# Tropical Revolving Storms: Formation, Structure and Aviation Hazards

> Aviation-focused explanation of tropical revolving storm formation, lifecycle, and hazards, including hurricanes, typhoons, and cyclones.

- **Subject:** 050 — Meteorology
- **Section:** 050-07 — Pressure Systems
- **Topic code:** 050-07-04
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/meteorology/pressure-systems/tropical-revolving-storms/

---

**Tropical Revolving Storms** (TRS) are large-scale thermal depressions that develop over warm tropical oceans. They are the most destructive weather systems on earth, deriving their energy from the release of **latent heat** of condensation.

![Hurricane Structure](https://upload.wikimedia.org/wikipedia/commons/4/4f/Hurricane-en.svg)
*Vertical cross section of the anatomy and structure of a Hurricane (TRS).*

## Classification

Terminology changes according to sustained wind speed:

1.  **Tropical Depression**: Winds < 33 kt.
2.  **Tropical Storm**: Winds between 34 and 63 kt.
3.  **Tropical Cyclone (TRS)**: Sustained winds **> 63 kt**. (This is when they receive names like Hurricane, Typhoon, etc.)

## Formation Conditions

For a TRS to form, three critical factors must coincide:

1.  **Sea Temperature**: Must be above **26.5°C** (26°C) in a deep layer (at least 60m). This is fundamental for the energy supply. For this reason, they do not form in the South Atlantic (water too cold).
2.  **Latitude**: Between **5° and 25°** North or South.
    *   **< 5°**: The Coriolis force is negligible, preventing cyclonic rotation.
    *   **> 25°**: Waters are usually too cold.
3.  **Atmospheric Instability**: Necessary to allow explosive development of convective clouds (CB) through the Trade Wind inversion.

## Storm Structure

A mature TRS has a typical diameter of about **500 km (270 NM)** and extends vertically to the tropopause.

*   **The Eye**:
    *   Center of the storm, circular, with a diameter of **30-50 km (10-20 NM)**.
    *   Zone of **minimum pressure** at the surface.
    *   **Calm winds** and skies often clear or with broken clouds (due to subsidence of warm air from above).
    *   It is the warmest region aloft.
*   **The Eye Wall**:
    *   Dense ring of Cumulonimbus immediately surrounding the eye.
    *   Contains the **most violent winds**, most severe turbulence, and heaviest precipitation.
    *   It is the most dangerous part for aviation and navigation.

## Regional Nomenclature and Seasons

| Name | Region | Main Season | Notes |
| :--- | :--- | :--- | :--- |
| **Hurricane** | North Atlantic, East Pacific | July - November | ~6 per year. |
| **Typhoon** | West Pacific, China Sea (Japan, Taiwan) | June - November | **Most active region** (~20/year). |
| **Cyclone** | Indian Ocean | Mar-Dec (Peaks: Apr-May, Oct-Nov) | ~12 per year. |
| **Cyclone** | South Pacific, Australia | December - April | ~9 per year. |

## Movement and Track

*   **Typical Track**: Initially move **Westwards** (Westerly) driven by the Trade Winds.
*   **Recurvature**: Upon reaching latitudes of approx. 20°, they tend to curve away from the equator (Northwards in the NH, Southwards in the SH) and accelerate towards the East.
*   **Prediction**: Their exact movement is erratic and very difficult to forecast accurately.

## Dangerous Semicircle

Wind speed in a TRS is not symmetrical due to the translational movement of the storm. The wind is stronger where the rotational speed adds to the speed of movement of the system.

*   **Northern Hemisphere**: The **RIGHT** side of the track is the dangerous one.
*   **Southern Hemisphere**: The **LEFT** side of the track is the dangerous one.
*   The opposite side is called the "navigable semicircle" (less intense winds), although it is still dangerous.

---

Free study notes from pilotsyllabus.com. Not a substitute for training at an approved ATO.