1) Polar Maritime and tropical air masses move towards each other - the warmer, less dense air from the south, rises above the colder, dense air from the north
2) Rising air is removed by the jet streams but as it rises, the earth's rotational spin causes it to twist
3) The twisting vortex produces a wave at ground level in the polar front -> this increases in size to form a depression
4) The warm front at the leading edge of the depression where warm, less dense air rises over the colder air ahead. THe cold front at the rear of the depression is where colder, dense air undercuts the warmer air ahead.
5) In between these fronts lies the warm sector - an area of warm and most air. As the depression moves eastwards, the cold front gradually overtakes the warm front to form an occlusion
6) In an occlusion, the colder air has completely undercut the warmer air so that is no longer touches the groud