What’s Tropical Cyclone (Typhoon)?
Tropical cyclones is one of the natural hazard for aviation. How do we call them depending on the region.
According to WMO (World Meteorological Organization):
[Ref Tropical cyclone (wmo.int)]
A tropical cyclone is a rapidly rotating storm that begins over tropical oceans, and they can vary in speed, size, and intensity.
Tropical cyclones are the second-most dangerous natural hazards, after earthquakes.
Classification of tropical cyclones
Tropical cyclones above a particular wind speed are classified for public safety.
Depending on the maximum sustained wind speed, tropical cyclones are designated as follows:
- Tropical depression is when the maximum sustained wind speed is less than 63 km/h.
- Tropical storm is when the maximum sustained wind speed is more than 63 km/h. It is then also given a name.
- Hurricane, typhoon, tropical cyclone, very severe cyclonic storm - depending on the basin - when the maximum sustained wind speed exceeds 116 km/h or 63 knots.
One question you may feel abstracting or confusing – how corilois force plays role in the formation of TC?
The Coriolis Effect Explained (youtube.com)
Next, you may come up to another question – What are the most dangerous parts of a tropical cyclone?
Also reference : 熱帶氣旋：在東邊、在西邊？ (hko.gov.hk)
Semicircle Effect
In the Northern Hemisphere, winds surrounding a tropical cyclone blow in an anticlockwise direction. Imagining if a tropical cyclone is divided into two semicircles along its direction of movement, winds on the right semicircle will be in the same direction of the storm’s translation motion while winds over the left semicircle will be in the opposite direction. As a result of the superposition of the cyclonic winds and the translational motion of the storm, winds on the right semicircle are usually stronger than those on the left semicircle, which are thus termed the dangerous semicircle and navigable semicircle respectively1.
Furthermore, in the western North Pacific and the South China Sea, subtropical ridge usually stays on the northeastern side of a tropical cyclone, resulting in a tighter pressure gradient and thus stronger winds in between them. As tropical cyclones over the region usually track northwestwards, the dangerous semicircle will overlap with the tight gradient over the northeastern flank of the storm such that winds over the right semicircle will strengthen further (as shown in Figure 1).
