What Is A Typhoon?
A typhoon is an intense area of low atmospheric pressure. Like all low pressure weather systems, the air rotates around the center of an area of low pressure in an anti-clockwise direction. The more intense the area of low pressure the higher the wind speeds near the center. A typhoon has sustained wind speeds near its center of at least 56 knots (about 105 kph).
What Makes A Typhoon?
Typhoons are made over the ocean; tyhpoons are not made over land.
To make a typhoon you need a lot of warm, moist air evaporating off the ocean surface and rising rapidly, creating the area of relatively low pressure - a weather system. The rapid rising of the air mass, and the subsequent condensation of the moisture at altitude, is the energy input for rotation of the weather system and a tropical depression (TD) is formed. When the upper atmospheric conditions are conducive - the warm moist air can disperse into an area of high-altitude subtropical drier air with moderate vertical wind-shear - then the rotation is accelerated and the weather feature forms into a tropical storm (TS). Assuming the tropical storm remains over an area of warm ocean - with a sea surface temperature between 26 and 30 degrees Celsius - then the weather system will most likely continue to increase in rotation speed until it reaches the status of a typhoon – sustained wind speeds in excess of 56 knots near its center. If the sea surface temperature is less than 26 degrees Celsius then a typhoon has insufficient energy and cannot sustain itself; if the sea surface temperature is greater than 30 degrees Celsius then the weather system becomes overpowered and will dissipate.
Where Are Typhoons Made?
In the northern hemisphere, tropical depressions can be made anywhere in the tropical region (5 - 22 degrees latitude) where the sea surface temperature is between 26 and 30 degrees Celsius, but only very rarely do they form close to the equator (0 - 5 degrees latitude) – the sea surface temperature is too high. Tropical depressions tend to move haphazardly until they reach tropical storm status when they tend to move in a westerly or north westerly direction. A typhoon will almost always move northwest until it reaches the subtropical zone (18 - 25 degrees latitude) when it will usually turn north and then finally northeast before dissipating over cooler water.
Typhoons with sustained wind speeds in excess of 115 knots (about 200 kph) are frequently referred to as Super Typhoons, equivalent to a category 4 storm in the Atlantic; a typhoon can have wind speeds near its center in excess of 150 knots, although this is quite rare.
What Affects A Typhoon Track?
The main factor affecting the track of a typhoon is its proximity to, and intensity of, an anti-cyclone (area of high pressure); anti-cyclones most frequently occur North of the sub-tropical region (above 25 degrees latitude) over a large landmass, such as Siberia, or sometimes over the North Pacific. If the anti-cyclone is intense then the typhoon will tend to track with a more westerly vector and increase in wind speed more rapidly.
Typhoons tend to follow the barometric contours of an anti-cyclone until they reach 25 degrees latitude, when they will usually start to dissipate. However, typhoons have been observed to move erratically and their paths are sometimes difficult to predict – Typhoon Gerald (1987) is a classic example of a typhoon with an unpredictable path, as it performed crazy-eights in the West Philippine Sea and South China Sea, south of Hong Kong, before finally dissipating over land in South East China. More recently, typhoon Lupit (22W) in 2009 made like a snake as it approached the Philippines, before finally turning North (see below image of Typhoon Lupit's track).
When a typhoon crosses a land mass it will lose power because: (a) the air mass will be forced higher and start to cool; and, (b) it will no longer have access to its fuel – warm, moist air. A typhoon crossing the high mountains of northern Luzon, for example, may lose as much as 50% of its power during the short, one hundred mile transit.
A tropical depression or tropical storm passing over land will typically deliver higher levels of precipitation than a fully formed typhoon.
Typhoon, Cyclone & Hurricane
A “typhoon” is the term used to describe the same weather feature as a “cyclone” or a “hurricane”: the term Typhoon is used for intense low pressure weather systems in the northwest Pacific; the term Cyclone is used when referring to an intense low pressure weather system over the Indian Ocean and the South Pacific; and, the term Hurricane is used when referring to an intense low pressure weather system over the Atlantic Ocean and northeast Pacific.
Of interest, only one hurricane has ever been recorded over the South Atlantic (March 2004), although weather systems that developed in the South Atlantic during March 2010 and March 2011 were classified as tropical cyclones for a few days each before dissipating. Some suggest that the fact that these weather systems have been recorded only within this century is additional evidence of Man-made climate change - no credible alternative explanation has been accepted.
Typhoon Names & Numbers
All intense low pressure weather systems are given numbers, sequencial within the current year, e.g. Typhoon 20W (20th typhoon this year in the West Pacific). They are given names once they become established tropical depressions with the potential to become typhoons e.g. Typhoon (Nesat) 20W; these are the names by which specific typhoons will become known internationally. Historically, typhoon names alternated between anglicized male and female names (interestingly, typhoons with women's names were most frequently the most powerful and destructive) but at the turn of the century a list of largely genderless Asian names was chosen, from which individual typhoon names are used in rotation. Some countries, notably the Philippines, have their own naming scheme for typhoons, which leads to some confusion in news reporting, e.g. Typhoon “Nesat” (2011) was called “Pedring” in the Philippines. The northwest Pacific can be expected to spawn around 25 named typhoons during a year.
The word “typhoon” originates from the ancient Greek language, where it referred to a weather related, mythological monster. In Cantonese/Chinese “typhoon” translates (phonetically) directly to “big wind”.
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