Hurricanes, typhoons and cyclones: these terms all conjure up an image of total destruction left by heavy winds and rain. While these are three different terms, they all mean the same thing: a tropical storm. Let's look at what tropical storms are, their formation and effects, and some examples.
Tropical storms definition
Tropical storms aren't exactly your light breeze. Tropical storms are extremely powerful and can travel very fast, up to 40mph/65 km/h! Tropical storms are also accompanied by heavy rainfall and strong winds. Let's define them properly.
A tropical storm is a low-pressure area with winds moving in a spiral motion around the 'eye of the storm', which is a calm, central point.
Tropical storms occur when warm, tropical air rises, creating an intense low-pressure area. Then, as the moist, warm air reaches high altitudes, powerful winds swirl around the calm, central point. This creates the famous 'eye of the storm'. As the warm air cools, it condenses, resulting in heavy rainfall and thunderstorms.
The intensity of a tropical storm is recorded using the Saffir-Simpson hurricane wind scale (SSHWS). Note that "tropical storm" is a general term applied to all storms, but it is also a term for the category between tropical depression and Category 1 storm: a storm with winds of 39-73mph/63-118 km/h.
Fig. 1 - Saffir-Simpson Scale details
Types of tropical storms
Now, when it comes to tropical storms, you have probably heard different terms. That would be correct..."huh"?! Well, don't fret; the explanation is actually very simple: all those terms are correct. There is no difference between them other than their location.
| Different types of tropical storms |
|---|
| Type | Formation location | Affected areas | Example |
| Hurricanes | North Atlantic Ocean and Northeast Pacific | US and Caribbean | Hurricane Katrina in 2005 |
| Cyclones | South Pacific and the Indian Ocean | South Asia | Cyclone Pam in 2015 |
| Typhoons | Northwest Pacific Ocean | East Asia | Typhoon Haiyan in 2013 |
| Table 1 |
Tropical storms form at different times of the year, depending on their location.
| Where and when do tropical storms occur? |
|---|
| Location | Months occurring | Peaks |
| North Atlantic Ocean, the Gulf of Mexico and the Caribbean Sea | June - November | early to mid-September |
| Northeast Pacific basin | Late May/early June - Late October/early November | late August/early September |
| Northwest Pacific basin | Occur all year round, but mainly July - November | late August/early September |
| North Indian basic | April - December | May and November |
| Southwest Indian basin | Late October/early November - May | mid-January and mid-February/early March |
| Southeast Indian/Australian basin | Late October/early November - May | mid-January and mid-February/early March |
| Australian/Southwest Pacific basin | Late October/early November - May | late February/early March |
Table 2 |
Global distribution of tropical storms
The global distribution, formation and development of tropical storms are related to global atmospheric circulation. The transportation of heat from tropical to polar latitudes is necessary, and the global atmospheric circulation is a global wind system providing that transportation.
Tropical storms occur in, you've guessed it, the tropics, in particular where the so-called intertropical convergence zone (ITCZ) is situated, where the northeast and southeast trade winds converge - roughly north of the Tropic of Capricorn and south of the Tropic of Cancer.
Most often, tropical storms form between 5o and 30o north and south of the Equator, i.e. low latitudes. Tropical storms most commonly occur in the warmest seasons, between summer and autumn. Furthermore, tropical storms need ocean temperatures of at least 27oC and a water depth of about 197-230 ft/60-70m to form a storm.
As you can see, tropical storms form under specific conditions. Tropical storms cannot develop along the Equator because the Coriolis effect is not strong enough here, which is needed for tropical storms to spin. Read more on the formation of tropical storms below.
For more in-depth information, read our explanation of global atmospheric circulation.
Formation of tropical storms
Now that you know what tropical storms are, let's look at their formation, which happens in a specific sequence.
When solar heating causes the ocean's surface temperature to increase to at least 27oC, the low-pressure conditions cause the warm air to rise, creating strong winds. Around a calm central eye of the storm, which can measure up to 30 mi/48 km across, the air spins upwards because of the Coriolis effect. As the warm air rises, it cools, and the water vapours condense to form cumulonimbus clouds, causing massive rainfall. When the air cools, the heat given off powers the storm. Cold air sinks, so when the air cools, it falls into the eye of the storm. That is why the eye of the storm has no clouds; it is drier and much calmer. The prevailing winds cause the storm to travel across the ocean, potentially making landfall at some point.
The Coriolis effect refers to the effect that the Earth's rotation has on weather patterns and ocean currents. This effect makes storms move counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
Cumulonimbus clouds are tall, large, dense grey clouds. They produce rain, hail or snow.
Remember: when tropical storms reach a land surface, their energy will decrease because the ocean no longer provides the storm with its much-needed heat energy and moisture.
Effects of tropical storms
Because tropical storms are so powerful, they almost always leave a path of destruction, especially when they are near or travel over land. Between 2001 and 2010, over 500 tropical storm disasters killed nearly 170,000 people, affected more than 250 million people and caused estimated damages at US$380 billion!
There are primary and secondary effects, which can differ per location.
| Primary and secondary effects of tropical storms |
|---|
| Primary | Secondary |
| Strong winds often cause widespread damage to infrastructure, buildings, trees and crops. | Heavy rains can lead to unstable slopes causing landslides/mudslides. |
| Due to precipitation, strong winds and rising sea levels can cause storm surges, resulting in extensive coastal flooding. | Water supplies can become polluted, causing diseases and potential further deaths. |
| Widespread flooding due to heavy rainfall. | Damages to houses can lead to uninhabitable homes. This forces people to live in make-shift shelters. |
| Localised tornadoes can occur. | Damaged infrastructure often means that basic services, such as powerlines, must be repaired. Road damage can also lead to basic provisions not reaching affected areas until they are fixed. |
| Table 3 |
Tropical storms and climate change
It should not come as a surprise that climate change also affects tropical storms, with the potential of tropical storms happening in more and different locations. An effect of climate change is the warming up of oceans. Warmer oceans can allow storms to extend further north and south of the Equator, causing bigger storms and putting more habitable land areas and people at risk. Low-lying coasts, where many people live, are more at risk than ever before.
Regardless of an increase or decrease in the number of tropical storms happening, they will most likely increase in intensity, causing more damage and potential loss of life. Another effect of climate change is rising sea levels. This will make storms more destructive and increase coastal flooding.
Examples of tropical storms
With an average of 80-100 tropical storms per year, it is no surprise to learn there have been many devastating storms over many years. Let's look at a few examples.
Cyclone Bhola
The 1970 Bhola Cyclone occurred in East Pakistan (present-day Bangladesh) on 11 November 1970. The storm formed over the central Bay of Bengal on 8 November, from where it travelled north, gaining strength. On 10 November, the storm reached winds speeds peaking at 115 mph/185 km/h before making landfall on East Pakistan's coast. The following storm surge destroyed many offshore islands, wiping out entire villages and destroying crops. The storm eventually dissipated on 13 November.
The Pakistani government, then led by Junta leader General Yahya Khan, was heavily criticised as relief operations were delayed.
The Bhola Cyclone went down in history as the deadliest tropical cyclone ever recorded and one of the deadliest natural disasters in the world. At least 300,000 and possibly as many as 500,000 people lost their lives.
Fig. 4 - Weather satellite image showing Cyclone Bhola at the Bay of Bengal on 12 November 1970
The 1970 Bhola Cyclone, also known as the Great Cyclone of 1970, was a Category 4 tropical cyclone on the SSHWS.
Typhoon Nina
Typhoon Nina formed on 30 July 1975 and gained strength as it moved west. Peak intensity was reached on 2 August, and on 3 August, it hit Taiwan. As the storm moved to China's southeastern shore, it weakened slightly. That said, it wreaked havoc in this region! The storm moved through central China at a slow pace with massive rainfall. This caused many dams to collapse, such as the Banqiao dam, causing the third-deadliest flood in history, affecting over 10 million people. At least 229,000 people died. Material damages were estimated at US$1.2 billion.
Fig. 5 - Map showing the path of Typhoon Nina
Typhoon Nina was a Category 4 super typhoon (SSHWS).
Hurricane Sandy
On 22 October 2012, south of Jamaica in the western Caribbean Sea, Sandy developed as a tropical wave, where it quickly gained strength and was upgraded to a tropical storm a mere six hours later. On 24 October, the eye of the storm began to develop and later that day, the National Hurricane Center (NHC) upgraded the storm to a hurricane. In the evening of the same day, Hurricane Sandy, with winds of about 85 mph/105 km/h, made its first landfall near Kingston. When Sandy made its second landfall west of Santiago de Cuba on 25 October, the storm intensified into a Category 3 hurricane. Over the next few days, Sandy fluctuated between being a Tropical Storm, a Category 1 hurricane and a Category 2 hurricane before dissipating on 2 November. Sandy left a trail of destruction, with 233 people killed and material costs estimated at US$68.7 billion.
Fig. 6 - Map showing the path of Hurricane Sandy
Now that you know more about tropical storms learn more about managing them in our explanation on tropical storm management. If you want more case studies, read our explanations on Hurricane Katrina, Cyclone Nargis and Typhoon Haiyan.
Tropical Storms - Key takeaways
- Hurricanes, cyclones, and typhoons are all names for the same thing: tropical storms. Which term is used depends on where you live.
- The global distribution, formation and development of tropical storms are related to global atmospheric circulation.
- For a tropical storm to form, the surface water needs to be at least 27oC, and the water depth needs to be 197-230 ft/60-70m.
- Tropical storms are affected by climate change. Climate change will be responsible for tropical storms occurring with ever-increasing intensity in more and different locations.
- Examples of tropical storms are Cyclone Bhola, Hurricane Sandy and Typhoon Nina.
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