A winter storm effecting Canada is essentially a mid-latitude cyclone (occuring between 30 and 60 North latitude) that forms in the winter. When cool arctic air and warm humid air from the tropics combine along a frontal boundary, the contrasting air masses provide energy, allowing the counterclockwise spinning low pressure system to intensify. As the air rises, cools, and descends, neighbouring air rushes in to fill the gaps, causing increasingly stronger winds.   When a cold front takes over a warm front there is a consequential counterclockwise rotation. Mid-latitude cyclones tend to move west to east, explaining why the Maritimes receive so many storms and Western Canada receives so few.   A mid-latitude cyclone goes through many different stages during its lifetime, which normally lasts from 3 to 10 days.   Step 1: Cyclogenesis. Cold and warm air masses meet and begin to conflict, causing the low pressure system to become more intense. Convergence of air at the surface must be coupled with divergence aloft in order or a wave cyclone to develop.   Step 2: Open Stage. In this stage the cyclone begins its counterclockwise rotation (in the Northern Hemisphere). A warm front to the east of the low pressure system begins to move northward, and the cold front begins to move southward to the west, creating the counterclockwise movement. Cool air is pulled from the north and west while warm air is pulled from the south.   Step 3: Occluded Stage (Closed Stage). If a cold front moves at 40 kmph while a warm front moves at 16-24 kmph, the cold front would therefore catches up with the warm front and abruptly lift the warm front, causing cumulonimbus clouds, heavy precipitation and often thunder and lightning. Once the warm front has been lifted, an occluded front is formed.   Step 4: Dissolving Stage. The warm front provides energy for the mid-latitude cyclone.