Tropical cyclones
A tropical cyclone is any low originating over tropical oceans. Hurricane is its top intensity class (AWH 17.3.5).
US classes by 1-minute sustained wind (AWH 17.3.5):
| Class | Wind |
|---|---|
| Tropical depression | up to 34 kt |
| Tropical storm | 35 to 64 kt |
| Hurricane | at least 65 kt |
Name of a hurricane-strength storm by basin (AWH 17.3.5):
| Basin | Name |
|---|---|
| North Atlantic, northeast Pacific | Hurricane |
| Northwest Pacific | Typhoon |
| North Indian Ocean | Severe cyclonic storm |
| Southwest Pacific, southeast Indian Ocean | Severe tropical cyclone |
| Southwest Indian Ocean | Tropical cyclone |
Formation (AWH 17.3.5.1), needs:
- Optimum sea surface temperature
- Low-level convergence
- Cyclonic wind shear
- Horizontal outflow (divergence) at high levels
Feedback: rising air condenses, releases latent heat, lowers surface pressure, draws in more low-level convergence.
Latitude:
- Usually forms between 5° and 20°.
- Unlikely within 5° of equator: Coriolis force too small to turn winds around a low, so they flow straight in and fill it.
Movement (Northern Hemisphere, AWH 17.3.5.2):
- Low latitudes: usually west to northwest.
- Nearing mid-latitudes, the prevailing westerlies take over and recurve it north, then northeast, then east-northeast.
- Between the two wind systems, motion can be erratic.
Structure (AWH 17.3.5.5):
| Zone | Weather |
|---|---|
| Spiral rain bands | Very heavy rain, ceiling and visibility near zero, strong gusty winds, violent turbulence. Between bands, ceilings and visibilities somewhat better, turbulence less. |
| Eye | Skies free of turbulent cloud, comparatively light wind. |
| Eye wall (wall of cloud around the eye) | Strongest winds and deluging rain. |
Decay (AWH 17.3.5.3):
- Takes on mid-latitude characteristics: extratropical, meaning "outside the Tropics".
- Well inland it weakens: moisture source lost, surface friction increases.
Traps:
- Reading the basin name as strength: typhoon and hurricane are both top-class; basin only sets the name.
- Assuming low-level convergence alone suffices; high-level outflow also required.
- Expecting a steady track; the storm may move very erratically until the westerlies gain control.
- Judging the eye as the worst part: eye winds are comparatively light, but ringed by the strongest winds.
- Using another name for a weakened storm: the AWH term is extratropical.
References
- FAA Aviation Weather Handbook (FAA-H-8083-28) 17.3.5 (tropical cyclones)
- FAA Aviation Weather Handbook (FAA-H-8083-28) 17.3.5.1 (tropical cyclone formation)
- FAA Aviation Weather Handbook (FAA-H-8083-28) 17.3.5.2 (tropical cyclone movement)
- FAA Aviation Weather Handbook (FAA-H-8083-28) 17.3.5.3 (tropical cyclone decay)
- FAA Aviation Weather Handbook (FAA-H-8083-28) 17.3.5.5 (tropical cyclone weather)