# Air masses

> Classification and properties of air masses (polar, tropical, maritime, continental) in aviation meteorology.

- **Subject:** 050 — Meteorology
- **Section:** 050-06 — Air Masses and Fronts
- **Topic code:** 050-06-01
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/meteorology/air-masses-and-fronts/air-masses/

---

An air mass is a large volume of air whose physical properties (temperature and humidity) are uniform in the horizontal plane.

![Air Masses](https://upload.wikimedia.org/wikipedia/commons/2/2f/Air_masses.svg)

## Classification

Air masses are classified using a three-letter system according to their moisture content, their region of origin, and their temperature relative to the surface.

### 1. Moisture Content (First Letter)
*   **m (Maritime):** Moist. Oceanic origin.
*   **c (Continental):** Dry. Land origin.

### 2. Region of Origin / Temperature (Second Letter)
*   **A (Arctic):** Arctic. Very cold.
*   **P (Polar):** Polar. Cold (origin in high latitudes, not necessarily at the poles).
*   **T (Tropical):** Tropical. Warm.
*   **E (Equatorial):** Equatorial. Very warm (does not typically affect Europe).

### 3. Relative Temperature (Third Letter)
*   **k (cold/kalt):** The air mass is **colder** than the surface over which it moves.
    *   Warmed from below $\rightarrow$ **Instability**.
    *   Cumuliform clouds, showers, good visibility (except in precipitation).
*   **w (warm):** The air mass is **warmer** than the surface over which it moves.
    *   Cooled from below $\rightarrow$ **Stability**.
    *   Stratiform clouds, fog, drizzle, poor visibility. Temperature inversions.

## Air Masses Affecting Europe

| Air Mass | Code | Origin | Typical Characteristics |
| :--- | :--- | :--- | :--- |
| **Arctic Maritime** | **mAc** | Polar caps / Arctic | Very cold. Becomes unstable moving south over the warm sea. Snow/Showers. |
| **Polar Maritime** | **mPc** | North Atlantic (Greenland/Canada) | Cold and moist. **The most common in Western Europe**. Unstable (warmed from below). Showers, Cu/Cb. |
| **Polar Continental** | **cPc** | Siberia / Russia (Winter) | Very cold and dry. Stable. "The Beast from the East". |
| **Tropical Maritime** | **mTw** | Azores High | Warm and moist. Stable (cooled from below). Stratiform clouds, drizzle. |
| **Tropical Continental** | **cTw** | North Africa / Turkey | Warm and dry. Stable. Heat waves in summer. |

## Key Concepts

*   **Stability at Source:** All air masses are stable in their region of origin (usually high-pressure zones).
*   **Modification:** Air masses modify their properties as they move.
    *   **Cold Air over Warm Surface:** Instability (convection). Example: Polar Air moving towards the equator.
    *   **Warm Air over Cold Surface:** Stability (inversion). Example: Tropical Air moving towards the poles.
*   **Orographic Effect:** When an unstable air mass (like mPc) hits mountains, the forced ascent intensifies the instability, creating more developed clouds, heavier precipitation, and a higher risk of icing on the windward side.
*   **Seasonal Shift:** In the northern hemisphere, frontal systems and air masses shift to the **North in summer** (following the sun) and to the **South in winter**.

## Polar Front Depressions
The Polar Front separates Polar air (cold) from Tropical air (warm). In a typical North Atlantic depression:
*   **Warm Sector:** Tropical Air (mTw).
*   **Ahead and Behind the depression:** Polar Air (mPc).

---

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