Tropical Cyclones
A tropical cyclone is a rapidly rotating storm system in the tropics that is characterised by a low-pressure centre, strong winds, and a spiralling arrangement of thunderstorms. Sea surface temperatures must be greater than 26.5˚C over a large area and for at least 60 metres depth. The typical diameter for these storms is around 500 km and they have a vertical depth of 10 - 15km. They are known regionally as hurricanes in the Atlantic and east Pacific, typhoons in the central and western Pacific, and cyclones in the Indian Ocean. 

Tropical cyclones are perhaps the ultimate example of organized tropical convection, with thunderstorms arranging into spiral bands around a central area of low pressure. They are well known for producing strong winds that can damage buildings and structures, although it is often the flooding associated with the storm surge and heavy rainfall that produces the most fatalities. Heavy rain can lead to inland flooding, especially in mountainous regions - where landslides are also a threat. In low-lying coastal regions, water can take many days to drain away - increasing the likelihood of disease and contamination of drinking water, which can lead to further problems long after the storm has passed through. 
Visible satellite image of Hurricane Isabel in 2003, just prior to making landfall in North Carolina as a category 2 storm. 
Credit: NASA/ MODIS.

Visible satellite image of Hurricane Isabel as a category 5 storm, about 400 miles north of Puerto Rico.
Credit: NASA/ MODIS.
Tropical Cyclone Formation

Several criteria need to be fulfilled in order for a tropical cyclone to form:
  • The latitude must be poleward of +/- 5˚, in order for the coriolis force to be large enough to allow storm rotation and organisation. 
  • The sea surface temperature must be greater than 26.5˚C, extending to a depth of at least 60 m, in order to lead to atmospheric instability and provide ample latent heat for storm development. 
  • Vertical wind shear must be low, so that the warm core centre of the storm can be maintained - allowing outflow of air at the top of the storm. 
  • A warm-core initial disturbance that provides low-level relative vorticity is required to help the storm organise in its developing stages - such as a tropical easterly wave. 
  • Conditional instability and a moist mid-troposphere are also important. 

If all of these criteria are fulfilled to some extent, then topical cyclone formation is possible. Once the storm has formed from a pre-existing initial disturbance, it can grow rapidly by positive feedback mechanisms involving latent heat release and the decrease in pressure in the storm's centre. The strength of a tropical cyclone depends on the sea surface temperature and the degree to which the atmosphere is conducive to tropical cyclone development. 

There are currently two competing theories for tropical cyclone growth: in the CISK theory, the vortex spins up through convective heating and mass convergence due to friction over the sea surface. In the WISHE thoery, spin-up is driven by surface fluxes of heat and moisture from the warm ocean that enhance convection, leading to a pressure drop near the centre.  

There are three typical stages to tropical cyclone formation:
Tropical depression - maximum sustained winds < 36 mph. 
Tropical storm - maximum sustained winds between 36 and 74 mph. 
Tropical cyclone (hurricanes, typhoons, cyclones) - maximum sustained winds > 74 mph. 

Tropical Cyclone Classification

Different ocean basins have slightly different classification systems. In the Atlantic and east Pacific, the Saffir-Simpson hurricane wind scale is used:

Table displaying the various stages of the Saffir-Simpson scale for hurricanes. Storm surge heights are given in feet. 
The origins of tropical cyclones are ultimately dependent on the sea surface temperature and atmospheric wind shear. Most tropical cyclones form in the northern hemisphere, since sea surface temperatures in the South Atlantic and eastern South Pacific are generally too cold to support their formation. Climatologically high values of vertical wind shear in the South Atlantic and central Pacific also further inhibit tropical cyclone formation in these regions. The most active region for tropical cyclogenesis is the western Pacific, where the highest sea surface temperatures in the world occur, typically above 30˚C for most of the year - this is one of the few locations where tropical cyclones can form year-round.  
Schematic illustrating tropical cyclone tracks with intensity. The highest frequency and most intense storms generally occur in the western Pacific. 
Credit: NASA. 
Tropical Cyclone Structure 

A well-developed tropical cyclone will typically have the following features: 
  • Cyclonic spiral convergent rainbands at the low-levels, which propagate outwards from the eye, and anticyclonic outward spiral cirroform clouds at the upper levels. 
  • Eye - typically a cloud free centre of 10-50 km in diameter. The eye is generally characterised by calm, clear conditions and light winds, and is where the lowest pressure occurs within the storm. Pressure can be lower than 900 hPa in the centre of a strong tropical cyclone. 
  • Eye wall - deep convection surrounding the eye, which slopes outward with height. The strongest winds within the storm occur here. 

Schematic illustrating the typical structure of a well-developed tropical cyclone. 

The majority of tropical cyclones stay out to sea, where their human impacts are small. However, a minority make landfall, upon which they can cause damage and destruction. Potential impacts include strong winds, storm surges, inland flooding, large waves, landslides, tornadoes and rip tides. 

However, landfalling tropical cyclones are immediately deprived of the latent heat source from a warm ocean that is needed to sustain their intensity. The land also poses other problems, such as enhanced frictional convergence, which promotes the filling of storm's central low pressure. The entrainment of much drier air into the storm results in strong downdrafts that weaken updrafts in thunderstorm bands. Mountainous terrain can disrupt the low-level circulation, meaning that surface air cannot be easily supplied to thunderstorm updrafts. 
Typhoon Haiyan at peak intensity before landfall in the Philippines on November 8th 2013. Haiyan was the strongest tropical cyclone ever recorded at landfall, with one-minute sustained winds of 195 mph, and gusts up to 235 mph. 
Credit: NASA/Terra sateliite.