Depression slides

1) Initiation: Depressions begin to form when a tropical air mass meets a polar air mass (polar front boundary between cold and warm air) to form a vortex of anticlockwise swirling air. Instability occurs on the polar front.

2) Growth and development

stage: The less dense warm air rises over cold

air. The boundary

between the two forms the warm

front. The colder advancing air to

the west is denser and

undercuts the warmer air. The boundary forms the cold front. In

between is a warm sector.

3) Occluded stage: The cold front moves faster than the warm front and eventually catches it up and lifts it away from the ground, forming an occluded front.

4) Decay stage: After occlusion the depression dissolves as the warm air has been lifted away. Warm air is now trapped on poleward side of polar front.

How depressio

ns are formed• A depression brings rapidly changing weather, which nevertheless follows a pattern.

• The stage a depression is at when it passes will influence the nature of its impact. Further variation is provided by the nature, type and temperature differences between the converging air masses which form the system.

• Key point: a depression is a weather system where a cold front is chasing a warm front.

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