Tropical cyclones: All you need to know about circular storms that occur in the tropics

A cyclone is a large air mass that circulates inwards around a calm center with low atmospheric pressure —clockwise in the Southern Hemisphere and counterclockwise in the Northern Hemisphere.

Tropical cyclones are one of the most severe forms of cyclones. Originating in the tropics (mostly over tropical seas), they can be defined as rapidly rotating storm systems with strong winds —between 74 mph (119 kmh) and greater than 156 mph (251 kmh)— and spiral-shaped thunderstorms (when seen from above). 

A common mistake is to think that there are several types of tropical cyclones when in reality, they are only called differently depending on their location. For example, if they form in the Atlantic Ocean or the central North Pacific or eastern North Pacific ocean, they are referred to as “hurricanes”; if they occur in the Indian or western Pacific oceans, they are referred to as “typhoons”. They are not common in the South Atlantic Ocean.

How is a tropical cyclone formed?

The development of a tropical cyclone is called tropical cyclogenesis. For a cyclone to form, there are a few requirements  — different meteorological events that must take place for the tropical cyclone to form and evolve.

First, tropical cyclones are more common in the summer. One reason for this is that hot air can rise up the temperature of tropical oceans more easily. And tropical cyclones need sufficiently warm ocean surface temperatures (about 79.7ºF or 26.5ºC) to generate a warm core. The warm core fuels tropical cyclones by giving heat energy to the storms above. This heat comes from the condensation of moist air and water vapors from the warm tropical waters.

 Tropical cyclones form in areas of low pressure, where the air is heated over the warm tropical ocean. This air rises, causing thundery showers to form. When these thunderstorms group together into large clusters, this creates a flow of very warm, moist, rapidly rising air, leading to the development of a center of low pressure, or depression, at the surface.

In order for these thunderstorms to transform into tropical cyclones, a number of other factors also need to be present. These include the convergence of winds near the ocean surface, and a low wind shear (the winds should not vary much with height), which allows the storm clouds to rise high. The storm system also needs to be a sufficient distance from the equator to allow it to spin or twist

Low-level disturbances, such as gust fronts or tropical waves, also benefit tropical cyclone formation; while weak vertical wind shears of less than 22 mph (35 kmh) are often necessary to fortify them as they keep the warm core above their surface circulation center.

The spiral shape of a tropical cyclone is due to the Coriolis effect, which deflects air moving from high-pressure areas to low-pressure areas to create the cyclone’s rotating motion. 

What are the 5 stages of a tropical cyclone?

The development of tropical cyclones can be divided into five stages. 

The first one is called “incipient disturbance” and it refers to the period in which all of the pre-conditions for tropical cyclone formation develop and eventually form a cluster of thunderstorm clouds.  

The second stage is called “tropical depression”, which can be defined as a period of intensification in wind strength. As thunderstorm clouds form a column, winds start whirling each time faster around it. The spiraling winds accelerate inwards and upwards, releasing heat and moisture as they go. The storm is called a “tropical depression” when it reaches surface wind speeds of about 38 mph (61 kmh). 

At this point, the storm generally becomes a self-sustaining system that no longer depends on external factors. As it reaches maximum sustained surface winds of between 39 mph (62 kmh) and 73 mph (115 kmh), it becomes a tropical storm. It qualifies as a tropical cyclone when it surpasses a wind speed of 74 mph (117 kmh).  A mature cyclone generally has a cylinder of deep thunderclouds surrounding a center that is relatively free from clouds. There will also be a relatively small area of intense horizontal winds at the surface.

At this point, it has a spiral pattern of deep cumulonimbus clouds and has a circular, calm eye with a diameter of roughly 12 to 37 miles (20 to 60 km). 

After this, tropical cyclones eventually enter a phase of decay. For example, if they encounter strong vertical wind shears, they can weaken because these winds tend to divert moisture and heat from the system’s center. All of the system’s energy spreads out in an area too big for it to survive. 

Tropical cyclones can also debilitate if they move over land or to a point with cooler waters (generally those cooler than 79.7ºF or 26.5ºC) because they would no longer be powered by the heat energy from the warm tropical oceans. The warm core of a tropical cyclone can easily dissipate if enough cool water vapor is added to the mixture. Additionally, dry air intrusions from a landmass help to neutralize moist air.

Tropical cyclones can take up to one week to go through all of these stages. They can survive a bit longer in the subtropics (usually between 30° and 40° latitude) if they move to that zone and turn into an extratropical cyclone — a process that can last from 1 to 3 days and results in a less destructive, colder-core cyclone.

Extratropical cyclones develop when cold air masses interact with warm air masses. As the area of cold air collides with the area of warm air, weather fronts form. As the fronts mature and strengthen, the denser, drier cold air masses move underneath the more buoyant warm air masses and help force the warm air to rise.

Extratropical cyclones cover a larger area and can last for 15-20 days, but are less violent in comparison to tropical cyclones, with lower wind speeds, and they often result in simple precipitation. In fact, more than 70% of rainfalls in parts of Europe and North America are due to the passage of an extratropical cyclone.

Occasionally, extratropical cyclones can become more intense and cause more damaging thunderstorms, gales, tornadoes, or blizzards.

Famous tropical cyclones

Tropical cyclones usually receive a name as soon as they are recognized as tropical storms by weather forecasters. In North America, this practice began in 1953. The goal is to help people differentiate such events from each other. Some storm warning centers also give an identifier, consisting of two digits and a letter, to every tropical depression. The digits start at "01" each season and the letter defines the region of the depression.

Tropical cyclones are classified as categories 1, 2, 3, 4, and 5 according to their wind speeds and their related damage potential in their landfall. 

Some of the most famous tropical cyclones were also some of the most destructive. For example, in 1992, Hurricane Andrew was a category 5 tropical cyclone whose winds of around 175 mph (280 kmh) caused extreme damage in Florida, Louisana, and the Bahamas. The estimated cost of the damage  was $27 billion in 1992 USD. Around 177,000 people lost their homes and 65 died.

Hurricane Katrina (2005) was also a category 5 tropical cyclone. With similar wind speeds, it caused damages totalling around $125 billion and led to the deaths of 1,836 people. 

Hurricane Harvey, which made landfall on Texas and Louisiana in August 2017, was a category 4 tropical cyclone and inflicted $125 billion in damage as well, along with 100 deaths and severe flooding in Texas.

Hurricane Sandy was the deadliest and most destructive hurricane of the 2012 Atlantic hurricane season. It was a category 3 hurricane and caused 233 fatalities across eight countries from the Caribbean to Canada.

In 1995, Hurricane Erin developed from a tropical wave into a category 2 hurricane as it moved over the northwestern Bahamas, hitting Florida in August 1995 with a wind speed of 85 mph (140 kmh). In all, 13 people died and the hurricane caused around $700 million in damages.

Another famous tropical cyclone is Hurricane Catarina, an extraordinarily rare South Atlantic tropical cyclone, which made landfall in South Brazil in March 2004 with wind speeds of 100 mph (155 kmh). This is the first and only tropical cyclone registered in the South Atlantic Ocean.