# Fronts

> Detailed guide on weather fronts: cold, warm, occluded, and stationary fronts for ATPL meteorology exams.

- **Subject:** 050 — Meteorology
- **Section:** 050-06 — Air Masses and Fronts
- **Topic code:** 050-06-02
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/meteorology/air-masses-and-fronts/fronts/

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A front is the boundary or transition zone between two air masses with different characteristics (mainly temperature and density).

![Cross section warm and cold](https://upload.wikimedia.org/wikipedia/commons/b/bc/Cross_section_through_a_cold_front_and_warm_front.jpg)
*Cross section showing the development and lifting of air in warm and cold fronts.*

## Types of Fronts

![Weather Fronts](https://upload.wikimedia.org/wikipedia/commons/c/ca/Surface_analysis.gif)

### 1. Warm Front
Occurs when a warm air mass advances and replaces a colder air mass. The warm air (less dense) slides over the cold air.

![Warm Front](https://upload.wikimedia.org/wikipedia/commons/4/40/Warm_front_symbol.svg)

*   **Symbol:** Red semicircles.
*   **Slope:** Gentle (1:150).
*   **Speed:** Slow (~10 kt, or 2/3 of the geostrophic wind).
*   **Cloud Sequence:** Ci $\rightarrow$ Cs $\rightarrow$ As $\rightarrow$ Ns $\rightarrow$ St. (Appear 800-1000 km ahead).
*   **Weather:** Continuous precipitation (light to moderate), drizzle, frontal fog. Stable air.
*   **Hazards:** Freezing Rain in the cold sector beneath the front.

### 2. Cold Front
Occurs when a cold air mass advances and pushes (wedges under) a warm air mass, forcing it to rise violently.

![Cold Front](https://upload.wikimedia.org/wikipedia/commons/c/c0/Cold_front_symbol.svg)

*   **Symbol:** Blue triangles.
*   **Slope:** Steep (1:50).
*   **Speed:** Fast (equal to the geostrophic wind component).
*   **Clouds:** Vertical development (Cu, Cb, TCU).
*   **Weather:** Heavy showers, thunderstorms, hail, gusty winds. Unstable air.
*   **Front Passage:**
    *   Wind: Sharp veer (usually from SW to NW).
    *   Pressure: Rises rapidly.
    *   Temperature: Drops.
    *   Visibility: Improves notably after passage (clean air).

### 3. Occluded Front
Forms when a cold front (faster) catches up to a warm front, lifting the warm sector off the ground.

![Occluded Front](https://upload.wikimedia.org/wikipedia/commons/2/2d/Occluded_front_symbol.svg)

*   **Symbol:** Alternating purple semicircles and triangles.
*   **Cold Occlusion:** The air behind the cold front is **colder** than the air ahead of the warm front. (Common in Europe in **Summer**). Acts like a cold front.
*   **Warm Occlusion:** The air ahead of the warm front is **colder** than the air behind the cold front. (Common in Europe in **Winter**). Acts like a warm front.

### 4. Stationary Front
Boundary between two air masses that are not moving. The wind blows parallel to the front. The weather is similar to that of a warm front (stratiform clouds, continuous rain).

## Polar Front Depression (Mid-Latitude Cyclone)

These are low-pressure systems moving East, driven by the Westerlies and the jet stream.

### Life Cycle
1.  **Frontogenesis:** Initial disturbance on the stationary polar front.
2.  **Mature Stage:** A wave forms with a well-defined warm sector between the warm and cold fronts. Minimum pressure.
3.  **Occlusion Stage:** The cold front catches the warm front, starting near the center of the low. The warm sector is lifted.
4.  **Frontolysis (Dissipation):** The depression fills and dies.

### Structure and Weather
*   **Warm Sector:** Zone between the warm and cold fronts. Tropical air (mTw). Generally humid weather, drizzle, stratiform clouds (St, Sc), fair/poor visibility. In summer, convection may occur.
*   **Passage of the Depression (East to West):**
    1.  Approach: Pressure falls, high clouds increase (Ci).
    2.  Warm Front: Continuous rain, low ceiling.
    3.  Warm Sector: Heavy rain stops, drizzle, Temp rises.
    4.  Cold Front: Heavy showers, thunderstorms, wind veers.
    5.  Behind Cold Front: Skies with broken clouds (Cu), good visibility, occasional showers.

## Seasonal Movement
The Polar Front shifts with the sun:
*   **Winter:** Further South (35°N - 50°N). Depressions affect Southern Europe.
*   **Summer:** Further North (50°N - 65°N). Depressions are weaker and further north.

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