Tropical cyclones

Why does warm water near the Equator not automatically create a cyclone?

Assemble the heat supply, rotating disturbance and deep-column environment.

Favourable conditions for tropical cyclogenesis

A tropical cyclone is a non-frontal synoptic-scale rotating low with organised deep convection originating over tropical or subtropical waters. A classical favourable ocean condition is water around 26.5°C or warmer through a substantial upper layer, commonly of order 50 m or more. A deep warm reservoir helps maintain sensible and latent heat supply as winds stir the ocean. The 26.5°C value is a useful conventional guide, not a universal physical switch independent of the temperature aloft.

A moist lower and middle troposphere and sufficient moist instability support sustained deep convection. A pre-existing low-level disturbance with convergence and cyclonic vorticity helps organise the circulation. Weak vertical wind shear between the lower and upper troposphere favours alignment of the vortex and convection; strong shear can tilt or disrupt it. Divergent outflow aloft allows rising air to leave the column. Condensation releases latent heat and helps maintain a warm core, while the coupled circulation draws in more moist low-level air.

Enough Coriolis effect is usually required, so genesis commonly occurs about 5–30 degrees from the Equator and is uncommon within roughly 5 degrees of it. Rare near-equatorial exceptions do not make the Equator a normal formation belt. The ingredients are favourable conditions, not a guarantee that every disturbance develops. Dry-air intrusion, shear, cooler water or land interaction can inhibit or weaken development.

When does a tropical depression become a named storm?

Use sustained wind with the stated averaging period and regional convention.

From disturbance to a mature revolving storm

A tropical disturbance is an area of enhanced organised convection that may not yet have a definite closed surface circulation. A tropical depression has a closed low-level circulation and maximum sustained winds below tropical-storm strength. Tropical-storm strength begins at 34 kt in the commonly used international scheme. Stronger systems reach hurricane, typhoon or tropical-cyclone intensity at 64 kt or more under the applicable regional classification.

The EASA learning sequence also includes severe tropical storm as an intermediate stage. In the western North Pacific JMA scheme, a tropical storm has maximum 10-minute sustained wind 34–47 kt, a severe tropical storm 48–63 kt, and a typhoon at least 64 kt. This intermediate label is not used identically in every basin. The Atlantic scheme calls 34–63 kt a tropical storm and at least 64 kt a hurricane, using a 1-minute sustained wind.

The generic term tropical cyclone can refer to the family of these rotating tropical systems, while some regional classifications use it as a particular intensity class. Always read the issuing centre, averaging period, units and legend. A gust is not the sustained classification wind, and a 1-minute estimate cannot be compared directly with a 10-minute threshold as though the averaging periods were identical. The appearance of an eye is also not a required threshold test for naming or classifying the system.

Which part can look calmer while the surrounding system remains extreme?

Separate the wind maximum, convective core and outer weather.

Meteorological conditions and dimensions

A mature tropical cyclone can have a relatively calm eye with subsiding warmer air and partly clear sky, surrounded by an eyewall of deep convective cloud. The strongest sustained winds and some of the most intense rain and turbulence are commonly near the eyewall, not at the pressure minimum in the eye. Spiral rainbands extend outward with heavy showers or thunderstorms separated by comparatively quieter intervals. Not all tropical cyclones have a clear eye.

The overall cloud and wind system commonly spans several hundred kilometres, of order 500–1,000 km, with considerable variation and some much larger systems. Eye diameters are often of order 20–60 km but can be outside that range. Convective tops can reach the tropical tropopause, around 15–18 km, with overshoots possible. Cloud extent, gale-wind radius, eyewall radius and eye diameter are different dimensions; do not substitute one for another.

Hazards include destructive sustained winds and gusts, severe turbulence, low-level and convective wind shear, very heavy precipitation and poor visibility, lightning and icing in appropriate cloud layers. High ice-water content can occur near deep convection aloft. Low cloud, squalls and rainbands can affect an airport well outside the eye and strongest-wind core. Coastal storm surge and flooding can affect the airport and access infrastructure. A gap between bands or a forecast centre miss does not establish that the route or alternate will remain usable.

Why do many storms first move westward and later recurve?

Use the surrounding deep-layer flow and hemisphere, not a fixed path template.

Typical movement and its uncertainty

At low tropical latitudes, the trade-wind circulation and subtropical ridge commonly steer systems westward with a poleward component. In the Northern Hemisphere this is often west to north-west; in the Southern Hemisphere west to south-west. If the cyclone reaches the poleward side of the ridge and the mid-latitude westerlies, it may recurve towards north then north-east in the Northern Hemisphere, or south then south-east in the Southern Hemisphere.

Some storms remain westward, stall, loop, accelerate or follow a different track as ridges and troughs evolve. The depth and intensity of the vortex affect which layer of the surrounding flow steers it. Forward movement is separate from the circulating wind: a centre moving at 10 kt can contain sustained winds many times that speed. As a system moves poleward it can lose tropical characteristics and become extratropical while retaining dangerous winds, precipitation and a large wind field.

Forecast tracks provide useful guidance, but the exact future position and timing cannot be assumed. Forecast uncertainty grows with time and changes in steering; intensity and size are uncertain too. Use the responsible tropical cyclone advisory centre, SIGMET and current forecasts with their issue and valid times. A centre-track line is not the edge of hazardous weather, and a forecast uncertainty cone is not a wind-radius or safe-clearance boundary. Plan for the whole weather system and viable alternatives rather than trying to thread a schematic path near it.

Where are hurricanes, typhoons and tropical cyclones found?

Place the principal basins and their affected coasts on a world map.

Source areas and naming conventions

Hurricane is the mature-system name in the North Atlantic, including the Caribbean and Gulf region, and the eastern and central North Pacific. Typhoon is used in the western North Pacific, including the South China Sea. Tropical cyclone is used generically and in regional schemes across the Indian Ocean, Australian region and South Pacific; the North Indian Ocean also uses cyclonic storm and stronger qualified categories. These names do not describe different physical storm engines.

Important source regions lie over the tropical Atlantic east of the Caribbean, the eastern North Pacific off Mexico and Central America, the western North Pacific east of the Philippines, the Bay of Bengal and Arabian Sea, the south-west Indian Ocean near Madagascar and the Mascarene region, waters north and west/east of Australia, and the South Pacific east of Australia. Tropical cyclones are very rare in the South Atlantic and south-eastern Pacific, where the usual environmental combination is unfavourable; rare events do not make these major basins.

Genesis is mainly away from the Equator, commonly within roughly 5–30 degrees latitude. The mature system can travel outside its source region, cross a regional boundary or transition into an extratropical low. Follow the responsible advisory centre and current classification rather than assuming the storm disappears at a basin boundary or a change of regional name.

Why is a global annual count not a forecast for one route?

Compare climatological averages, with the reference periods and definitions attached.

Seasonal occurrence and approximate frequency

Activity generally peaks in the local late summer and autumn, after the ocean has warmed, with important basin differences. The North Atlantic season is June–November, peaking around September; the eastern North Pacific mainly May–November. The western North Pacific has year-round activity, concentrated in the northern summer and autumn. The North Indian Ocean has pre-monsoon and post-monsoon peaks, broadly April–June and October–December; strong summer-monsoon shear often suppresses mature cyclone development even while monsoon depressions occur.

Southern Indian, Australian and South Pacific activity is mainly November–April, with much of the activity in January–March and occasional out-of-season storms. The table gives rounded teaching scales from identified agency climatologies. Named-storm-strength systems are not the same count as systems reaching hurricane/typhoon strength; the periods and basin boundaries also differ, so do not add every row to create a precise world total. WMO gives approximately 85 tropical storms globally each year, with roughly 45 intensifying to the stronger hurricane/typhoon-equivalent group.

Actual annual activity varies markedly with ENSO, ocean temperatures, shear and the regional circulation. Australia illustrates the reference-period issue: the Bureau’s older long record gave about 11 systems per season, while its 2023 statement gave around 9 since 2000–01. A long-term average or an official season boundary is not a promise of safe conditions on a particular date. Read current warnings for the route, destination and alternates.

Can you apply the ideas?

Four original questions. Decide from the facts, then review the explanation.