What is a Typhoon?
A Typhoon is the name given to Tropical Revolving Storms that affect the South China Sea and Asia Pacific. In Cantonese the word Typhoon literally means 'big wind' giving an indication of one of its most obvious dangers.
Of course in other parts of the world these revolving storms are called different things. Around the gulf of mexico, the islands of the central pacific and the Indian ocean they are known as Hurricanes. In India, Cyclones, and to the north of Australia the somewhat colloquial 'willy-willies'.
Each storm is made more individual by giving it a name. These names start at the letter A at the beginning of each storm season for each region. One of the most famous recently was Hurricane Katrina. Some of the largest to hit Hong Kong have been called Ellen (12 killed, 10 missing, 333 injured), York (killed 2 injured 500), Hope (12 killed, 266 injured) and in 2012, Vicente.
The worst recorded Typhoon was an un-named storm in 1937 that killed a staggering 11,000 people.
How is a Typhoon formed?
A typhoon needs two essential conditions to form. A sea water temperature of greater than 27 degrees and a geostrophic force found above 4 degrees of latitude.
The Hong Kong typhoon season runs June - November and is linked to the south easterly monsoon. The wind of the forms from the trade winds flowing up from the equator. These winds become warm and moist as they travel over the warm ocean. The warm air rises and becomes unstable leading to the formation of towering thunderstorms.
This massed group of thunderstorms release enough latent heat into a pre-existing 'cold-core' disturbance to create the warm core unique to tropical storms. The core around which the thunderstorms revolve is called the vortex, or 'eye' of the storm.
In order to gain the rotation around the core so that it becomes a 'revolving storm' a geostrophic force is needed.
The force that does this is the Coriolis effect. The Coriolis effect is due to the rotation of the earth and will turn air to the right in the northern hemisphere. It is also proportional to the sine of the lattitude i.e it is greatest at the poles and zero at the equator. Therefor we reach the second requirement for the formation of a Typhoon, it can only be formed where there is sufficient coriolis force to start the rotation, which is above 4 degrees of latitude. It also means that Typhoons can never cross the equator.
Once the 'engine' is started it will continue to increase in power and the storm may extend from 100 - 1000km across. It is estimated that a storm generates about 20 million megawatts of mechanical power, enough to power Hong Kong for over a decade.
On the Move
Although the wind speeds around the storm being very high the actual storm itself only moves at 10-15 kts.Initially the storms move westward by the low-lattitude easterlies combined with a small movement polewards. As they drift into higher latitudes they will start to be effected by westerlies which will halt the westward drift. Eventually the track will turn more polewards then reverse to go eastwards.
It is important to note that, despite modern advances, Typhoons do not always follow a predicted path. Despite the best efforts of forecasters Typhoons can deviate and cause major damage and disruption to unprepared areas. From an aviation perspective this is one of the greatest considerations for pilots, airline operators and airports.
The weather associated with a tropical storm is fairly easy to guess, after all it is the Cantonese name for 'Big Wind'.
Wind speeds around airports pose particular threats to aircraft with wind gusting over 60kts. Below is the wind speed recorded during Typhoon York on Sept. 16 1999.
The Wind also poses significant threats to Maritime areas and populated areas as trees are blown over, windows smashed and flying objects are sent flying.
The thunderstorms bring with them the associated heavy rain, reducing visibility, surface flooding and lightning. Severe turbulence in and around thunderstorm activity, severe icing and lightning strikes are all dangerous to aircraft attempting to penetrate the line of storms to get to an airfield however the greatest danger arises when attempting to operate into and out of an airfield effected by a typhoon.
Want to know more?
- Typhoon Operations - Human Factor Considerations
- perspectives of different performers in the management of operations during a typhoon