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examBoard: Cambridge
examType: IGCSE
lessonTitle: Causes of Tropical Cyclones
Tropical cyclones are among the most powerful and destructive weather systems on Earth. These massive storms bring intense winds, torrential rainfall and storm surges that can devastate coastal communities. Understanding what causes these extreme weather events is crucial for predicting their behaviour and preparing for their impacts.
Key Definitions:
Tropical cyclones are known by different names around the world:
Despite these different names, they are all the same weather phenomenon.
Tropical cyclones are categorised based on their wind speeds:
The Saffir-Simpson Hurricane Wind Scale is commonly used to categorise Atlantic hurricanes.
Tropical cyclones don't just appear randomly. They require specific conditions to form and develop. Let's explore the key factors that contribute to their formation:
Sea surface temperatures must be at least 26-27°C to a depth of about 50 metres. This warm water provides the energy and moisture that fuels the storm through evaporation and condensation.
The Coriolis effect (caused by Earth's rotation) must be strong enough to create the spinning motion. This is why tropical cyclones rarely form within 5° latitude of the equator, where the Coriolis effect is too weak.
Minimal vertical wind shear (change in wind direction or speed with height) is needed. High wind shear disrupts the vertical structure of developing cyclones, preventing them from organising.
A moist layer in the middle troposphere is essential. Dry air can weaken a developing tropical cyclone by promoting evaporative cooling and downdrafts.
A pre-existing weather disturbance or weak low-pressure system is needed to provide the initial rotation and convergence of air.
The atmosphere must be unstable enough to support thunderstorm activity, allowing for deep convection (rising air) to occur.
Warm ocean water is the primary energy source for tropical cyclones. Let's explore why this factor is so crucial:
Tropical cyclones operate like giant heat engines, converting the thermal energy from warm ocean waters into the kinetic energy of winds. This process works through a series of steps:
The warmer the water, the more evaporation occurs, providing more energy to fuel the cyclone. This is why tropical cyclones rapidly weaken when they move over cooler water or land, as their energy source is cut off.
Hurricane Katrina provides a clear example of how warm ocean waters can intensify a tropical cyclone. As Katrina passed over the unusually warm waters of the Gulf of Mexico (30-32°C), it rapidly intensified from a Category 3 to a Category 5 hurricane in just 12 hours. This rapid strengthening occurred because the warm Gulf waters provided abundant energy for the storm. Katrina eventually made landfall as a Category 3 hurricane, causing catastrophic damage to New Orleans and surrounding areas, with over 1,800 deaths and $125 billion in damage.
The Coriolis effect is a crucial factor in the formation and behaviour of tropical cyclones. This phenomenon is caused by Earth's rotation and affects the direction of moving objects (including air) on the planet's surface.
The Coriolis effect causes moving objects to appear to deflect:
This effect is strongest at the poles and weakest at the equator. In fact, it's virtually non-existent within about 5° latitude of the equator, which explains why tropical cyclones rarely form in this region despite warm ocean temperatures.
The Coriolis effect determines the direction of tropical cyclone rotation:
This consistent rotation pattern helps meteorologists identify and track developing tropical systems worldwide.
Tropical cyclones develop through a series of stages, from initial disturbance to a fully formed storm. Understanding this process helps explain why only some weather disturbances develop into powerful cyclones.
Tropical cyclones form in specific regions where the necessary conditions exist:
Note that tropical cyclones are absent near the equator (within 5° latitude) and in the South Atlantic (except for rare cases).
Research suggests climate change is affecting tropical cyclones in several ways:
While the total number of cyclones may not increase, the proportion of intense storms (Categories 4-5) is expected to grow.
Typhoon Haiyan (known as Yolanda in the Philippines) demonstrates the perfect combination of factors that create an extremely powerful tropical cyclone. Forming over exceptionally warm Pacific waters (29-30°C), Haiyan encountered minimal wind shear and a moist atmosphere, allowing it to rapidly intensify. The storm reached estimated sustained winds of 315 km/h (195 mph), making it one of the strongest tropical cyclones ever recorded at landfall. Haiyan's catastrophic impact on the Philippines resulted in over 6,300 deaths, primarily from the massive storm surge that reached up to 7 metres in height. The storm's extreme intensity has been partially attributed to the unusually warm ocean temperatures in the region, potentially influenced by climate change.
To summarise, tropical cyclones form and strengthen when specific environmental conditions align:
Understanding these causes helps meteorologists predict when and where tropical cyclones might form, giving communities valuable time to prepare for these powerful storms.
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