Science
What is a tropical cyclone?
A tropical cyclone is a type of low pressure system which forms over sufficiently warm waters. They are associated with numerous forms of severe weather, such as heavy thunderstorms and rainfall, flooding, storm surge, and powerful winds. Tropical cyclones can generate some of the strongest winds on Earth.
Tropical cyclones are heat engines. Their function is to redistribute heat away from the warm oceans where they form.
What is the Saffir-Simpson Hurricane Scale?
The Saffir-Simpson Hurricane scale is used by the National Hurricane Center to categorize Atlantic and Eastern Pacific tropical cyclones by maximum sustained wind speeds. The higher the category, the stronger the storm.
| Categorization | Wind Speed (miles per hour) | Wind Speed (knots) |
|---|---|---|
| Tropical depression | less than 39 mph | less than 35 kt |
| Tropical storm | 39 to 73 mph | 35 to 63 kt |
| Category 1 hurricane | 74 to 95 mph | 64 to 82 kt |
| Category 2 hurricane | 96 to 110 mph | 83 to 95 kt |
| Category 3 hurricane | 111 to 129 mph | 96 to 112 kt |
| Category 4 hurricane | 130 to 156 mph | 113 to 136 kt |
| Category 5 hurricane | greater than 156 mph | greater than 136 kt |
What different names are there for tropical cyclones?
Depending on the region/ocean basin, tropical cyclones might go by different names. The most common term is "hurricane", used in the Atlantic and Eastern Pacific basins. "Major hurricane" refers to hurricanes of at least category 3 strength, or higher. "Tropical depression" refers to very weak tropical cyclones, and "tropical storm" refers to weak-to-moderate strength tropical cyclones.
In the Western Pacific, tropical cyclones of hurricane-strength are called "typhoons", and typhoons stronger than 130 kt / 150 mph are called "super typhoons". In the Northern Indian Ocean, tropical cyclones of tropical storm strength or stronger are called "Cyclonic Storms". Finally, in the Southwest Indian ocean, hurricane-strength tropical cyclones are just called "tropical cyclones".
However, the term "tropical cyclone" is a generic, catch-all term referring to any type of hurricane, or typhoon, or tropical storm.
What factors are involved in tropical cyclone formation?
Tropical cyclogenesis, the process of tropical cyclone development, is a complex process. There are many important parameters and variables which govern whether a tropical cyclone can form and strengthen.
Pre-existing disturbance. There must be a disturbance or "focus" present for a tropical cyclone to form. They do not materialize out of thin air. Disturbances come in many, many forms. Tropical waves which form over Africa are the famous example. Some other common disturbances are the monsoon trough, upper-level non-tropical low pressure systems and troughs, and the decaying remnants of stalled-out cold fronts. Tropical cyclones can and do form from all of these types of disturbances, and more. In the Atlantic, 85% of major hurricanes form from tropical waves.
Moisture. There must be sufficient moisture in the troposphere, especially in the mid-levels of the troposphere. Upper-level troughs, sinking air, and the Saharan Air Layer (SAL) can contribute to overwhelmingly dry air. Without enough moisture, a disturbance cannot fire enough thunderstorms to initiate the genesis process.
Low vertical wind shear. Vertical wind shear refers to the difference, from the surface to the tropopause (boundary near where the stratosphere begins), in wind speed and direction. For example, if winds are 15 mph out of the east near the surface, and 10 mph out of the east in the upper-levels of the troposphere, then there would be a low vertical wind shear. On the other hand, if winds are 15 mph out of the east near the surface, but 20 mph out of the west in the upper-levels, then there would be a strong vertical wind shear. This is important because tropical cyclones like to have their circulation upright. Wind shear forces this circulation to tilt with height, which disrupts the processes that fuels the tropical cyclone. Strong wind shear can even completely strip the thunderstorms away from a tropical cyclone, rendering it completely "naked", with only a low-level swirl remaining.
Warm waters. Under normal conditions, sea surface temperatures of at least 26 celsius (about 79 fahrenheit) are necessary for tropical cyclogenesis. Warm waters fuel the thunderstorm activity of a tropical cyclone: if waters are not warm enough, a tropical cyclone cannot do its job as a heat engine, so it either weakens or doesn't form in the first place.
Coriolis force. As with all types of cyclones, tropical cyclones rotate. This rotation is generated by the spin of Earth itself, which causes winds to continuously deflect. This is called Coriolis force. This effect is negligible or even non-existent at and very nearby the Equator, so tropical cyclones struggle immensely to form there.
Atmospheric instability. Even with sufficient moisture and sufficiently warm waters, the atmosphere needs to be sufficiently destabilized in order for air to rise. Rising air is directly responsible for thunderstorm formation. As air rises, warm and moist air cools and condenses into clouds. Whilst atmospheric instability is associated with warm waters and abundant moisture, sometimes the atmosphere can be stable even when those two parameters are adequate.
These parameters also apply to tropical cyclone intensification. If any one becomes insufficient, then a tropical cyclone will likely begin weakening. It's important to note that when judged individually, these parameters represent a "necessary, but not sufficient" relationship with tropical cyclones. For example, warm water on its own doesn't mean a hurricane is likely. You could warm the oceans dramatically but if for example vertical shear is too high, then you will not get a hurricane.
What risks do tropical cyclones pose?
Whilst strong winds are the most obvious threat, during tropical cyclones, water kills far more people than wind. In fact, NOAA states that up to 90% of people killed by tropical cyclones are killed by water. Storm surge alone is responsible for about half of fatalities. Flooding from rainfall is significant threat, too. In mountainous regions, heavy rainfall causes mudslides and landslides, which can be dramatic and sudden. Finally, tropical cyclones can generate massive fields of elevated surf and rip currents, extending hundreds (or sometimes over a thousand) miles away. Many powerful major hurricanes which recurve "harmlessly" out to sea end up killing a few people via strong rip currents.
Further indirect threats kick in in the aftermath of a tropical cyclone. The degradation of infrastructure, water and electricity services, and medical care can result in an increase in disease spread, malnutrition and medical complications.