In comparison storms in Germany are a gentle breeze

by Ruth Bücker,  2011/06/11

"Vince", "Bret", "Cindy" or "Franklin" – about 30 to 100 hurricanes occur every year in the hurricane season from June to November. “Adrian”, which formed above the Pacific Ocean and brought along severe rainfalls, was the first hurricane to rage this year and it will be followed by further hurricanes, cyclones and typhoons carrying the above-mentioned names. They cause immense devastation and destruction and in the worst case they claim numerous lives.

Weather experts forecast particularly intensive hurricane development over the Atlantic Ocean and the Pacific Ocean for 2011 and correspondingly large will be the threat above all to countries in the Gulf of Mexico and the Caribbean. But what exactly is a hurricane, a typhoon or a cyclone? What is the difference between these destructive storms and how are they caused?

Difference hurricane, typhoon, cyclone

At best, tropical thunderstorms leave behind some uprooted trees. In most cases, however, the consequences are far more severe. Photo: humedica

What is still often called “Orkan” in German-speaking countries has mostly been replaced by the English term “hurricane” and this term describes storms with a wind speed of a minimum of 117 kilometres per hour.

The terms typhoon and cyclone are used to describe the same phenomenon and hence they also describe tropical thunderstorms of the above mentioned wind speed. The factor that decides if such storms are called hurricane, typhoon or cyclone, is the region of their origin:

  • if the storm forms over the Atlantic Ocean or the Northeast Pacific Ocean, it is called a hurricane
  • a typhoon is a tropical thunderstorm that develops over the Northwest Pacific
  • storms developing over the Indian Ocean and around Australia are called cyclones.

When the air whirls: formation

A simplified explanation is that the storms are caused by whirling air. There are several factors that need to come together for a hurricane, a typhoon or a cyclone to develop.

The basic fact – which applies to all three kinds of storms, hurricanes, typhoons and cyclones alike – is that they can only evolve above water. When such a tropical thunderstorm develops, the following factors came together:

Cyclone Nagris raged in 2008 and left nothing to this family in Myanmar except the skeleton of their house. Photo: humedica

  • the surface of the water has warmed up to at least 26 degrees down to a depth of 50 metres, large quantities of water evaporate and rise up in the form of water vapour
  • a tower of clouds forms from this warm, humid and sun-heated air, which rises upwards
  • the rising warm air on the one hand causes barometric pressure to decrease near the ground and, on the other hand there is a lack of this rising air immediately above the ocean’s surface
  • as a consequence, ever more warm and humid air is sucked up from the surrounding area and once again rises upwards
  • at the top of the tower of clouds that has formed, the air is hurled outwards, where it sinks back to the ocean’s surface, is once more sucked up into the current of rising air and in this way a consistent vertical air circulation develops
  • due to the earth’s rotation, these air currents are furthermore caused to rotate horizontally and an eddy of air is formed
  • heated up by additionally released heat that is generated when the clouds form, upwards suction gets even stronger, and so gets air circulation

When all these factors come together, they result in hurricanes. The centre of the circulating eddy, which can reach a diameter of several hundreds of kilometres, is called the “eye”.

Measuring and categorising tropical storms

Images of destruction after cyclone Nagris in Myanmar, where humedica could offer immediate medical aid. Photo: humedica

Depending on the wind speed, the strength of tropical thunderstorms is categorised according to the Saffir-Simpson Scale. This scale was introduced by Herbert Saffir and Bob Simpson at the beginning of the 1970s and its approach is to categorise storms according to the wind speed measured.

The scale goes from 1 to 5, with category 5 being assigned to the most powerful winds.

Category 1: wind speeds from 118-154 km/h (weak thunderstorm)
Category 2: winds speeds from 155-177 km/h (moderate thunderstorm)
Category 3: wind speeds from 178-211 km/h

Category 4: thunderstorms with wind speeds of more than 211-250 km/h
Category 5: thunderstorm is an extraordinarily destructive monster hurricane that destroys everything in its way with wind speeds of more than 250 km/h.

There are several institutes worldwide that monitor the development of tropical storms and give out warnings, such as for example the National Hurricane Center or the US Joint Typhoon Warning Center.

When Alex, Igor and Nicole are raging

In order to differentiate them, the storms are given friendly-sounding, alternately male and female names like “Lorenzo” or “Henriette”, “Wilma” or “Hector”, or “Ophelia” and “Tammy”.

If a tropical storm develops, it is assigned a name by the weather bureaus, which is chosen from a corresponding list for each ocean. For example, there exists a list of names for six consecutive years for the Atlantic Ocean and the Northeast Pacific Ocean.

This list includes one name for each letter from A to W and the storms that occur in a year are named in this alphabetical order. Hence, the first hurricane developing over the Atlantic Ocean in 2011 will be called Arlene. The first tropical storm over the Pacific Ocean was called “Adrian” and occurred not long ago. The second one will be called Bret. They will be followed by Cindy, Don, Emily and others, until the last name of the year will be reached with Whitney.

As there are six name lists for the Atlantic Ocean and the Northeast Pacific Ocean, the names are used again after the six years. An exception is made for the names of exceptionally strong storms, which are taken out of the list and replaced by another name (the name “Katrina”, for example, will not be used again).

When hurricanes, typhoons and cyclones hit land, they cause dramatic damage, claim human lives and destroy their victims’ basis of life. When reaching the coastline, they can cause up to six metre high waves that penetrate far inland and can cause massive destruction and extensive floods.

Furthermore, the massive storm surge is also accompanied by extremely strong wind speeds and torrential rains. Life-threatening phenomena are above all larger objects which are whirled around by the wind, such as torn branches or bricks of roofs uncovered by the storm.

It is always the already poor classes of the population who are affected worst by natural disasters. Support us to help the people in their misery. Thank you very much! Photo: humedica

This article contains information of the National Weather Service, the Joint Typhoon Warning Centers, the page Wetter-Klimawandel (weather climate change), of the very detailed and recommendable internet platform Naturgewalten (Forces of Nature) and of Diercke Weltatlas .

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