Tropical Cyclones (Typhoons)
FoundationTropical cyclones form over warm seas (≥26.5°C). Structure: outer spiral rainbands, the eyewall (strongest winds), and the calm eye at the centre. In Hong Kong we call them typhoons — bringing gales, torrential rain and storm surges.
- Formation: warm sea + moist air + rotation
- Structure: rainbands → eyewall (strongest) → eye (calm)
- Hazards: wind, rain, storm surge; heed warning signals
§Text Passage
1A tropical cyclone is a giant storm born over warm seas. The water must be at least 26.5°C to supply enough energy; evaporating moisture rises and condenses, releasing vast latent heat that keeps driving the winds stronger, while the Coriolis effect from Earth's rotation spins the whole system. Because the Coriolis force is almost nil at the equator, cyclones generally form over tropical waters at least about five degrees of latitude away; the northwest Pacific is warmest in late summer and autumn, and storms forming east of the Philippines most often approach the South China coast between July and September.
2A mature cyclone has a clear structure. Outermost are the spiral rainbands curling inward ring by ring, bringing squalls from time to time; the middle layer is the eyewall, the ring of cloud around the eye where the fiercest wind and rain concentrate; at the very centre lies the eye, where sinking air brings strangely calm, almost cloudless weather. That calm is only a pause: as the storm moves on, the opposite eyewall strikes soon after and the wind reverses direction abruptly — never assume the storm is over.
3The cyclone brings a triple hazard: gales tear down scaffolding, signs and large trees; torrential rain floods urban districts and triggers landslides on the slopes; and the storm surge raises the sea abnormally, pushing seawater over low-lying coasts — often the deadliest of the three. When Super Typhoon Mangkhut hit Hong Kong in 2018, the Observatory hoisted Signal No. 10, the surge severely flooded coastal areas such as Heng Fa Chuen and Tai O, and over 60,000 fallen trees were reported citywide; back in 1962, Typhoon Wanda's storm surge claimed more than a hundred lives.
4Before a typhoon arrives, follow the Observatory's warning signals from T1, T3 and T8 up to T10 — the higher the number, the greater the danger — and secure doors and windows early, stock drinking water and food, and keep away from the shore, slopes and temporary structures. Never go outside during the eye's calm; residents of low-lying areas should follow official advice and evacuate to safety early.
1Summary 重點摘要
Tropical cyclones form over warm seas (≥26.5°C). Structure: outer spiral rainbands, the eyewall (strongest winds), and the calm eye at the centre. In Hong Kong we call them typhoons — bringing gales, torrential rain and storm surges.
2Vocabulary cards Vocabulary
The calm, relatively cloud-free centre of the storm.
The ring of cloud around the eye — where winds and rain are fiercest.
Abnormally raised sea level driven by winds and low pressure — floods coasts.
Outer bands of rain spiralling in, bringing squalls and showers.
A rotating storm system born over warm seas.
The Earth's rotation deflects moving air — it makes cyclones spin.
Heat released when water vapour condenses — the cyclone's power source.
The name for tropical cyclones in the NW Pacific — Hong Kong's usage.
3Step by step Steps
- FormationA tropical cyclone needs several conditions at once: warm seas of at least 26.5°C supply the energy; seawater evaporates, rises and condenses, releasing vast latent heat that drives the air ever faster upward; and the Coriolis effect from Earth's rotation spins the whole system. This is why seas near the equator, though warm, rarely produce cyclones — the Coriolis force there is nearly zero, so cyclones generally form over tropical waters at least about five degrees of latitude from the equator.
- Three-layer structureA mature tropical cyclone has three layers. On the outside, spiral rainbands curl inward ring by ring, bringing squalls from time to time. In the middle sits the eyewall, the ring of cloud around the eye, where the fiercest wind and rain concentrate. At the very centre is the eye itself: air sinks there, so the weather turns strangely calm and almost cloudless, with the sky even visible. The eye's calm easily misleads people into thinking the storm is over — in fact the danger is not yet past.
- HazardsA cyclone brings a triple hazard. Gales tear down scaffolding, signs and large trees, and wind-blown debris can injure people. Torrential rain floods urban districts and triggers landslides on slopes. Storm surges raise the sea abnormally and push seawater over low-lying coasts, drowning streets and basements. Of the three, the surge is often deadlier than the wind itself, and most historical mass casualties came from it — the cyclone's number-one killer.
- PreparednessBefore a typhoon reaches Hong Kong, follow the Observatory's warning signals, from T1, T3 and T8 up to T9 and T10 — the higher the number, the greater the danger. Secure doors, windows and scaffolding early, stock up on drinking water and food, and keep away from the shore, slopes and temporary structures. The calm of the eye is only a pause — never go outside during it; residents of low-lying areas should follow official advice and evacuate to safety early.
- Tracks and seasonIn late summer and autumn the northwest Pacific is at its warmest, supplying cyclones with abundant energy. They usually form east of the Philippines and travel around the edge of the subtropical high, entering the South China Sea towards the South China coast most often between July and September. In a typical year several tropical cyclones stray into Hong Kong's warning zone, creating a distinct 'typhoon season' when residents must stay alert and watch forecasts closely.
- Hong Kong case studiesSuper Typhoon Mangkhut in 2018 brought Signal No. 10 to Hong Kong: sea, land and air transport were paralysed, the storm surge severely flooded low-lying coastal estates such as Heng Fa Chuen and Tai O, over 60,000 fallen trees were reported citywide, and many districts suspended schools and work for days. Back in 1962, Typhoon Wanda's storm surge claimed more than a hundred lives. Both cases teach the same lesson: the surge is often a typhoon's deadliest element, and coastal protection cannot be neglected.
4Diagrams Diagrams
5Worked example Example
Why is it calm when the eye passes — yet you still can't go outside?
- The eye is the storm's centre with sinking air → calm
- But the eye MOVES — the opposite eyewall arrives soon after
- Eyewall winds can be several times stronger than conditions in the eye
- So the calm is only a pause — the wind direction reverses abruptly
- Answer: Inside the eye, sinking air brings calm — but the eye travels with the storm, and the opposite eyewall strikes soon after, with winds several times stronger than in the eye and the wind direction reversing abruptly. The calm is only an illusion: stay indoors until the Observatory hoists down all signals, or the suddenly returning wind and rain could injure you — never dash outside.
6Common mistakes Mistakes
7Quick check Quick check
Q1What minimum sea temperature does a cyclone need to form?
Q2Order the cyclone structure from outside in.
Q3Name three hazards of tropical cyclones.
Q4Why do tropical cyclones rarely form right on the equator?
Q5Give one severe Hong Kong typhoon and its impacts.
8Recap Recap
- Formation: warm sea + moist air + rotation
- Structure: rainbands → eyewall (strongest) → eye (calm)
- Hazards: wind, rain, storm surge; heed warning signals
- Needs seas ≥26.5°C; latent heat from condensing vapour drives it
- Calm in the eye is only a pause — the opposite eyewall follows