How are hurricanes measured

Hurricanes - the heat engines of the tropics

The energy and destructiveness of a hurricane are impressive. But nature causes much stronger wind currents on earth: the hurricanes. This is the name of the adult specimens of the tropical cyclones in the Atlantic and Eastern Pacific.

A tropical storm has wind speeds between 50 and 120 kilometers per hour. In a hurricane, on the other hand, the air flows faster than 120 kilometers per hour. In this country one speaks of a hurricane from this wind speed. The winds on the North Sea coast are not much stronger. Hurricanes, on the other hand, can cause wind speeds of more than 250 kilometers per hour. On the five-part Saffir-Simpson scale for classifying intensity, this corresponds to the highest category.

These cyclones are not called hurricanes everywhere. “Typhoons” swirl over the western Pacific and “tropical cyclones” over the Indian Ocean. It is characteristic of these tropical cyclones that they arise over warm sea water: They form from a water temperature of around 26.5 degrees Celsius. In the vicinity of Europe, the sea never gets so warm on a large scale - that's why there are no hurricanes here. However, temperature is not the only condition for a hurricane to develop. Three other factors have to be added: little wind, thunderclouds and the right latitude.

If a strong wind blows over the sea, the trade wind for example, then not even the precursors of the hurricanes can form - cloud towers are simply blown away. The Passat also sometimes transports very dry air from West Africa to the Atlantic, which is also not conducive to the development of a hurricane. But if there is calm or if there is already a weak wind vortex at high altitude, then a tropical cyclone may arise. Meteorologists recognize this early on in satellite images from the fact that thunderclouds clump together over the tropical waters to form so-called clusters.

Areas of origin of the tropical cyclones

From a thermodynamic point of view, tropical cyclones are huge heat engines: warm, moist air rises in the storm clouds of the cyclone. The evaporated water condenses and the heat of condensation drives the air up even more, up to a height of about 15 kilometers. It loses moisture due to rain. Outside the cyclone, the dry air sinks and flows back to its center either directly or via a detour.

In order for the disordered thunderstorm cluster to form a cyclone, it must be at a distance of at least five degrees of latitude from the equator. Because only then is the Coriolis force, which deflects the air and sets it in rotation, large enough. This force is created by the rotation of the earth. If you move from the north or south pole along a degree of longitude towards the equator, then the speed of rotation with respect to the earth's axis increases. That is why the wind is diverted - to the right in the northern hemisphere and to the left in the southern hemisphere. At the equator the Coriolis force is practically zero, at the north and south poles it reaches its maximum value. In a tropical cyclone, however, it is not only the Coriolis force that acts on the air masses. In addition, the low air pressure in the center exerts a suction inwards, while the centrifugal force drives the air outwards. The interaction of these three forces determines how strongly the air masses of a hurricane rotate.

The inner workings of a tropical cyclone

After the formation of a hurricane, a windless, cloud-free area soon opens up in its center: the eye. It is bounded on the outside by a rotating circle of thunderstorms, in which the highest wind speeds of the hurricane occur. The air rises around the eye, but inside it sinks slowly and warms up through compression. The diameter of the cloud hole is between four and more than a hundred kilometers. Commercially available barometers no longer show the air pressure - it is that low. For example, Hurricane Wilma set a new record for the Atlantic in October 2005: 882 hectopascals. A strong hurricane low near Iceland typically comes to values ​​around 950 hectopascals. The normal air pressure is 1013 hectopascals.

As hurricanes move across the ocean, their intensity fluctuates. This has internal and external causes. One of the most important external factors is wind shear: if the wind increases sharply with height, it hinders the development of the cyclone. Because not only the inflow of warm air on the sea surface is important for him, but also the outflow at an altitude of about 15 kilometers. Too much external wind can interfere with this outflow of air. In the northern hemisphere, this air flow rotates clockwise because of the Coriolis force, i.e. exactly opposite to the vortex at the very bottom.

Another external factor is the sea temperature - and not just that on the surface. The water temperature at a depth of 50 meters is also important, because the extreme hurricane winds churn the sea up into this layer. The mostly cooler water that comes up this way weakens the hurricane. In the main season, however, in the Caribbean, for example, there is often lukewarm water down to a depth of 100 meters and more. Best conditions for hurricanes.

When do the hurricanes occur in the Atlantic?

The cyclones can also temporarily weaken themselves. Usually a process is responsible for replacing the eye wall with a new one: the collapsing clouds of the old wall fill the center, while the new wall is formed further out. Meanwhile, the wind speed drops by a few percent. After a few hours the eye opens again and the hurricane gains momentum again.

The train speed of a hurricane as a whole is quite slow. Usually he is not faster than a well-trained cyclist. Sometimes it stops or accelerates to 20 to 30 kilometers per hour. The danger of a hurricane does not come from its train speed, but from other side effects. The swell of tropical cyclones can be enormous - in the Gulf of Mexico, for example, pressure probes on the sea floor could measure waves almost 30 meters high between the trough and crest of the wave.

On the coast, however, it is less the waves that cause the greatest devastation than the storm surge. It arises from the fact that the hurricane is driving enormous masses of water in front of it - on its right flank, where the speed of movement and the speed of rotation add up. Depending on the intensity of the hurricane and the shape of the coast, the storm surge can reach heights of more than six meters. The low air pressure inside the wind turbulence, on the other hand, hardly contributes to the flooding: this “negative pressure” can raise the sea level by a maximum of one meter.

As soon as the cyclone has crossed the coast, it literally runs out of breath: Because there is no energy supply from the warm sea, the wind subsides within a few hours and the clouds gradually dissolve. Before that, however, huge amounts of rain fall. They often add up to hundreds of liters per square meter and create floods that exceed all other damage.