# Tropical climatology

> Tropical climatology for aviation: ITCZ, monsoons, and typical weather patterns near the equator.

- **Subject:** 050 — Meteorology
- **Section:** 050-08 — Climatology
- **Topic code:** 050-08-02
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/meteorology/climatology/tropical-climatology/

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Tropical climatology is dominated by the **Intertropical Convergence Zone (ITCZ)**, the trade winds, and the monsoons.

![ITCZ January-July](https://upload.wikimedia.org/wikipedia/commons/d/d7/ITCZ_january-july.png)
*Seasonal average displacement of the Intertropical Convergence Zone (ITCZ) between January and July.*

## Intertropical Convergence Zone (ITCZ)

Also known as the **Doldrums**, **Thermal Equator**, or **Equatorial Trough**.

*   **Definition**: It is a zone of low pressure where the trade winds from the Northern Hemisphere (NE) and the Southern Hemisphere (SE) converge.
*   **Cause**: Intense solar heating near the equator forces warm, moist air to rise (convection).
*   **Associated Weather**:
    *   Clouds of great vertical development (**Cu**, **Cb**) and frequent thunderstorms.
    *   Heavy precipitation, severe turbulence, and icing (above 16,000 ft).
    *   Cloud tops can reach **50,000 - 60,000 ft**, making it difficult to fly over.
*   **Seasonal Movement**: Follows the sun (thermal equator).
    *   **NH Summer (July)**: Shifts North.
    *   **NH Winter (January)**: Shifts South.
    *   The displacement is much greater over **continents** than over oceans (due to the thermal inertia of water).

## Trade Winds

Persistent winds blowing from the subtropical high-pressure belts (**Horse Latitudes**, ~30° N/S) towards the ITCZ.

*   **Northern Hemisphere**: Northeast Trades (**NE Trades**).
*   **Southern Hemisphere**: Southeast Trades (**SE Trades**).
*   **Characteristics**:
    *   They are low-level winds (up to ~10,000 ft).
    *   They pick up moisture as they pass over oceans (moist in lower layers, dry aloft due to subtropical subsidence).
    *   Upon crossing the equator, they change direction due to the Coriolis force (e.g., SE becomes SW).

## Monsoons

Seasonal changes in the prevailing wind direction, caused by the displacement of the ITCZ and land-sea thermal differences.

### Southwest Monsoon (SW Monsoon) - Summer
*   **Season**: June, July, August.
*   **Mechanism**: The ITCZ moves North. The SE trades cross the equator, turn right (Coriolis), and become SW winds.
*   **Weather**: **Warm, moist, and unstable** air (long maritime track).
    *   Cb clouds, strong thunderstorms, heavy rains, low ceilings.
*   **Affected Regions**: India, Southeast Asia, West African Coast (Guinea, Ghana).

### Northeast Monsoon (NE Monsoon) - Winter
*   **Season**: December, January, February.
*   **Mechanism**: The ITCZ moves South. The wind blows from the **Siberian High** towards the equator.
*   **Weather**: **Cool and dry** air (continental origin).
    *   Clear and stable weather over India and Thailand.
    *   *Exception*: If it travels over the sea (e.g., Bay of Bengal towards Sri Lanka), it picks up moisture and can bring rains.

### Northwest Monsoon (NW Monsoon)
*   It is the extension of the NE Monsoon crossing the equator into the Southern Hemisphere, turning NW. Affects northern Australia in January (austral summer).

## Other Tropical Phenomena

*   **Easterly Waves**: Low-pressure troughs moving from East to West in the trade winds. They originate in West Africa and can evolve into tropical cyclones in the Atlantic/Caribbean.
*   **Harmattan**: Dry and dusty NE trade wind blowing from the Sahara over West Africa in winter.
*   **Pampero**: Outbreak of cold polar air in South America (Argentina, Uruguay, Brazil) with severe storms.
*   **0°C Isotherm**: In the tropics, it is found very high, typically at **15,000 - 17,000 ft**.
*   **Sandstorms**: Require winds > 20 kt and instability. Drastically reduce visibility.

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