Why can't hurricanes cross the equator?

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.

How does the earth move?

Every morning we see the sun rise, move across the sky and set again in the evening. To us it looks like the sun is moving around the earth. Until the late Middle Ages, many people actually believed that the earth stood still in the middle of the universe and that everything revolved around it.

Today we know that it is exactly the other way round: We experience day and night because the earth is turning. And the earth is neither still nor in the center, but revolves around the sun.

The gravitational pull of the sun holds the earth tight, like on a long leash. More precisely: an almost 150 million kilometers long line. This is the distance at which the earth orbits the sun.

The time it takes the earth to orbit is called a year. During this time, the earth covers a distance of around 940 million kilometers. This means that it races through space at a speed of over 100,000 km / h! (That's nearly thirty kilometers per second.)

By the way, the earth's orbit is not exactly circular, but rather elongated: At the beginning of January, the earth is closest to the sun. Half a year later, at the beginning of July, the gap is greatest. The earth is then a few million kilometers further from the sun than it was in January. But this has nothing to do with the change of the seasons: the difference is so small that the amount of sunlight hardly changes. (And besides, when the earth is closer to the sun in January, it is winter here in the northern hemisphere.)

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 around 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 parallel, 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. This 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.


In August 2005, the southeastern United States experienced a disaster: Hurricane Katrina raced over the coast, killing almost 2,000 people. Like all hurricanes, Katrina was a tropical cyclone. In other regions of the world they are also called typhoon or cyclone. Storm surges, torrential rains, landslides and floods are their consequences. But how does such a hurricane come about?

A hurricane occurs where warm water evaporates and moist, warm air rises quickly and high. Cold air is sucked down to compensate. A thunderstorm is approaching. As a result of the Coriolis force, the cold and warm air masses begin to turn as if in a spiral. By rotating, they suck in even more warm, moist sea air. The cyclone is getting stronger and stronger: it can reach a diameter of several hundred kilometers and travel thousands of kilometers. Its air masses can reach speeds of up to 300 kilometers per hour. Only in the center there is no wind: that is the eye of the hurricane. It can take over a week for the storm to subside.

In order to form such a cyclone, the water must have a temperature of at least 27 ° Celsius. In addition, the Coriolis force is required, which causes the air masses to rotate. In the direction of the poles the water is too cold, in the direction of the equator the Coriolis force is too low. For this reason, hurricanes only occur in a strip in the tropics, which is roughly between the 5th and 20th parallel.

Tornadoes, also known as “tornadoes”, are smaller, but much faster than hurricanes. They form in hot and humid regions when warm and cold air meet during a thunderstorm. Like a huge trunk, they descend from a thundercloud to the ground. Inside this trunk there is very little air pressure, which sucks in the air masses and whirls them around. Such tornadoes can be very small, but can also have a diameter of up to 1.5 kilometers and are clearly visible from a distance because they pull dust and water vapor far upwards. The ghost is over after a short time.

Where the tornado races along, however, it leaves a swath of devastation. The dangerous air eddies are particularly common in the American Midwest. There is even a real “tornado street” there: because cold and warm air masses from north and south collide here unhindered, several hundred tornadoes race through this area every year.

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, seafarers used this fact: 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.

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 flow, 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.

Gulf Stream

The first seafarers to cross the Atlantic were puzzled on their return: Why was their ship faster on the route from America to Europe than the other way around? Today we know the solution: a sea current in the North Atlantic propelled the ship on its way to Europe - the Gulf Stream.

The Gulf Stream is a powerful ocean current in the Atlantic. It is up to 200 kilometers wide. The amount of water that it transports exceeds the amount of water that flows into the sea from all the rivers on earth by more than a hundred times. The Gulf Stream is fed by warm ocean currents near the equator. The Gulf Stream begins north of the Bahamas. From here it initially moves over 1,000 kilometers north along the American east coast.

Westerly winds and Coriolis force force the current northeast at North Carolina level. On its way towards Europe, the Gulf Stream continues to lose speed. It no longer moves in a straight line, but meanders forward. Parts of the stream split off and flow back. The ice-cold Labrador Current finally gets in his way from the north; the Gulf Stream continues to lose strength and heat. Evaporation increases the salt content and density of the water until the Gulf Stream finally descends east of Greenland. Parts of its water masses flow from here as deep currents in the direction of the South Atlantic and Indian Ocean.

The Gulf Stream is very important for Europe: it acts like central heating on our climate. Without its warmth, the winters in Western and Central Europe would be much harder. It is only because of him that ports in Northern Europe are ice-free all year round - except on the Baltic Sea, where the current does not reach. We owe even the fact that palm trees and lemon trees thrive on England's south-west coast to the mighty and warm Gulf Stream.