What determines the Coriolis force and from which strength a storm becomes a hurricane: the most important answers to tropical cyclones

Cyclone, Hurricane, Typhoon What you need to know about these storms

Tropical cyclones keep making headlines worldwide. With wind speeds of up to 195 kilometers per hour, Cyclone Idai broke through southeastern Africa in March, leaving a picture of devastation. While the Southeast African country of Moçambique was hit again by a tropical cyclone shortly thereafter, recent hurricane “Dorian” in the Bahamas made for sad headlines. Although the terms “cyclone” and “hurricane” are likely to be familiar to most, when asked what is the difference between a hurricane, a cyclone, and a typhoon, many are likely to falter.

Are the three terms one and the same weather phenomenon or are there serious differences? The most important questions and answers at a glance:

Cyclone, Hurricane, Typhoon: What are the differences?

Hurricanes, typhoons and cyclones are all tropical cyclones and nothing more than different terms for one and the same weather phenomenon. Whether a tropical cyclone is called a cyclone, hurricane or typhoon depends on which region the storm is occurring.

  • As hurricanes, tropical cyclones are called, which occur in the North Atlantic, the Caribbean and the Gulf of Mexico, as well as those in central and eastern North Pacific.
  • The same weather phenomenon is called typhoon in the western North Pacific.
  • Cyclones, in turn, occur in the Indian Ocean and the western South Pacific.

What are tropical cyclones?

A tropical cyclone is a low pressure area with organized vertical air movements – called convection in meteorology, heavy thunderstorms and a closed bottom wind circulation around the center. Usually they arise only in the tropics and subtropics.

The so-called Coriolis force causes tropical cyclones to rotate cyclically, and it also determines which direction a storm will turn. Because of the earth’s rotation, trade winds are deflected to the right in the northern hemisphere, and trade winds on the southern hemisphere to the left. As a result, tropical cyclones turn clockwise in the southern hemisphere and counterclockwise in the northern hemisphere.

When is a storm a hurricane, typhoon or cyclone?

To be classified as a hurricane, typhoon or cyclone, a storm near its center must reach wind speeds of at least 119 kilometers per hour. If a hurricane reaches a wind speed of 179 kilometers per hour, this is called a “strong hurricane”. A typhoon with a wind speed of 241 kilometers per hour is called “Supertaifun”.

On the Beaufort scale, the world’s most widely used system for describing wind speeds, a storm is called a hurricane, typhoon or cyclone when its wind force is more than 11.

On the Saffir-Simpson hurricane scale , hurricane strength varies between category 1 and 5. While category 1 hurricanes reach wind speeds of 119 to 153 kilometers per hour, wind speeds in a hurricane of level 5 are more than 249 kilometers per hour.

The wind speed has nothing with the speed of movement to do the storm, which is much deeper and is only 15 to 30 kilometers per hour.

How do such storms arise?

Prerequisite for the emergence of a tropical cyclone are strongly heated, moist air masses. Therefore, such storms can form only in intense sunlight above the sea at a water temperature of over 26 degrees Celsius. Such conditions are usually found only in the tropics and subtropics.

Emergence of tropical cyclones

Emergence of tropical cyclones

Due to strong sunlight, the seawater heats up and evaporates. From at least 26.5 degrees Celsius, the conditions for the formation of a tropical cyclone are given. Warm, humid air rises and cools upwards.

The water vapor condenses and storm clouds form. Through them air rises to a height of 15 to 18 kilometers. There, the clouds form an anvil, through which the air flows away to the side.
As the humid masses of air rise, the pressure on the surface of the sea decreases, forming a depression. In order to compensate for the negative pressure, humid air from the environment flows and rises, at the height of which more spring clouds emerge.

Finally, more and more air rises through the storm clouds upwards, whereby the suction of the low pressure area is getting bigger. The incoming air is thereby accelerated. The rotation of the earth produces the Coriolis force. It deflects the movement of air in the thunderstorms so that a whirlwind arises from it.

How are tropical cyclones structured?

At the center of a cyclone depression is the so-called eye. It is free of cloud and wind and usually 20 to 60 kilometers wide. The clouds of a cyclone low turn around the eye and form a cloud wall of 12 to 16 kilometers in height. The cloud wall is the most active thunderstorm zone of a hurricane. Wind speeds can reach over 200 kilometers per hour and rainfall over 100 millimeters per hour.

At what time of year do tropical cyclones develop?

  • In the Caribbean and America, the hurricane season lasts from
    1 June to 30 November. The highlight of the season takes place in August and September.
  • The typhoon season in the western North Pacific usually extends from May to November.
  • The cyclone season in the Southwest Pacific and Australia usually lasts from November to April. In the Bay of Bengal and the Arabian Sea, on the other hand, the tropical cyclones usually occur from April to June and from September to November. The eastern coast of Africa is usually hit by cyclones from November to April.

Which regions are the hardest hit?

Because of the favorable water temperatures there, most tropical cyclones occur within a zone that lies around the equator between the southern and the northern latitude 30 degrees. The Coriolis force, the distracting force of the Earth’s rotation, however, is strong enough to initiate a rotational movement of the cyclones only from 5 degrees north and south latitude. Therefore, the equatorial area is practically excluded as a tropical tropics storm zone.

Around two-thirds of all tropical cyclones form in the northern hemisphere. They are most common in the Western Atlantic, the Eastern North Pacific, the Western North Pacific, the Northern and Southern Indian Ocean, and the Southwest Pacific off Australia. There are virtually no tropical cyclones in the South Atlantic and Southeast Pacific, where the sea surface temperature is too low.

The hurricanes of the North Atlantic are particularly dangerous for the Caribbean, Central America and the southern United States. The typhoons in the Northwest Pacific often have an impact on China, Japan, South Korea, Southeast Asia and numerous islands in Oceania. Cyclones in the Indian Ocean mainly hit India, Bangladesh, Sri Lanka, Burma and Pakistan as well as eastern Africa. Cyclones in the Pacific pose a threat especially to Australia and the islands in Oceania.

How long does a tropical cyclone survive?

The lifetime of a tropical cyclone can be several days or weeks. Important for the survival of the storm is the supply of warm, moist air. This supply is interrupted as soon as the cyclone low hits land or a cool ocean current. If the cloud vortex hits land, it is additionally slowed down by the soil friction. As a result, the low pressure area weakens and finally dissolves.

What were the strongest tropical cyclones in the recent past?

The strength of a tropical cyclone can be measured in several ways, such as with the highest wind speeds reached or with the low air pressure caused. The record for the highest ever measured wind speed outside a tornado is held by the cyclone “Olivia”, which sailed over Western Australia in 1996. The record speed of 408 kilometers per hour, measured on Barrow Island off the Australian west coast, was later reviewed and confirmed by scientists.

But not only the wind force, but also the air pressure says something about the strength of tropical cyclones, because a tropical cyclone is basically a low pressure area. The lower the pressure generated by the storm, the stronger it is. The record for the lowest pressure generated (870 hectopascals) was set by the typhoon “Tip” in 1979, which corresponds approximately to the normal air pressure at 2000 meters altitude. At sea level, the mean air pressure is 1013.25 hectopascals.

What are the dangers of such storms?

Tropical cyclones develop a strong suction and pressure force that can damage buildings, vehicles and infrastructure and pose a significant threat to humans and nature. If the force of the storm is large enough, unsecured buildings can be completely destroyed. Entrained and flying objects represent another danger and can cause additional damage.

The dangerous sequel of tropical cyclones include heavy rain, which is often so severe that it leads to flooding or landslides. Particularly at risk are coastal areas and islands, where flooding and coastal erosion can be triggered by storm surges and waves. On the high seas, during a tropical cyclone, waves can reach up to 30 meters, which can be dangerous for ships and oil rigs. In addition, tornadoes can occur as a side effect in tropical cyclones.

Are there early warning systems?

Meteorologists around the world are using modern technologies such as satellites, weather radar and computers to track tropical cyclones as they develop. The goal is to be able to predict when and where a tropical depression will go through and in what intensity. Official warnings are issued by the national meteorological services of the countries concerned. However, tropical cyclones are often hard to predict as they can suddenly change course or weaken.

The World Meteorological Organization (WMO) enables the timely and worldwide dissemination of information on tropical cyclones. Thanks to international cooperation and coordination, hurricanes can now be monitored very early on from their formation stage.

How to protect yourself from hurricanes, typhoons and cyclones?

It is not recommended to be outside during a tropical cyclone. Persons who are confronted with such a natural event should – if possible – find a shelter. As a safety measure it is also recommended to nail boards in front of the windows. Not every pane of glass withstands a tropical cyclone. Vehicles should be left in the garage so that they do not fly away or get damaged. Important information or security measures to be observed are announced via the radio. In the case of a tropical cyclone, it is therefore particularly important to start the radio. In addition, it is advisable to create water and food supplies for several days and in case of an evacuation to fill the car tank with fuel.

Incidentally, valuables are best kept in the dishwasher during a tropical cyclone. This is absolutely waterproof – both against the outside and against the inside. The city of Brookhaven in the US state of New York recommends in their “Hurricane Survival Guide” to clear out the dishwasher and store all the valuables in it. If the storm water masses enter the house, important items and documents in the dishwasher are initially protected.

The best protection is provided by sufficiently secured buildings with an adapted construction method. Building codes have therefore been adopted in some countries. Fasteners in timber constructions must be secured by steel bands called “hurricane straps”. As a precaution, shutters, so-called “storm shutters”, must be installed. Windows and doors should be protected with safety glass.

How do tropical cyclones get their names?

“Idai”, “Haiyan”, “Michael” – in the news, cyclones, typhoons and hurricanes are usually called by their names. But who determines what name a tropical cyclone receives? The naming of big storms has a long tradition. Australian meteorologist Clement Wragge is considered the first to give storms names; he started at the end of the 19th century. He first used women’s names and later first names of politicians he did not like.

In 1953, when the US Weather Service decided to use memorable women’s names, this was quickly adopted worldwide. After the protests of politicians and women’s rights organizations, women’s and men’s names have been used alternately since 1978/79. In the North Pacific, it is also possible for storms to get the names of animals, flowers, trees or food.

Today, the World Meteorological Organization (WMO) in Geneva sets the name lists for hurricanes. The weather services, which are responsible for the respective sea area, then give the names. For the naming of storms on the Atlantic and the Northeast Pacific, for example, the National Hurricane Center in Miami in Florida is responsible.

What effects does climate change have?

In 2017, the Atlantic experienced such an active hurricane season as it had for years. Ten hurricanes followed in succession, with hurricane Harvey being particularly destructive, with a few thousand gallons of rain per square meter in a matter of days. Because of such events, there is currently a controversial debate in the scientific community as to whether anthropogenic climate change is leading to more and more severe tropical cyclones. The first volume of the major report, presented by the Intergovernmental Panel on Climate Change (IPCC) in 2013, provides sound answers.

Observations in the North Atlantic show that since the beginning of satellite measurements in the 1960s, the strongest hurricanes there have become more frequent and have increased in intensity. For other oceans, there are still no clear statements in this respect, since the observation series there are often not long or accurate enough. In general, however, it can be stated that it is extremely difficult to establish relationships between man-made climate change and the emergence of tropical cyclones. The activity of the storms is also influenced by natural processes, such as the natural variations in ocean temperatures.

However, studies on Hurricane Harvey suggest, for example, that the likelihood of such a large amount of rain may have been increased by a few percent due to man-made warming of the atmosphere, as warmer air can absorb more water vapor. However, studies on climatic trends in a single cyclone are very demanding and should be treated with caution.

According to the IPCC, forward-looking statements about the development of tropical cyclones over the next few decades are still fraught with uncertainty. According to the climate report, the number of tropical cyclones could easily decrease or remain at the current level. The intensity of the storms could increase by a few percent. However, what this means for the individual regions is difficult to say.