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Low-level shear

INGREDIENTSWind turning in the lowest kilometre — the spin a storm can tighten into a tornado.

Low-level shear is the same idea as deep-layer shear, squeezed into the lowest kilometre of the atmosphere: how much the wind strengthens and turns between the ground and roughly 1 km up. In the Tornado Outlook it's the Spin meter — the specifically tornado ingredient.

>Invisible rolls of air

When the ground-level wind is slow (friction brakes it) and the wind just above is fast, the layer between them starts to roll — like a pencil rolled between two hands moving at different speeds. The lowest kilometre fills with long, invisible, horizontally spinning tubes of air.

Now add a storm. Its updraft inhales those horizontal rolls and bends them upright: horizontal spin becomes vertical spin, feeding rotation right at the storm's base — exactly where a tornado would have to form. Deep shear makes the whole storm rotate high up; low-level shear delivers the rotation to the ground floor.

>Reading the number

under 5 m/s — barely any low-level turning; tornadoes very unlikely.
5–8 m/s — some spin available.
8–12 m/s — a genuine tornado ingredient; with the other meters lit, this is a day to keep the radar open.
over 12 m/s — strong low-level spin, the signature of serious tornado outbreaks.

Watch how it changes toward evening: as the sun sets, the ground-level wind often slows while winds just above stay strong — low-level shear grows at dusk, which is one reason many European tornadoes arrive in the early evening.