The polar vortex is analyzed at 500 millibars. The polar vortex occurs above the core of the coldest polar air. Since frigid air is dense, heights are lower aloft because cold air has a lower thickness than warmer air. At the surface of the polar air mass will be high pressure, but low heights will occur aloft at 500 mb since the air is compacted due to high density air near the surface. The polar vortex can often be located over Canada since the coldest surface air is often found over high latitude icy/land locations. The polar vortex aloft propagates toward where the polar air mass moves. Below is a 500 mb image of the 500-mb polar vortex. 
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