Mesocyclone
At a glance
The rotating updraft inside a supercell. If it tightens and stretches to the ground, you get a tornado.
Deep dive
A mesocyclone is a vertical column of rotating air, typically 2–10 km across, embedded in a storm's updraft. It arises when horizontal vorticity — generated by environmental shear — is tilted into the vertical by the updraft.
Radar detects mesocyclones as a velocity couplet on Doppler scans: inbound and outbound velocities adjacent across the scan. The NWS term is mesocyclone detection (MD); a tighter/faster couplet extending to low levels flags a TVS (Tornadic Vortex Signature).
Not every mesocyclone produces a tornado — the low-level environment (low LCL, strong 0–1 km SRH) has to support the stretching that collapses the circulation to the ground.