Key Terms

📖 Key Terms

tropical storm — An intense low-pressure weather system formed over tropical oceans where sea surface temperature is ≥26°C, producing extreme sustained winds (>119 mph), heavy rainfall, and storm surge. The same type of storm is called a hurricane in the North Atlantic and Caribbean, a typhoon in the western North Pacific, and a cyclone in the Indian Ocean and Southern Hemisphere.

storm surge — A dome of ocean water pushed onshore by a tropical storm's winds and by the low atmospheric pressure beneath the storm, which allows sea level to rise. Surges are typically the deadliest component of a tropical storm — they can raise sea level by several metres and push seawater kilometres inland with enormous force. Typhoon Haiyan produced a 7.5 m surge at Tacloban. Surge height is amplified over shallow continental shelves.

Coriolis effect — The apparent deflection of moving air (or water) caused by the Earth's rotation. In the Northern Hemisphere, air is deflected to the right of its direction of travel; in the Southern Hemisphere, to the left. The Coriolis effect causes tropical storms to rotate anticlockwise (Northern Hemisphere) or clockwise (Southern Hemisphere). The effect is zero at the equator, which is why tropical storms cannot form there — no deflection means no rotation, and no rotation means no organised storm structure.

eye — The calm, cloudless centre of a mature tropical storm, typically 20–65 km in diameter. Air in the eye is sinking and being compressed by surrounding atmospheric pressure, suppressing cloud formation. Atmospheric pressure reaches its minimum value here. As the eye passes over a location, there is a period of eerie calm — followed by the eyewall arriving from the opposite direction.

eyewall — The ring of towering cumulonimbus cloud immediately surrounding the eye, containing the storm's most intense winds, heaviest rainfall, and most violent lightning. Wind speeds are highest in the eyewall. The 195 mph winds of Typhoon Haiyan were eyewall winds.

latent heat — The energy absorbed when a liquid evaporates (becomes vapour) and released when a vapour condenses (becomes liquid). In tropical storm formation, evaporation from the warm ocean surface absorbs latent heat; that energy is released into the atmosphere when the vapour condenses high above the surface. This released heat warms the rising air column, driving the positive feedback loop that intensifies the storm.

trade winds — Persistent surface winds that blow from subtropical high-pressure zones toward the equatorial low, deflected westward by the Coriolis effect. Trade winds are responsible for the initial westward steering of tropical storms after they form.

managed retreat — A land management approach that accepts periodic flooding or coastal inundation as natural, prioritising ecological benefit or upstream water retention over hard engineering defences. Used on the Somerset Levels by the Environment Agency before 2014; highly controversial with local farmers and residents who favoured active dredging and pumping.

rhynes — The network of man-made drainage channels (ditches) on the Somerset Levels, maintained since medieval times. When rhynes are poorly maintained or blocked by sediment, the drainage capacity of the Levels is significantly reduced, increasing flood risk.

storm surge vs flooding from rainfall — Two distinct processes often confused. A storm surge is seawater pushed onshore by atmospheric forces; it is saltwater, fast-moving, and can arrive far more rapidly than a river flood. Rainfall flooding occurs when precipitation exceeds the capacity of rivers, drains, or soil to absorb and channel it. Tropical storms cause both simultaneously, but the surge is typically more immediately deadly.