Correct Option
The correct option is Warm air moves northwards and cold air moves southwards..
Explanation
Extra-tropical cyclones, also known as temperate cyclones or wave cyclones, originate along the polar front. This front represents the boundary between the warm subtropical air mass and the cold polar air mass. The formation process is described by the Polar Front Theory.
Detailed Analysis
Initially, the front is stationary. The formation of a cyclone begins when a local drop in pressure occurs along this front, creating a disturbance.
- Air Mass Movement: As the pressure drops, the equilibrium is disturbed, setting the air in motion. The warm air from the south pushes northwards into the region of cold air. Simultaneously, the cold air from the north pushes southwards into the region of warm air.
- Cyclonic Circulation: This opposing movement creates a wave-like undulation on the front and establishes a cyclonic circulation (anticlockwise in the Northern Hemisphere).
- Front Formation: The part of the front where warm air advances northwards is termed the warm front, while the section where cold air advances southwards is termed the cold front.
Therefore, the initiation of the cyclone is characterized by the northward movement of warm air and the southward movement of cold air.
Key Takeaway: The genesis of an extra-tropical cyclone involves the development of a wave along the polar front, driven by the northward intrusion of warm air and the southward intrusion of cold air, leading to cyclonic circulation.