<Li> Air above the cap cools </Li> <P> This creates a warmer, moister layer below a cooler layer, which is increasingly unstable (because warm air is less dense and tends to rise). When the cap weakens or moves, explosive development follows . </P> <P> In North America, supercells usually show up on Doppler radar as starting at a point or hook shape on the southwestern side, fanning out to the northeast . The heaviest precipitation is usually on the southwest side, ending abruptly short of the rain - free updraft base or main updraft (not visible to radar). The rear flank downdraft, or RFD, carries precipitation counterclockwise around the north and northwest side of the updraft base, producing a "hook echo" that indicates the presence of a mesocyclone . </P> <Table> <Tr> <Td> Wind shear (red) sets air spinning (green) </Td> <Td> The updraft (blue)' bends' the spinning air upwards </Td> <Td> The updraft starts rotating with the spinning column of air </Td> </Tr> </Table>

Where does a tornado form relative to the primary features of a supercell thunderstorm