Tropical Revolving Storms - ATPL 050 — Meteorology

Nothing in the mid-latitude catalogue prepares a pilot for the tropical revolving storm: a heat engine running on warm ocean water, with the most violent winds and weather any routine operation can encounter. This lesson covers what it takes to build one, the ladder of names it climbs as it strengthens, the anatomy of the mature storm (eye, eye wall, spiral bands), how it moves and how unreliably that movement can be forecast, and the world tour of basins, local names and seasons that the examination expects by heart.

What it takes to spin up a storm

A tropical revolving storm is fuelled by the latent heat of water evaporated from a hot sea, and it forms only where every item on a short list is satisfied:

A sea surface of at least about 26 to 27 °C, warm through a deep layer: the fuel supply.
At least about 5° of latitude from the equator — closer in there is too little Coriolis force to set the air rotating, which is why no storm forms on the equator itself.
An initial disturbance — a trough of the equatorial low-pressure belt or an easterly wave, providing the low-level convergence that starts the deep convection.
Weak vertical wind shear through the depth of the troposphere, so the growing tower of convection is not torn apart, plus a deep, moist, unstable layer to feed it.

The ladder from disturbance to storm

As the circulation organises and the mean wind climbs, the system is renamed at fixed wind thresholds, a ladder the examination quotes by name and number:

Tropical disturbance — a first area of organised convection, no strong circulation yet.
Tropical depression — a closed circulation with mean winds up to 33 kt.
Tropical storm — mean winds 34 to 47 kt; the system is named.
Severe tropical storm — mean winds 48 to 63 kt.
Tropical revolving storm — mean winds of 64 kt and more: the full storm, called hurricane, typhoon or tropical cyclone depending on the ocean.

Anatomy of a mature storm

The mature storm is a ring machine:

The eye — at the centre, a core of subsiding air 20 to 50 km across: nearly calm, warm, largely cloud-free, where the surface pressure is extremely low, below 950 hPa in a deep storm.
The eye wall — around the eye, a ring of the deepest cumulonimbus, carrying the most violent winds, torrential rain, severe turbulence and wind shear: the most dangerous airspace in meteorology.
Spiral rain bands — outward from the wall, cumulonimbus and heavy showers winding in toward the centre; the whole system measures some 400 to 800 km across, with cloud from near the surface to the tropopause.
The storm surge — at the coast the destruction is finished by the sea itself, driven ashore by the wind and the low pressure.

How the storm moves

Embedded in the tropical easterlies on the equatorward flank of the subtropical high, a storm first tracks westward to west-northwestward at some 10 to 15 kt, in the northern hemisphere (the mirror image, west to west-southwestward, in the southern). Many storms then curve poleward around the western flank of the subtropical high and recurve into the westerlies — turning toward the northeast in the northern hemisphere and usually accelerating as they go. But the track is the least trustworthy thing about the storm: it can stall, loop or lurch, and the exact movement can only rarely be forecast, the operational consequence of which closes this lesson.

Where and when: basins, names and seasons

The same storm answers to a different name in each ocean, and the examination expects the full atlas:

Hurricane — the North Atlantic (Caribbean, Gulf of Mexico, the waters west of Africa) and the northeast Pacific off Mexico; season June to November, peaking August-September.
Typhoon — the northwest Pacific: the China Sea, the Philippines, Japan; most active July to November, but possible in any month, the busiest basin on Earth.
Tropical cyclone — the north Indian Ocean (Bay of Bengal, Arabian Sea), with a double season before and after the monsoon, around May-June and October-November; and the southwest Indian Ocean near Madagascar and the Australian and South Pacific waters, December to April, peaking January to March.

Worldwide the count runs to roughly 80 storms a year, the seasons falling in the late summer and autumn of each hemisphere, when the sea is warmest. The one tropical ocean that stays quiet is the South Atlantic: too cool and too sheared for storms to form.

Keeping out of its way

Everything above condenses into one operational rule. Because the exact movement of a tropical revolving storm can only rarely be forecast, the utmost care is necessary anywhere near one: plan to avoid the whole system by a wide margin, and never attempt to penetrate it. No altitude clears the weather (the cumulonimbus reaches the tropopause), and a track forecast a day old may already be wrong by hundreds of kilometres. Diversion fuel, alternates well clear of the forecast track, and respect for the storm's ability to change its mind are all part of the flight plan; the wider hazards of flying near deep convection have a module of their own.

What the exam asks

Tropical-storm questions are a memory test with numbers attached: the formation list, the wind-speed ladder, the dimensions, and the basin-name-season atlas. The overview, following the learning objectives:

State the conditions necessary for the formation of tropical revolving storms

Name the stages of development: tropical disturbance, tropical depression, tropical storm, severe tropical storm, tropical revolving storm

Describe the meteorological conditions in and near a tropical revolving storm, and state its approximate dimensions

State how a tropical revolving storm generally moves in its area of occurrence

List the areas of origin and occurrence, the specified names (hurricane, typhoon, tropical cyclone), the expected times of occurrence and the approximate frequency

State that the movement can only rarely be forecast exactly, and that utmost care is necessary near a tropical revolving storm