What is wind
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. 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. This happens all the faster, the greater the temperature difference between the air layers. 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 over the land warms up faster than over the 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 easterly winds transport icy air masses from the pole to the arctic circle.
With wind speeds of over 200 kilometers per hour, Kyrill raged in Europe on Friday night. Peak values of the hurricane were measured on the Feldberg in the Black Forest and on the Brocken in the Harz Mountains. 47 people were killed in the hurricane and many were injured. The damage is estimated at billions.
It was the worst hurricane since Lothar: Kyrill uprooted trees, covered roofs and smashed cars. Several people were killed by falling trees, and over a hundred were injured. The power grid collapsed in numerous regions. At Berlin Central Station, hurricane gusts tore a ton-heavy piece of iron from the facade. The station had to be evacuated, nobody was injured. Everywhere in Germany the fire brigade and police were in constant use, in many places there were disaster alarms.
The rail traffic had to be stopped at times. Overhead lines were broken, fallen trees blocked the tracks. Kyrill also completely mixed up air and shipping traffic. The hurricane hit the German North Sea coast less badly than expected, and the feared storm surge did not materialize. In contrast, the forest in North Rhine-Westphalia suffered severe devastation. Millions of trees have been knocked over or torn from the ground with their roots. Kyrill also left a swath of devastation in Great Britain, France and the Netherlands.
On Friday morning the authorities gave the all-clear, the storm and storm surge warnings were lifted. After the hurricane chaos, the clean-up work is now in full swing.
Storms of the century Kyrill and Lothar
The images are the same: shaved forests, bent electricity pylons, crushed cars. In December 1999, hurricane Lothar swept across Central Europe even more violently than Kyrill. Especially in northern France, Switzerland, southern Germany and Austria, Lothar reached top speeds of 270 kilometers per hour and caused the worst storm damage: 110 people died; total damage amounting to more than 6 billion US dollars was caused. And now another storm of the century with Kyrill?
Climate researchers suspect that such violent winter storms will occur even more frequently in the future due to climate change. Because the storms that sweep over Europe in the winter months have their origin over the North Atlantic. The warming of the oceans now ensures that more water evaporates there. This in turn favors the formation of hurricane lows like Lothar and Kyrill.
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.
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 delimited 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.
The effect of sunlight
It's unimaginably hot inside the sun: it's 15 million degrees. 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 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.
Wind strength and wind speed
When smoke can rise vertically and there is hardly a breath of air to be felt, then there is no wind. In a hurricane, on the other hand, the wind is so violent that it pulls heavy objects with it. Wind can vary in strength - and the strength of the wind is indicated on the “Beaufort scale”, which ranges from wind force 0 with complete calm to a hurricane with wind force 12.
The scale is named after the British Sir Francis Beaufort, who used a similar scale a good 200 years ago. At that time, the wind strength was determined by, for example, observing the height of the waves on a ship or the effect of the wind on the sails and then reading off the appropriate wind strength in a table. Today, every wind strength has a certain wind speed. For example, wind force 0 means that the wind is blowing less than one kilometer per hour. So it is imperceptible - there is no wind. If, on the other hand, the wind has a speed of 39-49 kilometers per hour, i.e. almost as fast as a car drives in the city, then large branches are already moving. Such a strong wind has wind force 6. At wind speeds of more than 62 kilometers per hour, there is talk of a storm. And a hurricane is on the way when the wind speed exceeds 118 kilometers per hour: This corresponds to the highest value on the scale, wind force 12. In this case, severe devastation is to be expected.
The strongest wind that has ever been measured on the earth's surface was blowing in April 1996 at a whopping 408 kilometers per hour over the island of Barrow Island in Western Australia. Such a violent storm can blow railroads off the rails and collapse buildings like houses of cards. The storm also wreaked havoc on Barrow Island.
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 cover 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 lies approximately 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 they 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.
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.
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, the winds follow a very specific pattern. This global wind system (also called planetary circulation) is mainly influenced by the radiation from the sun and 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 latitude 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 be only 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.
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