Tropical Cyclone Formation

It takes a balance of six different atmospheric features to come together for a tropical cyclone – including tropical storms and hurricanes – to form. This formation process is also known as tropical cyclogenesis.

Content:
Tropical Cyclogenesis Factors
Warm Sea Surface Temperatures
Low-Level Disturbance
Instability
Mid-Level Moisture
The Coriolis Force
Weak Vertical Wind Shear
How Does It Actually Form?
What affects the strength?
Changes in Ocean Temperatures
Encountering Other Air Masses (Saharan Dust)
Encountering Land Masses
Vertical Wind Shear
Interaction With Other Weather Systems

These features are sufficiently warm sea surface temperatures (at least 26.5°C (79.7°F)), atmospheric instability, high humidity (moisture) in the lower to middle levels of the troposphere, enough Coriolis force to develop a low-pressure center, a pre-existing low-level focus or disturbance (like the ITCZ or a tropical wave), and low vertical wind shear.

These conditions are necessary for the formation of a tropical cyclone, but even when present, do not guarantee a formation of one.

Tropical Cyclogenesis Factors

Warm Sea Surface Temperatures

Sea surface temperatures during the hyperactive 2005 Hurricane Season on August 27th, with Hurricane Katrina in the Gulf of Mexico. Image: Encyclopedia Britannica
Sea surface temperatures during the hyperactive 2005 Hurricane Season on August 27th, with Hurricane Katrina in the Gulf of Mexico. Image: Encyclopedia Britannica

Tropical cyclones are known as warm-core systems or heat engines – drawing their energy from the moist air that lies above warm oceans through convection.

In typical circumstances, ocean temperatures of 26.5°C (79.7°F) spanning through at least a 50-meter depth is considered the minimum to maintain a tropical cyclone. However, in recent years, tropical storm systems have formed in areas with more marginal sea surface temperatures, particularly in the North Atlantic Ocean due to higher instability.

Low-Level Disturbance

Diagram showing the structure of a tropical wave. This is one of the low-level disturbances from which a tropical cyclone can form. Approximately 60 of these waves form every year, with 30-40 affecting T&T. Tropical waves are the origin of approximately 60% of Atlantic Tropical Cyclones and 85% of major Atlantic Hurricanes (Category 3 and above).
Diagram showing the structure of a tropical wave. This is one of the low-level disturbances from which a tropical cyclone can form. Approximately 60 of these waves form every year, with 30-40 affecting T&T. Tropical waves are the origin of approximately 60% of Atlantic Tropical Cyclones and 85% of major Atlantic Hurricanes (Category 3 and above).

While warm waters are important, there also needs to be a pre-existing atmospheric disturbance to focus on the development of a tropical cyclone. This can be a disturbance within the Intertropical Convergence Zone, a tropical wave, the tail end of a frontal system, an outflow boundary from an organized thunderstorm system, or any other low-level feature with sufficient spin (vorticity) and convergence to begin tropical cyclogenesis.

Without a low-level to surface focus, even with optimal upper-level conditions and atmospheric instability, no organized convection and surface low-pressure system will be able to form.

Tropical cyclones can form when smaller circulations within the Intertropical Convergence Zone merge.

Instability

Diagram of atmospheric stability. For tropical cyclones to form, an unstable atmosphere is required. Image: North Carolina Climate Office.
Diagram of atmospheric stability. For tropical cyclones to form, an unstable atmosphere is required. Image: North Carolina Climate Office.

Atmospheric stability is a measure of the atmosphere’s tendency to discourage or defer vertical motion, which trigger convective activity such as cloud, shower and thunderstorm development to name a few processes.

In unstable conditions, a parcel of air will be warmer than surrounding air at an altitude. This hot air will inherently rise as it is less dense than the surrounding air, which begins the convection process.

Effects of atmospheric instability in moist atmospheres include thunderstorm development, which over warm oceans can lead to tropical cyclogenesis.

To measure instability, a number of indices are used, but utilizing a calculation called the lapse rate (the rate at which temperature falls with altitude) can tell us how unstable the atmosphere is at a point in time.

Dry adiabatic lapse rate: In a dry atmosphere, a temperature decrease of less than 9.8°C per kilometer would be stable, while a greater temperature decrease per kilometer would be considered unstable.

Moist adiabatic lapse rate: In a moist atmosphere, a temperature decrease of less than 6.5°C per kilometer would be stable, while a greater temperature decrease per kilometer would be considered unstable. Lapse rates between 6.5-9.8 °C per kilometer would be considered conditionally unstable – meaning more heat is needed.

In environments with marginally favorable sea surface temperatures, cooler air temperatures at higher altitudes can lead to conditions favorable for tropical cyclogenesis as a result of an unstable atmosphere. A recent example of a tropical cyclone that maintained itself over cooler waters was Epsilon of the 2005 Atlantic hurricane season.

Mid-Level Moisture

Low to mid-level moisture across the Atlantic Ocean on May 10th, 2020 showing mostly dry conditions across the region. Image: University of Wisconsin - Madison
Low to mid-level moisture across the Atlantic Ocean on May 10th, 2020 showing mostly dry conditions across the region. Image: University of Wisconsin – Madison

The atmosphere is layered like a cake, with each layer’s temperature, wind speed and moisture content having an impact on our day-to-day weather.

When considering how tropical cyclones form, moisture at the mid-levels of our atmosphere (near the 500-millibar (mb) level, at approximately 5.9 kilometers above ground level) is particularly important.

At the 500-mb level, the air temperature in the tropics average at −7 °C but because the air is also quite dry at this level, the air has the opportunity to cool further as it moistens. This new, cooler temperature is called the wet-bulb temperature, and it provides a more favorable temperature to support convection.

In the tropics, at the 500-mb level, −13.2°C is required to initiate convection if the water temperature is 26.5 °C, and this temperature requirement increases or decreases proportionally by 1 °C in the sea surface temperature for each 1 °C change at 500-mb. 

Under a cold cyclone, 500 millibar temperatures can fall as low as −30 °C, which can initiate convection even in the driest atmospheres. This also explains why moisture in the mid-levels of the troposphere, roughly at the 500 mb level, is normally a requirement for development.

The Coriolis Force

Schematic representation of flow around a low-pressure area (in this case, Hurricane Isabel, north of the Greater Antilles with Puerto Rico and Hispanola to the south) in the Northern hemisphere. The pressure gradient force is represented by blue arrows, the Coriolis acceleration (always perpendicular to the velocity) by red arrows.

In nearly all tropical cyclones, these low-pressure systems require a minimum distance of approximately 500 kilometers (310 miles) from the equator (about 4.5 degrees from the equator) is normally needed for tropical cyclogenesis.

This all has to do with complex rotational forces that the Earth imparts on the rotation of incipient tropical cyclones as winds begin to flow in toward the lower pressure created by a pre-existing disturbance.

Weak Vertical Wind Shear

Diagram showing the effect of windshear on a tropical cyclone. Image: National Science Teachers Association.
Diagram showing the effect of wind shear on a tropical cyclone. Image: National Science Teachers Association.

In our layered atmosphere, winds move at different speeds and directions as altitude increases. Generally, vertical wind shear of less than 10 meters/second (20 knots, 22 MPH) between the surface and the tropopause is favored for tropical cyclone development.

Weaker vertical wind shear makes the storm grow faster vertically into the air, which aids in development, convection, and allow systems to strengthen.

When wind shear is too strong, convection is disrupted, allowing energy to spread across a larger area but keeping the storm weak. This occurs with showers and thunderstorms in our region.

When wind shear is too weak, the tropical cyclone can fall apart as the mid-level circulation and core become displaced from the surface circulation. This also causes the mid-levels of the system to become too dry, disrupting development.

How Does It Actually Form

When all of these six factors come together, convection is the process that rules. The warm ocean heats the air above it, causing moisture-laden air to rise rapidly.

As the air rises, it cools, condenses, and forms towering cumulonimbus clouds near the center, with smaller, but still intense towers circulating the center of the system as you move further away from the low-pressure center.

Diagram showing the structure of a tropical cyclone formation. Image: NOAA Scijinks
Diagram showing the structure of a tropical cyclone formation. Image: NOAA Scijinks

Rising warm air causes the pressure to decrease at higher altitudes. Warm air is under higher pressure than cold air, so moves towards the ‘space’ occupied by the colder, lower pressure, air. So the low pressure ‘sucks in’ air from the warm surroundings, which then also rises. A continuous flow of warm and wet air continues to create clouds and rain.

The rising air begins to spin around the low-pressure surface center, in an anti-clockwise motion in the Northern Hemisphere). This rapidly rising air creates intense low pressure, causing very strong winds. In hurricanes, this becomes the eye of the storm.

Structure of a hurricane. Diagram: BBC Bitesize
Structure of a hurricane. Diagram: BBC Bitesize

Air is ejected at the top of the storm – which can be 15 km high – and falls to the outside of the storm, out and over the top, away from the eye of the storm. As this happens, it reduces the mass of air over the ‘eye of the storm’ – causing the wind speed to increase further. Some ejected air also cools and dries, and sinks through the eye of the storm, adding to the low pressure at the center.

In the center is the eye of the hurricane, about 45 km across (30 miles) across. Often there will be no clouds in the eye. Seen from below, it will seem calmer, with a circle of blue sky above. The eye is formed because this is the only part of the hurricane where cold air is descending.

Once the storm moves over land, it rapidly loses its structure and begins to dissipate.

What affects the strength of a tropical cyclone?

These large low-pressure systems are particularly sensitive to changes in its environment as it moves. In some cases, these changes allow the cyclone to strengthen into a more powerful tropical storm or hurricane. In other cases, these environmental conditions cause the cyclone to weaken or dissipate.

Changes in Ocean Temperatures

As mentioned above, tropical cyclones require warm sea surface temperatures, and at depth, to support tropical cyclone formation.

Strengthen: If a developing or developed tropical cyclone encounters warmer sea surface temperatures, above 26.5°C (79.7°F), such as moving over the Gulf Stream or just generally warmer waters, the cyclone may strengthen.

Warmer ocean temperatures contribute more moisture and heat to a tropical cyclone, allowing for strengthening.
Warmer ocean temperatures contribute more moisture and heat to a tropical cyclone, allowing for strengthening.

Weaken: If a cyclone encounters cooler water, below 26.5°C (79.7°F), such as when a tropical storm or hurricane moves into the North Atlantic which usually has cooler waters, the tropical system will gradually weaken. In addition, if a cyclone moves too slowly, it may churn up cooler water below the sea surface, contributing to its demise. Cooler water does not allow the needed moisture and heat needed to develop a storm, and can even remove moisture from the air due to cooler overall air.

Cooler ocean temperatures limits moisture and heat to a tropical cyclone, causing weakening.
Cooler ocean temperatures limits moisture and heat to a tropical cyclone, causing weakening.

Encountering Other Air Masses

A general rule for other air masses – a moist air mass can allow for strengthening, while drier air masses will limit strengthening or even cause dissipation.

Strengthen: If a tropical cyclone encounters an air mass that is moist, conditions will generally allow for strengthening, provided that other factors remain favorable.

Saharan Dust (dry air) limited the intensification of then-Tropical Storm Dorian as it traversed the Eastern Caribbean in August 2019. Saharan Dust and dry air are seen in the yellows, oranges, and reds in the above Saharan Air Layer Tracking Product.
Saharan Dust (dry air) limited the intensification of then-Tropical Storm Dorian as it traversed the Eastern Caribbean in August 2019. Saharan Dust and dry air are seen in the yellows, oranges, and reds in the above Saharan Air Layer Tracking Product.

Weaken: If a tropical cyclone encounters an air mass that is drier such as a plume of Saharan Dust which is more common in the Atlantic, this can weaken a system. As the dry air is entrained, cloud development ceases and cloud cover thins, eventually disappearing due to evaporation into the drier air. The loss of moisture robs latent heat from the storm, and this is how many cyclones dissipate.

Encounters With Land

Once a tropical cyclone interacts with any landmass, it is robbed of the heat and moisture from the ocean needed to sustain its strength.

A tropical cyclone typically weakens as it moves over or encounters a landmass.
A tropical cyclone typically weakens as it moves over or encounters a landmass.

Once a tropical cyclone is over land, the cyclone no longer has an unlimited supply of empowering moisture. Also, the land can be warm and dry, or cold, but in either case, it will remove moisture and energy from the storm. The “Brown Ocean” effect (strengthening overland) has allowed for the strengthening of tropical cyclones in rare cases, but this are exceptions, not the norm.

Mountainous topography can facilitate rapid disruption of a tropical cyclone but also dangerous rainfall and landslides on the windward sides of islands.
Mountainous topography can facilitate rapid disruption of a tropical cyclone but also dangerous rainfall and landslides on the windward sides of islands.

Particularly in the Caribbean, tropical cyclones can interact with our mountainous islands. This can disrupt the low-level circulation which aid in weakening. It can also increase friction, which slows the winds of the tropical cyclone. Mountains also remove large amounts of moisture in the form of precipitation due to orographic effects. Such settings are one of the main causes of cyclone-related flooding and landslides.

Most strong storms lose their strength very rapidly after landfall and become disorganized areas of low pressure within a day or two. There is, however, a chance they could regenerate if they manage to get back over open, warm water

Vertical Wind Shear

Vertical wind shear is the difference in wind speed or direction between the upper and lower atmosphere.

Strengthen: Weaker vertical wind shear makes the storm grow faster vertically into the air, which aids in development, convection, and allow systems to strengthen.

Diagram showing the effect of vertical wind shear on convection.
Diagram showing the effect of vertical wind shear on convection.

Weaken: While strong vertical wind shear assists in extra-tropical thunderstorm development, it generally tears apart or dissipates tropical cyclones. Fast winds in the upper-troposphere will shear off the top of the developing tropical cyclone, as the near-surface trade winds push the base of the storm in the opposite direction. Note that vertical wind shear can spread out a storm’s latent heat release over a larger area (as represented by the red boxes), limiting the buildup of the storm.

Interaction With Other Weather Systems

Sinking air or subsidence from high pressure such as the subtropical ridge can also inhibit development. Hurricanes are vertically stacked systems, so they need to have air rise from the surface to the upper levels. Sinking air from high pressure hinders thunderstorm development, which is a critical element in hurricane strengthening.

Upper-level pressures can act to strengthen or weaken a tropical cyclone as they do for a mid-latitude cyclone. They also influence the path of the storm, by drawing it into a trough of low pressure or steering it away from a ridge of high pressure.

It can be weak enough to be consumed by another area of low pressure, disrupting it and joining to become a large area of non-cyclonic thunderstorms. (Such, however, can strengthen the non-tropical system as a whole.)

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