Changing Weather Associated with Fronts

Since fronts are born out of the clash of contrasting air masses, they naturally bring contrasting weather conditions.

• Think of it this way: when warm, moist, gentle air meets cold, dense, aggressive air — the atmosphere is bound to produce drama.
• The exact sequence of weather depends on which type of front is advancing — warm front or cold front.

Weather Associated with Warm Fronts

👉 Imagine warm, light, moist air advancing towards a region of cold, dense air.

• Because warm air is lighter, it cannot push cold air away.
• Instead, it gently overrides the cold air, sliding upward along the sloping warm front surface.

Process

• As warm air rises, it cools adiabatically → condensation begins → cloud formation.
• The type of clouds and precipitation depend on the stability and humidity of warm air:
  • If warm air is stable and less humid → condensation occurs high up → clouds form at higher altitude → precipitation requires more lifting.
  • If warm air is moist and unstable → even slight lifting triggers condensation → precipitation starts earlier.

Weather Characteristics of a Warm Front

• Precipitation: Long duration, moderate intensity, widespread → because slope is gentle.
• Cloud sequence (from top downward):
  1. Cirrus
  2. Cirro-stratus
  3. Alto-stratus
  4. Nimbo-stratus

👉 This classic sequence is a telltale sign of a warm front approaching.

Arrival of the Warm Sector

Once the warm front has passed, the observation point enters the warm sector. Suddenly:

• Temperature rises. 🌡️
• Specific humidity increases. 💧
• Air pressure drops. 📉
• Clouds disappear → clear sky appears.
• Precipitation stops → weather feels settled (at least temporarily).

Weather Associated with Cold Fronts

👉 Now, flip the situation: cold air advances aggressively into warm air territory.

• Cold, dense air is like a bulldozer — it stays at the ground and forces the warm air to rise violently.

Process

• If the cold front passes quickly, the weather clears soon after.
• If the front becomes stationary, conditions worsen:
  • Sky overcast with cumulonimbus clouds.
  • Frontal thunderstorms form (if cold air is moist and unstable).

Weather Characteristics of a Cold Front

• Precipitation: Heavy, but short-lived.
• Temperature: Drops suddenly after passage.
• Pressure: Rises sharply.
• Humidity: Both specific and relative humidity decrease.
• Wind Direction: Shifts abruptly (can change anywhere from 45° to 180°).
• Storms: Precipitation often accompanied by lightning, thunder, and sometimes hailstorms.

After the Passage of Cold Front

• Clouds vanish. ☁️❌
• Rain stops. 🌧️❌
• Weather clears, but cold north-westerly winds set in. 🌬️❄️

👉 Thus, cold fronts = short, violent drama, followed by a crisp, clear cold atmosphere.

✅ Comparison: Warm vs. Cold Front Weather

Feature	Warm Front	Cold Front
Air Movement	Warm air slides gently over cold air	Cold air bulldozes under warm air
Slope	Gentle (1:100–1:400)	Steep (1:50–1:100)
Cloud Sequence	Cirrus → Cirrostratus → Altostratus → Nimbostratus	Cumulonimbus dominates
Precipitation	Long-lasting, widespread, gentle/moderate	Short-lived, heavy showers, often with thunder & hail
Temperature Change	Increases with arrival of warm sector	Drops sharply after passage
Pressure Change	Falls gradually	Rises suddenly
Weather After Passage	Clear sky, warm & humid	Clear but cold, NW winds