Föhn Winds Formation Notes

Föhn Winds

Definition

• Föhn winds are dry winds that descend the leeward side of a mountain.

Mountain Sides

• Windward Side: The side of the mountain closer to the sea.
• Leeward Side: The side of the mountain furthest from the sea, more inland.

Föhn Wind Formation

1. Warm Moist Air Rises:

   • Warm, moist air from the sea is forced to rise along the windward side of the mountain.
   • Air cannot penetrate the mountain, so it is forced upwards.
   • Warm air always rises.

2. Cooling at Dry Adiabatic Lapse Rate (DALR):

   • As the warm moist air rises, it cools. The cooling rate is known as the Dry Adiabatic Lapse Rate (DALR).
   • $DALR = 1^{\circ}C$ per 100 meters.
   • Example: If the air is initially $50^{\circ}C$, after rising 200 meters, it will be $48^{\circ}C$.

3. Change to Wet Adiabatic Lapse Rate:

   • As air reaches the top of the mountain, cooling rate changes to Wet Adiabatic Lapse Rate (WALR).
   • $WALR = 0.5^{\circ}C$ per 100 meters.

4. Condensation and Cloud Formation:

   • Air cools and condenses to form clouds at the top of the mountain.
   • Condensation turns moisture into a cloud.

5. Rainfall at the Windward Side:

   • The windward side experiences rainfall due to cloud formation.

6. Cold Dry Air Descends (Föhn Wind):

   • The remaining air is now cold and dry, having lost its moisture through condensation.
   • Cold air descends down the leeward side of the mountain.

7. Warming as it Descends:

   • As the cold, dry air descends the leeward side, it warms up at the Dry Adiabatic Lapse Rate (DALR).
   • The air gains heat as it goes down the mountain.
   • $DALR = 1^{\circ}C$ per 100 meters.