# General global circulation

> Understand the general global atmospheric circulation model: Hadley, Ferrel, and Polar cells for aviation meteorology ATPL studies.

- **Subject:** 050 — Meteorology
- **Section:** 050-02 — Wind
- **Topic code:** 050-02-03
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/meteorology/wind/general-global-circulation/

---

Global atmospheric circulation is the large-scale movement of air that distributes heat across the Earth's surface.

![Global Circulation](https://upload.wikimedia.org/wikipedia/commons/9/9c/Earth_Global_Circulation_-_en.svg)

## Three-Cell Model

To explain the general circulation, a three-cell model is used in each hemisphere:

### 1. Hadley Cell (Equator - Subtropics)
*   **At the Equator:** Warm air rises (Equatorial Low Pressure), creating storms and the Intertropical Convergence Zone (ITCZ).
*   **Aloft:** Air flows towards the poles.
*   **In the Subtropics (~30° N/S):** Air descends (**subsidence**), creating permanent **Subtropical High Pressure** zones (e.g., the Azores High). This descent generates dry and clear weather (desert zone).
*   **At the Surface:** Air returns to the Equator as the **Trade Winds**.

### 2. Ferrel Cell (Subtropics - Temperate Latitudes)
*   Located between 30° and 60° latitude.
*   It is a cell "induced" by the movement of the other two.
*   **Surface Winds:** Flow from the subtropics towards the poles, deflected by Coriolis to the east, forming the **Westerlies**.

### 3. Polar Cell (Temperate Latitudes - Poles)
*   **At the Poles:** Cold, dense air descends, creating **Polar High Pressure**.
*   **At the Surface:** Air flows towards the equator (Polar Easterlies).
*   **At ~60° Latitude:** Polar air collides with warmer air from the Ferrel cell, rising and creating **Subpolar Low Pressure** zones (Polar Front).

---

## Pressure Systems and Isobaric Surfaces

### Interpretation of Isobaric Surfaces
Constant pressure surfaces (e.g., 500 hPa) are not flat:
*   **"Bulging Up"**: Indicate **High Pressure** (or warm air). The pressure surface is at a higher altitude.
*   **"Bulging Down"**: Indicate **Low Pressure** (or cold air). The pressure surface is at a lower altitude.

### Wind Speed Comparison (Gradient)
For the same isobar spacing (same pressure gradient):
*   **Around a HIGH:** The wind is **faster** (Super-geostrophic). Centrifugal force adds to the gradient force.
*   **Around a LOW:** The wind is **slower** (Sub-geostrophic). Centrifugal force opposes the gradient force.

---

## Buys Ballot's Law (Flight Application)

![Ley de Buys Ballot](https://www.pasionporvolar.com/wp-content/uploads/2015/08/buys-ballot-1.png)

*   **Tailwind (Northern Hemisphere):** If you fly with a tailwind, the **Low Pressure** is on your **left**.
*   **Headwind (Northern Hemisphere):** If you fly with a headwind, the **Low Pressure** is on your **right**.
*   **Crossing a system:**
    *   If you pass from a Low to a High (in the NH), the wind will **Veer** (turn clockwise).

---

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