Edexcel GCSE Geography B (1GB0) — Hazardous Earth
Have you ever noticed that a massive storm is called a hurricane in the USA, but a typhoon in Japan? These are regional names for the exact same weather hazard: tropical cyclones. Their source area is restricted to tropical oceans between and North and South of the equator. There is a distinct "equator gap" between and where cyclones never form because the is negligible and cannot induce the necessary rotation. Cyclones follow a clear seasonal pattern, peaking in late summer and autumn when ocean waters are at their highest temperature. This timing is dictated by the migration of the , a low-pressure belt that shifts north and south with the thermal equator. As global temperatures rise, the isotherm moves further from the equator, potentially expanding these source areas toward the poles.
A tropical cyclone acts like a giant, self-sustaining heat engine, but it requires a very specific type of fuel to start. First, the sea surface temperature (SST) must be at least to provide sufficient thermal energy for rapid evaporation. The ocean must also have a deep layer of warm water of at least so that the storm's churning does not bring cold water to the surface and kill the energy supply. Additionally, the atmosphere must have low wind shear. If wind speeds change too rapidly with height, it will tear the vertical storm structure apart before it can intensify into an extreme low-pressure system (where central pressure often falls below ).
Why do these giant storms only form in specific belts around the Earth? The answer lies in global atmospheric circulation. Cyclones are born in the rising arm of the Hadley Cell. Intense solar heating near the equator causes warm, moist air to rise rapidly, a process known as convection. This rising air creates the ITCZ, forming a permanent band of low atmospheric pressure. This steep pressure gradient forcefully draws in surrounding air, providing the constant supply of moisture and instability required to trigger cyclogenesis.
Water is one of the few substances that releases massive amounts of hidden energy when it changes state, and this energy is the driving force behind a cyclone. As warm, moist air rises, it cools and condenses to form towering cumulonimbus clouds. This condensation releases latent heat, which powers the storm and causes even more air to rise. You can remember this causal mechanism with the Edexcel "3Cs" mnemonic: Air Cools, Condenses, and Clouds form. As air rushes in to fill the extreme low pressure at the surface, the Coriolis effect deflects it, causing the storm to spin. Cold air sinks in the centre to form the calm, cloudless eye, surrounded by the eyewall where the most intense winds and heaviest rain occur. The intensity of the resulting storm is measured using the Saffir-Simpson scale, categorising storms from 1 to 5 based on wind speed.
A spinning top moves across a table depending on how you push it; similarly, a cyclone's path is steered by prevailing global winds and the Coriolis effect. The storm track initially moves westward, driven by the equatorial trade winds (easterlies). As the storms move, the Coriolis effect deflects their path, causing them to track poleward (away from the equator). Upon reaching latitudes of around North or South, they encounter Westerly winds that cause them to "recurve" and track eastward, creating a distinct hook shape. A cyclone will eventually dissipate when it loses its source of energy. This happens if it makes landfall, where friction slows it down and it is cut off from the warm ocean, stopping the release of latent heat. Dissipation also occurs if the storm moves over cold water (below ), which deprives it of the thermal energy needed for evaporation.
Students often think tropical cyclones form directly on the equator. You must state they form between 5° and 30° N/S because the Coriolis effect is zero at the equator.
When asked to 'Explain' cyclone formation in a 4- or 6-mark question, examiners expect a step-by-step causal chain starting with SSTs above 26.5°C leading to evaporation, condensation, and the release of latent heat.
Always clearly distinguish between the 'eye' (calm, high pressure, descending air) and the 'eyewall' (intense winds, rising air) when describing the physical structure of a cyclone.
If asked to 'Analyze the distribution', ensure you mention both the spatial element (specific latitudes) and the temporal element (late summer/autumn timing linked to ocean warming).
When analysing storm tracks, remember to explain both forces at play: prevailing winds (like trade winds) steer the initial westward direction, while the Coriolis effect causes the poleward deflection.
Source area
The specific geographic region over warm tropical oceans (between 5° and 30° N/S) where the conditions for cyclogenesis are met.
Coriolis effect
The force resulting from the Earth's rotation that causes moving air to be deflected, creating the spinning vortex of a cyclone and causing the poleward deflection of its track.
Intertropical Convergence Zone (ITCZ)
A permanent low-pressure belt encircling the Earth near the equator where trade winds meet and air rises.
Sea surface temperature (SST)
The temperature of the top layer of the ocean, which must exceed 26.5°C for a tropical cyclone to form.
Wind shear
The change in wind speed or direction with altitude; low wind shear is required so the storm's vertical structure is not destroyed.
Hadley Cell
A large-scale atmospheric convection cell in which air rises at the equator and sinks at medium latitudes, driving tropical weather.
Convection
The process where warm, moist air rises rapidly because it is less dense than the surrounding cooler air.
Latent heat
The hidden thermal energy released when water vapour condenses into liquid water droplets, which acts as the 'fuel' for a cyclone.
Eye
The central area of a tropical cyclone characterised by high pressure, descending cold air, calm winds, and clear skies.
Eyewall
The dense bank of cumulonimbus clouds surrounding the eye, containing the storm's most intense winds and heaviest rainfall.
Saffir-Simpson scale
A 1 to 5 rating scale used to categorise the intensity of tropical cyclones based on their sustained wind speeds.
Storm track
The specific path or trajectory followed by a tropical cyclone as it is steered by global winds and deflected by the Coriolis effect.
Trade winds
The prevailing easterly winds found in the tropics that initially steer tropical cyclones in a westward direction.
Put your knowledge into practice — try past paper questions for Geography B
Source area
The specific geographic region over warm tropical oceans (between 5° and 30° N/S) where the conditions for cyclogenesis are met.
Coriolis effect
The force resulting from the Earth's rotation that causes moving air to be deflected, creating the spinning vortex of a cyclone and causing the poleward deflection of its track.
Intertropical Convergence Zone (ITCZ)
A permanent low-pressure belt encircling the Earth near the equator where trade winds meet and air rises.
Sea surface temperature (SST)
The temperature of the top layer of the ocean, which must exceed 26.5°C for a tropical cyclone to form.
Wind shear
The change in wind speed or direction with altitude; low wind shear is required so the storm's vertical structure is not destroyed.
Hadley Cell
A large-scale atmospheric convection cell in which air rises at the equator and sinks at medium latitudes, driving tropical weather.
Convection
The process where warm, moist air rises rapidly because it is less dense than the surrounding cooler air.
Latent heat
The hidden thermal energy released when water vapour condenses into liquid water droplets, which acts as the 'fuel' for a cyclone.
Eye
The central area of a tropical cyclone characterised by high pressure, descending cold air, calm winds, and clear skies.
Eyewall
The dense bank of cumulonimbus clouds surrounding the eye, containing the storm's most intense winds and heaviest rainfall.
Saffir-Simpson scale
A 1 to 5 rating scale used to categorise the intensity of tropical cyclones based on their sustained wind speeds.
Storm track
The specific path or trajectory followed by a tropical cyclone as it is steered by global winds and deflected by the Coriolis effect.
Trade winds
The prevailing easterly winds found in the tropics that initially steer tropical cyclones in a westward direction.
