UK context#synoptic#shear
Jet stream
At a glance
A high-altitude river of fast air that separates cold and warm airmasses. The UK sits under or near it most of the year — which is why our deep-layer shear is rarely the limiting factor.
Deep dive
The polar jet at ~9–11 km altitude is the main driver of UK storm weather. Ridging (meridional jet) north of the UK gives plume-like southerly flow; zonal jet directly overhead brings fast-moving frontal systems with squall lines.
Jet streaks — localised regions of faster wind — set up divergence aloft at their exit regions (left exit, right entrance for cyclonic jet) which favours upward motion and storm initiation downstream.
See bulk-shear for how this translates to deep-layer storm organisation.