly than the country. As a result, the sea water acts as a buffer for temperatures. The climate is therefore mild near the coast. In the interior of the country, this heat balance is missing and the climate is continental, with temperatures fluctuating much more than in the maritime climate near the sea.

Ocean currents

Ocean currents traverse all five oceans like giant rivers. They transport huge amounts of water around the globe, similar to a conveyor belt. In doing so, they ensure an exchange of heat, oxygen and nutrients all over the world. Warm water from the equator flows towards the poles, cold water from the polar regions sinks to the sea floor and flows back to the equator. This cycle balances the temperatures in the water and on land. Icebergs, ships or rubbish can also be transported by the current.

The ocean currents are driven by the different salinity and temperature of the sea water. Where sea water freezes, salt is released. The sea water under a layer of ice is therefore particularly salty - and at the same time denser and heavier. It sinks down and pulls more water with it. At a depth of several thousand meters, the water flows back into warmer regions. There it rises again and the cycle closes.

On the surface of the water, winds also set the water in motion. The wind creates a current on the surface. This current does not move exactly in the direction of the wind, but is deflected by the Coriolis force: In the northern hemisphere, the Coriolis force directs the water to the right when viewed in the direction of the current, and to the left in the southern hemisphere. The winds are also influenced by the Coriolis force.

The various influences, such as temperature differences in the water, wind and the Coriolis force, create a pattern on the surface and in the depths of the oceans that is composed of many individual currents: a worldwide cycle, also known as the "global conveyor belt" becomes.