Deep-layer shear

Deep-layer shear is the difference between the wind near the ground and the wind about six kilometres up — not just in speed, but in direction. It's the Engine meter in the Tornado Outlook, and it decides whether a storm stays a brief rain shower or becomes an organized machine.

>Why storms need it

A storm with no shear is a fountain: air goes up, rain falls back down — straight through the updraft that fed it. The storm drowns its own engine within an hour. That's the classic summer "pulse" storm.

With strong shear, the storm grows tilted. The updraft leans downwind, so rain falls beside the rising air instead of into it. Intake and exhaust are separated, like a proper engine — and the storm can run for hours. Shear also makes the updraft start to rotate, which is the first step on the road to a supercell.

>Reading the number

The outlook computes it as the vector difference between the 10 m wind and the ~5.5 km wind:

• under 10 m/s — messy, short-lived storms.
• 10–15 m/s — some organization; brief strong cells possible.
• 15–20 m/s — supercell territory: storms can rotate and persist.
• over 20 m/s — strong engine; long-lived rotating storms likely if storms form at all.

The catch: shear without fuel is just a windy day. It's the combination — fuel to build the updraft, shear to organize it — that earns the outlook's "organized storms" level.