Edexcel GCSE Geography A (1GA0) — The Physical Environment
Every time you boil a kettle, you see liquid water turn into steam, but ocean storms do exactly the opposite on a massive scale.
Tropical cyclones are intense low-pressure systems that begin over warm oceans. The initial trigger happens at the Intertropical Convergence Zone (ITCZ), a belt near the equator where trade winds meet. Here, the rising arm of the Hadley Cell forces warm, moist air upwards.
For a cyclone to form, the sea surface temperature (SST) must be at least 27°C to a depth of 50–70 metres. As the warm air rises, it cools and condenses to form towering cumulonimbus clouds. This condensation releases latent heat, which acts as the storm's engine, rapidly lowering the central atmospheric pressure (typically below 950mb).
Air rushes in to fill this low-pressure void, but the Coriolis effect deflects these winds, initiating the storm's spin. The system also requires low wind shear so the developing clouds are not torn apart. The storm will only dissipate if it loses its heat and moisture source by making landfall, moving over colder water (below 26.5°C), or encountering high wind shear.
You can snap a piece of chalk easily, but try snapping a diamond; similarly, the outside of a cyclone is violently destructive, but its centre is perfectly calm.
Tropical cyclones are massive systems, often measuring up to 640km in diameter and reaching 10km into the atmosphere. The very centre is called the eye, which is 30–50km wide. Unlike the rest of the storm, the eye features descending (sinking) air, meaning it has no clouds, no rain, and light winds.
Immediately surrounding this calm centre is the eyewall. This ring of clouds contains rapidly rising air, creating the storm's most intense winds and heaviest rainfall. Spiralling outwards from the eyewall are vast rain bands that deliver intense bursts of precipitation.
To be classified as a tropical cyclone, sustained winds must exceed 119 km/h. Intensity is measured using the Saffir-Simpson Scale, ranging from Category 1 up to Category 5 (over 252 km/h). As they approach land, extreme winds and low atmospheric pressure create a storm surge, pushing a dangerous wall of seawater onto the coast.
Why do some regions face devastating hurricanes every autumn, while others never see them at all?
Tropical cyclones only form in specific tropical latitudes, between 5° and 30° north and south of the equator. They fundamentally cannot form between 0° and 5° because the Coriolis force is too weak to create the necessary spin (at the equator, ).
Globally, around 80 of these storms occur each year, with the Pacific Ocean experiencing the highest frequency (the North Atlantic accounts for only 15%). Their regional names change depending on where they form: they are called hurricanes in the Atlantic and Northeast Pacific, typhoons in the Northwest Pacific, and cyclones in the Indian and South Pacific Oceans.
They are highly seasonal, forming in late summer and autumn when ocean temperatures are highest (June to November in the Northern Hemisphere). Once formed, easterly trade winds steer them westward at roughly 300–400 miles per day before they eventually curve towards the poles.
Historical records show that while the total number of storms each year isn't growing, the amount of destructive energy they carry is soaring.
Analysis of historical data reveals that while global frequency remains steady at around 80 storms per year, their magnitude and intensity have increased by 70% over the last 30 years. Global warming and climate change directly influence this; for every 1°C increase in sea surface temperatures, wind speeds are predicted to rise by 3–5%.
Warmer atmospheres also hold more moisture—roughly 7% more for every 1°C rise. This has caused a 4% increase in atmospheric water vapour over 25 years, leading to far heavier rainfall during storms. Additionally, global sea levels have risen by approximately 19cm since 1901, drastically increasing the destructive height of storm surges.
We are also seeing a shift in temporal patterns and spatial distribution. Warming oceans mean temperatures stay above 27°C for longer, extending the cyclone season. Furthermore, there is a poleward shift; cyclones can now form at higher latitudes, evidenced by Hurricane Catarina in 2004, which was the first recorded hurricane in the South Atlantic.
Students often state that tropical cyclones form exactly on the equator because it is the warmest region, but they actually cannot form between 0° and 5° because the Coriolis effect is too weak to create spin.
Remember that air in the eye of the storm is descending (sinking), which is why it has no clouds, whereas the surrounding eyewall consists of violently rising air.
In 'Explain' questions about storm formation, examiners specifically look for the term 'latent heat' to explain how thermal energy from the ocean is converted into wind energy.
When asked to 'Analyse' changing trends, do not just say 'storms get worse due to climate change'; use specific evidence from the data, such as 'wind speeds increase by 3-5% for every 1°C rise in sea surface temperature'.
Low-pressure
An atmospheric condition created when warm air rises, drawing in surrounding air and often leading to unsettled, stormy weather.
Intertropical Convergence Zone (ITCZ)
An intense low-pressure belt near the equator where trade winds meet and warm air rises, acting as a trigger for tropical cyclones.
Trade winds
Tropical winds that blow from east to west, responsible for steering cyclones toward landmasses from their oceanic source areas.
Hadley Cell
A major global atmospheric circulation system where warm air rises at the equator and sinks at approximately 30° north and south.
Sea surface temperature (SST)
The temperature of the top layer of the ocean, which must be at least 27°C to a depth of 50-70m to form a tropical cyclone.
Latent heat
The thermal energy released when water vapour condenses into liquid water; it acts as the primary 'engine' driving a tropical cyclone.
Coriolis effect
The deflection of air movement caused by the Earth's rotation, creating the anti-clockwise or clockwise spin of a cyclone.
Wind shear
The variation in wind speed or direction at different altitudes; tropical cyclones require low wind shear to remain intact.
Eye
The calm, clear central area of a tropical cyclone, typically 30-50km wide, characterised by descending (sinking) air and high pressure.
Eyewall
A ring of towering cumulonimbus clouds surrounding the eye, containing the storm's rapidly rising air, most intense winds, and heaviest rainfall.
Saffir-Simpson Scale
A 1-5 scale used to categorise tropical cyclone intensity based on their sustained wind speeds.
Storm surge
A rapid and dangerous rise in coastal sea level caused by a combination of extremely low atmospheric pressure and strong winds pushing seawater ashore.
Tropical latitudes
Regions of the Earth located between 5° and 30° north and south of the equator, providing the warm ocean temperatures required for tropical cyclones.
Historical data
Past records and measurements of weather events used by geographers to identify long-term trends and changes.
Magnitude
The size, extent, or intensity of a tropical cyclone, often relating to its destructive power or the energy it releases.
Global warming
The long-term heating of the Earth’s climate system, primarily due to human activities, leading to rising sea surface temperatures.
Climate change
Long-term shifts in temperatures and weather patterns globally, which influence the frequency, intensity, and distribution of tropical cyclones.
Temporal patterns
Trends or changes that occur over time, such as shifts in the length or timing of the tropical cyclone season.
Put your knowledge into practice — try past paper questions for Geography A
Low-pressure
An atmospheric condition created when warm air rises, drawing in surrounding air and often leading to unsettled, stormy weather.
Intertropical Convergence Zone (ITCZ)
An intense low-pressure belt near the equator where trade winds meet and warm air rises, acting as a trigger for tropical cyclones.
Trade winds
Tropical winds that blow from east to west, responsible for steering cyclones toward landmasses from their oceanic source areas.
Hadley Cell
A major global atmospheric circulation system where warm air rises at the equator and sinks at approximately 30° north and south.
Sea surface temperature (SST)
The temperature of the top layer of the ocean, which must be at least 27°C to a depth of 50-70m to form a tropical cyclone.
Latent heat
The thermal energy released when water vapour condenses into liquid water; it acts as the primary 'engine' driving a tropical cyclone.
Coriolis effect
The deflection of air movement caused by the Earth's rotation, creating the anti-clockwise or clockwise spin of a cyclone.
Wind shear
The variation in wind speed or direction at different altitudes; tropical cyclones require low wind shear to remain intact.
Eye
The calm, clear central area of a tropical cyclone, typically 30-50km wide, characterised by descending (sinking) air and high pressure.
Eyewall
A ring of towering cumulonimbus clouds surrounding the eye, containing the storm's rapidly rising air, most intense winds, and heaviest rainfall.
Saffir-Simpson Scale
A 1-5 scale used to categorise tropical cyclone intensity based on their sustained wind speeds.
Storm surge
A rapid and dangerous rise in coastal sea level caused by a combination of extremely low atmospheric pressure and strong winds pushing seawater ashore.
Tropical latitudes
Regions of the Earth located between 5° and 30° north and south of the equator, providing the warm ocean temperatures required for tropical cyclones.
Historical data
Past records and measurements of weather events used by geographers to identify long-term trends and changes.
Magnitude
The size, extent, or intensity of a tropical cyclone, often relating to its destructive power or the energy it releases.
Global warming
The long-term heating of the Earth’s climate system, primarily due to human activities, leading to rising sea surface temperatures.
Climate change
Long-term shifts in temperatures and weather patterns globally, which influence the frequency, intensity, and distribution of tropical cyclones.
Temporal patterns
Trends or changes that occur over time, such as shifts in the length or timing of the tropical cyclone season.
