How does a hurricane come about


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 humid 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 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.


It's one of the worst natural disasters in the United States. Hurricane Katrina hit the American Gulf Coast on Monday morning (August 29, 2005). At 280 kilometers per hour and accompanied by torrential rain, the tropical cyclone hit the coast and the city of New Orleans.

Katrina's gusts of wind simply swept people, cars and houses away. The US states of Florida, Alabama, Georgia, Mississippi and Louisiana suffered severe damage from the hurricane. Worst of all, however, is the city of New Orleans, with a population of just under 500,000. In the afternoon, the city's dams, some of which are below sea level, broke at 150 meters. The masses of water shot into the center and flooded a good 80 percent of the area. Entire residential areas are now completely under water. However, due to the power outages, the floods cannot be pumped out. An airport has also been flooded and had to be closed. Many access roads are closed - New Orleans is almost completely cut off from the outside world. The rescue measures are in full swing, but disaster control is completely overwhelmed. The city is sinking into brackish water and chaos. Looting and acts of violence have already been reported.

The warnings of the storm came too late for many. Only a million people were able to leave the New Orleans area in time. Thousands of people who could no longer escape are now stuck in the "Louisiana Superdome" football stadium and hope that the masses of water will drain away soon. It is not yet known how many people lost their lives to Katrina. It is expected that damage will run into billions.

Where did Katrina come from?

A tropical storm developed over the Bahamas on August 23, which meteorologists named "Katrina". The storm grew into a hurricane and hit the southern tip of Florida on August 25, killing nine people. Overland, Katrina weakened slightly. However, shortly after reaching the Gulf of Mexico, Katrina regained momentum and became a hurricane again. Katrina reached her maximum strength and hit the south coast of Louisiana on the morning of August 29 with wind speeds of 280 km / h. This made it one of the worst storms ever recorded in the Gulf of Mexico - with devastating consequences. Even meteorologists hadn't expected such a destructive force.


A tornado razed the small town of Joplin in the US state of Missouri on Sunday. Over 100 people were killed, hundreds were injured, and many are still missing. The cyclone swept through the middle of the city, aerial photographs show a ten kilometer long and one kilometer wide swath of devastation.

The storm wreaked havoc in many areas of Missouri, but the worst hit was Joplin: three quarters of the city's 50,000 residents are in ruins. The tornado covered roofs and tore entire buildings to pieces. Houses, churches, supermarkets and gas stations were badly damaged or completely destroyed. A seven-story hospital was badly hit, and its medical equipment was thrown up to a hundred kilometers away. Because of the risk of collapse, the clinic was evacuated and the patients were taken to emergency shelters. The rescue measures are made even more difficult by the failure of the electricity and telephone network. Auxiliary workers report a tremendous amount of destruction. Three quarters of the city are practically completely wiped out.

The residents were warned too late about the storm: because the tornado was covered by rain and hail, its destructive power could not be predicted. Jay Nixon, governor of Missouri, has now declared the region a state of emergency. Warnings were issued against further storms in Missouri.

The tornado road

More than 1,000 tornadoes pass through the United States each year. 500 to 600 of the dreaded hurricanes alone take the route of the notorious "tornado alley". The "road of tornadoes" runs through the US states of Texas, Oklahoma, Kansas and Nebraska. There the conditions for tornadoes are particularly favorable: warm, humid air that rises over the Gulf of Mexico meets unhindered, dry, cold air from the north in the plains of the Great Plains. Violent thunderstorms arise here, from which many tornadoes develop. But not only the United States of America, Germany also has some of the dreaded windpants every year. But because the Alps prevent warm, humid air from the Mediterranean from reaching us, tornadoes are much rarer here.

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 affects 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.

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. 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 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.

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. Then the excess water 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?

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.