Fronts

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

Cross section showing the development and lifting of air in warm and cold fronts.

Types of Fronts

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.

• 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.

• 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.

• 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.