What a tropical revolving storm is and how it forms
1The lesson in plain words
A tropical revolving storm is an intense low fed by a warm ocean. Learn its names, its wind categories and what it needs to form.
Names and categories
A tropical cyclone, also called a tropical revolving storm (TRS), is an intense low-pressure system over warm tropical seas. Its strength is its maximum sustained wind; US advisories use a 1-minute mean. In those advisories, a tropical depression has winds up to 33 knots; a tropical storm has 34–63 knots and gets a name. At 64 knots or more it is a hurricane in the Atlantic and eastern and central North Pacific, and a typhoon in the north-west Pacific. Other basins use regional labels and wind-averaging periods.
The Saffir-Simpson scale
Hurricanes are graded on the Saffir-Simpson Hurricane Wind Scale: category 1 is 64–82 knots, 2 is 83–95, 3 is 96–112, 4 is 113–136 and 5 is 137 knots or more. Category 3 and above is a major hurricane. The scale counts only peak wind. A large category 1 storm can spread rough seas over a far wider area than a small category 4.
Structure
Winds spiral in towards the centre: anticlockwise in the northern hemisphere, clockwise in the southern. A well-developed cyclone may have a calm eye, typically a few tens of kilometres across, ringed by the eyewall, where the strongest winds and heaviest rain are. Rain bands spiral outwards. Hurricane-force winds may reach about 20 nautical miles from the centre of a small storm and over 150 in a large one; tropical-storm-force winds reach much further.
What it needs to form
Most storms form between about 5° and 30° of latitude, from a pre-existing disturbance such as a tropical wave: a trough drifting west in the trade winds. They usually develop over seas of at least 26.5 °C, with warm water about 60 m deep. Other ingredients are low vertical wind shear (little change of wind with height), moist air and enough Coriolis effect, the turning force of Earth's rotation. That is why they almost never form within about 5° of the equator.
Life and death
A storm's strengthening rate varies widely. Rapid intensification, 30 knots or more in 24 hours, is hard to forecast. Storms weaken over cooler water, over land or in strong shear. Many become extratropical at higher latitudes: their core loses its warmth relative to surrounding air. Dangerous winds and seas can remain as the storm accelerates.
2Key data
- Tropical depression
- Maximum sustained wind 33 kn or less
- Tropical storm
- 34–63 kn; receives a name
- Hurricane / typhoon
- 64 kn or more (1-minute mean in US advisories)
- Major hurricane
- Category 3 or more: 96 kn and above
- Sea temperature to form
- Usually ≥26.5 °C, with warm water about 60 m deep
- Rapid intensification
- Increase of 30 kn or more in 24 hours
3Key points
- 1
US advisories use 1-minute means: depression ≤33 kn; tropical storm 34–63 kn; hurricane ≥64 kn.
- 2
Northern hemisphere storms turn anticlockwise; southern hemisphere storms turn clockwise.
- 3
Typical formation conditions: warm sea (26.5 °C or more), low shear, moist air, a disturbance and Coriolis.
- 4
The Saffir-Simpson category measures peak wind only, not the size of the storm.
- 5
Storms that become extratropical can stay dangerous and often speed up.
4Frequent mistakes
Wrong: A category 1 hurricane is always less dangerous to a vessel than a category 4.
Right: Category counts only peak wind. A big category 1 can cover a much larger area with gales and high seas.
Wrong: Tropical storms form right on the equator, where the sea is warmest.
Right: They need the Coriolis effect, which is usually too weak within about 5° of the equator.
Wrong: Once a storm is called extratropical, the danger is over.
Right: The name changes, not the marine danger. Extratropical storms can stay large and violent and often accelerate.
5Practice case & exam keys
In September, east of the Caribbean, the advisory describes a system with maximum sustained winds of 60 knots, a well-defined circulation and a sea surface temperature of 29 °C. Another crew member says it is ‘only a tropical storm’. How do you classify it, and what could happen next?
Show the answer
At 60 knots it is a tropical storm, close to the 64-knot hurricane threshold. Warm water, the peak of the Atlantic season and a well-organised circulation favour strengthening, possibly rapid. Treat it as a storm that may become a hurricane within a day and plan to keep well clear of its whole wind field, not just its centre.
Exam keys
- US advisories use 1-minute means: depression ≤33 kn; tropical storm 34–63 kn; hurricane ≥64 kn.
- Saffir-Simpson: category 1 starts at 64 kn, category 3 (major) at 96 kn, category 5 at 137 kn.
- Typical formation conditions: sea at least 26.5 °C, low shear, moist air, a disturbance, latitude over about 5°.
- Rotation: anticlockwise in the northern hemisphere, clockwise in the southern hemisphere.
Finish this unit
When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.
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