# The principal pressure areas

> Learn about the Earth's principal pressure areas and their role in global weather patterns for pilots.

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

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![Atmospheric Circulation and Pressure Areas](https://upload.wikimedia.org/wikipedia/commons/7/79/Atmospheric_circulation.svg)
*Global atmospheric circulation showing the principal high and low-pressure belts.*

## Types of Anticyclones (High Pressure)

![Anticyclone Diagram](https://upload.wikimedia.org/wikipedia/commons/9/9c/High_Pressure.jpg)

There are five main types: warm, cold, temporary cold, ridges, and blocking.

### Cold Anticyclones
Form over continents in winter and polar areas.
*   **Characteristics:** Strong temperature inversions, clear air, sharp frosts, and very cold surface.
*   **Variation with Height:** They weaken with increasing altitude.
    *   At surface: Isobars curve upwards (high pressure).
    *   Aloft: Due to low temperature, pressure decreases rapidly, so at high levels they may become low-pressure areas (isobars curved downwards).

### Warm Anticyclones
Associated with warm air and subsidence aloft (e.g., Azores High).
*   **Characteristics:** Stable weather, often warm. Usually quasi-stationary or permanent.
*   **Variation with Height:** They intensify with altitude.
    *   Isobaric surfaces curve upwards at **all levels** (both at surface and aloft).

## Pressure Systems in the Northern Hemisphere

| System | Seasonality | Location / Effect |
| :--- | :--- | :--- |
| **Azores High** | Year-round | Subtropical High. 30ºN (Winter) - 35ºN (Summer). Affects Europe and N. Africa. |
| **Icelandic Low** | Year-round | Deep low pressure over Iceland/Greenland. Stronger in winter. |
| **Siberian High** | **Winter Only** | Very cold high pressure over Asia and Eastern Europe. |
| **Siberian Low** | Summer Only | Weak thermal low pressure over Asia. |
| **Mediterranean** | Variable | Frontal depressions in Winter. Anticyclonic and hot in Summer. |

## Weather Map Terminology

*   **Ridge:** An elongated extension of a **high-pressure** system (inverted U-shape). Good weather.
*   **Trough:** An elongated extension of a **low-pressure** system (V-shape). Bad weather, convergence.
*   **Col:** A neutral zone between two highs and two lows. Calm and variable winds.

## Identification on Maps (Southern Hemisphere)
In the Southern Hemisphere:
*   **Low Pressure (Cyclone):** Winds rotate **clockwise**.
*   **High Pressure (Anticyclone):** Winds rotate **counter-clockwise**.
*   **Fronts:** Cold and warm fronts are associated with depressions.

## General Rules of Pressure and Height
*   **High Pressure:** Isobaric surfaces bulge upwards.
*   **Low Pressure:** Isobaric surfaces bulge downwards.
*   **Warm Anticyclone:** High pressure at surface and aloft.
*   **Cold Depression:** Low pressure at surface and aloft.

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