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 in wild confusion, but the winds follow a very specific pattern. This global wind system (also called planetary circulation) is influenced above all by solar radiation and the Coriolis force.

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

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 formed at this point. From here, cold air on the ground flows towards the equator. As soon as this air mass has warmed up sufficiently, it rises again: A whole series of lows develops around the 60th parallel, the subpolar low pressure trough. The air that rises here flows back up towards the pole.

Between the polar cell and the Hadley cell, approximately between latitudes 30 and 60 degrees, the air masses of the polar regions and the trade wind 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 40 and 60 degrees latitude in Europe is called the west wind zone. At altitude, the wind also comes from the west: at the border of the polar cell, strong high-altitude winds flow, which are turned by the Coriolis force and directed towards the east – the so-called jet streams.

Three large wind circuits have thus built up in each hemisphere: the Hadley cell, the Ferrel cell and the polar cell. Why there are just three has to do with the speed of the Earth’s rotation. What would happen if the earth rotated much slower can be simulated with the computer: Then the warm air would simply rise at the equator, cool down at the pole, sink again and flow back at the ground. There would be only one large wind cell in each hemisphere. But the faster you let the Earth rotate in the computer model, the more wind cells split off. When simulating the actual rotation speed of the Earth, the computer also comes to the conclusion: there are exactly three large wind cells on each hemisphere.

How does wind develop?

A fresh wind often blows on the coast. If it blows particularly strongly, there is also talk of a stiff breeze. But it is not only at the seaside – everywhere on earth air is in motion. Only in a few places on earth is there not the slightest breeze, such as in the calme zone at the equator – named after the French word for calm: “calme”. This windless region used to be feared by seafarers because sailing ships could not get off the ground for weeks at a time. But why is it that sometimes there is a calm and sometimes a violent storm sweeps over the land?

Wind is created primarily by the power of the sun. When the sun’s rays heat up the ground, the air above it also heats up. The warm air expands and thus becomes thinner and lighter: the air mass rises. This creates low pressure near the ground. Where it is cold, on the other hand, the air sinks and high pressure forms on the ground. To compensate for the difference in pressure between neighbouring air masses, colder air flows to where warm air is rising. 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 blows.

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

Which way the wind blows is always indicated by the compass direction. In our latitudes it is often from the west, we live in the so-called west wind zone. The hot trade winds, on the other hand, blow reliably 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?

Aircraft flying from New York to Frankfurt have a good tailwind. The wind that drives them blows from west to east at an altitude of about 10 kilometres. This strong air current, which can reach speeds of up to 500 km/h, is called a jet stream. Its 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 investigate it mathematically in 1835. The cause of the Coriolis force is the rotation of the earth on its own axis: at the equator, the earth rotates eastwards at 1670 kilometres per hour, and the speed continues to decrease towards the poles. When air masses flow from the equator to the north pole, they take the momentum eastwards and then move faster than the earth’s surface. Seen from the Earth’s surface, it looks like they are deflected from their northward course to the east – i.e. to the right. Conversely, air masses flowing from the pole to the equator are overtaken by the Earth’s surface, 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 their way to the Pole are deflected from their southern course to the east, i.e. to the left – just like the air masses on their northern course towards the equator, which are deflected to the west. Thus, the Coriolis force leads to a rightward deflection in the northern hemisphere and a leftward deflection in the southern hemisphere, and the closer one gets to the poles, the stronger the deflection.

In this way, the Coriolis force influences the global wind system, the major air currents on Earth. Thus it has a great influence on the weather: In our latitudes, for example, the air flows towards the North Pole and is therefore deflected towards the east. At our latitudes, for example, the air flows towards the North Pole and is therefore deflected towards the East. Here, the wind usually comes from the West, from the Atlantic, and therefore brings rather humid air with moderate temperatures. The jet streams also owe their direction to the Coriolis force.

Even tropical cyclones with a diameter of several 100 kilometres are formed with the help of the Coriolis force. This is because it causes hot, humid air to start rotating until it grows into a destructive vortex. However, 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

t is unimaginably hot inside the sun: a full 15 million degrees prevail here. At the surface of the sun it is still 5,600 degrees Celsius. This makes the sun white-hot and it appears to our eyes as a white sphere.

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 heat into space. Part of its radiation also reaches the earth. This energy warms our atmosphere, the earth’s soil and the oceans.

The sun heats up the area around the equator the most, because there its rays strike a relatively small area perpendicularly. In contrast, the sun’s rays reach the poles at a flatter angle. Here, the solar energy is therefore distributed over a larger area; and it remains cooler in these regions. In this way, the different strengths of the sun’s rays create different climatic zones. Seasons and weather are also the result of different levels of solar radiation.

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

This happens through radiation from the Earth into space. About one third of the sun’s energy is immediately reflected back by the atmosphere, land surface, bodies of water and ice masses. The Earth first 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. In the past, sailors took advantage of this fact: they aligned the routes of their sailing ships according to the wind direction. With the support of the east wind, a safe crossing from Europe across the Atlantic to North America was possible. It is from this crossing – “passata” in Italian – that the reliable winds got their name: Trade winds. Because they transport dry, hot air, they dry out the soil. Large deserts such as the Sahara in northern Africa, the Kalahari in southern Africa, the Australian deserts or the Atacama in South America lie within the range of the trade winds.

The trade winds originate at the equator. There, the sun’s rays hit the earth perpendicularly 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 on this journey, it sinks back down after a while and creates high pressure on the ground. In this way, a whole series of high-pressure areas form 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 effect on the weather in Europe.

At the equator itself, the rising air masses have created areas of low air pressure. This low pressure draws in air masses from the subtropical high pressure belt, the trade winds. However, these do not blow directly from the high to the low, but are deflected by the Coriolis force. This is why the trade winds always blow from the north-east in the northern hemisphere and from the south-east in the southern hemisphere. These trade winds meet at the equator. Due to the strong solar radiation, the air rises again so that there is almost no wind. This closes the cycle of the trade winds, which are part of a global wind pattern.

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

What are climate zones?

In the morning cloudy to partly cloudy with showers. In the afternoon the sun will shine, with temperatures between 16 and 22 degrees”, this may be the weather forecast for southern Germany. The forecast is interesting for us because the weather is constantly changing. It is different with the climate, because that remains. Climate means the average weather of 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, temperatures are freezing and there is little precipitation. Between the equator and the poles, there are areas where it can be very changeable, just like here. But why is it that the climate on Earth is so different?

The sun’s radiation is not equally strong everywhere on earth. How intensively it heats the earth depends on the angle of the sun’s radiation and thus on the latitude. Because the sun is almost vertical all year round near the equator, the earth is heated very strongly here. Towards the poles, the sun’s rays strike at an increasingly shallow angle: The same solar energy is distributed over an ever larger area. Therefore, the greater the distance to the equator, the cooler it becomes. This creates regions with different climates, the climate zones.

According to the strength of the sun’s rays, 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 the 40th parallel, the temperate zone of our latitudes and the polar regions around the North and South Poles. Like belts, these climate zones run around the earth in an east-west direction.

However, the climate does not only depend on the latitude, other influences also play a role. For example, there is snow on Mount 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. Mountain climates are therefore always cooler than lower-lying areas.

The distance to the sea also affects the climate: Water can store solar heat longer than land. It also warms up more slowly than the land. As a result, seawater acts as a buffer for temperatures. The climate is therefore mild near the coast. Inland, 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

Like huge rivers, ocean currents cross all five oceans. They transport enormous masses of water around the globe, similar to a conveyor belt. In this way, they ensure an exchange of heat, oxygen and nutrients all over the earth. Warm water from the equator flows towards the poles, cold water from the polar regions sinks to the ocean 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.

Ocean currents are driven by the different salinity and temperature of seawater. Where seawater freezes, salt is released. The seawater under a layer of ice is therefore particularly salty – and at the same time denser and heavier. It sinks to the bottom, dragging other masses of water with it. At a depth of several thousand metres, the water flows back into warmer regions. There it rises again and the cycle closes.

At the water surface, additional winds set the water in motion. The wind causes 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 steers the water to the right, as seen in the direction of the current, and in the southern hemisphere it steers it to the left. Winds are also influenced by the Coriolis force.

The various influences, such as temperature differences of the water, wind and the Coriolis force, create a pattern at the surface and in the depths of the oceans that is made up of many individual currents: a worldwide cycle that is also called the “global conveyor belt”.