Jet Streams and Clear Air Turbulence

Two jets, two origins

The polar front jet forms along the same polar front that produces mid-latitude depressions, so its position is variable, tracking the front itself. It is generally stronger and lower in winter, when the temperature contrast between polar and tropical air is greatest, and it is typically found roughly between FL300 and FL400, with winter cores commonly quoted as exceeding 200 kt.

The subtropical jet forms at the poleward edge of the Hadley cell, where air that has risen near the equator and travelled poleward aloft is deflected strongly eastward. It sits at a generally higher and steadier altitude than the polar front jet, is less tied to any single surface feature, and also strengthens in winter as the temperature gradient at its latitude sharpens.

• Polar front jet: tied to the polar front, more mobile, typically FL300 to FL400, strongest in winter
• Subtropical jet: tied to the edge of the Hadley cell, generally higher and steadier, also winter-favoured

Locating clear air turbulence around the core

Clear air turbulence is produced by wind shear, not by wind speed alone, so it concentrates on the cold, poleward side of the jet core, where the horizontal temperature and wind gradient is steepest, rather than directly in the core itself where the flow is comparatively smooth. Vertical shear is also strong immediately above and below the core, so changing level without changing side is not a reliable fix on its own.

Jet curvature and the entrance and exit regions of the jet add further along-stream acceleration or deceleration, which increases shear rather than reducing it. Significant weather charts commonly mark turbulence in exactly these locations: the cold side of a jet axis, and around ridges, troughs, and jet entrances and exits, rather than along the smooth core itself.

Seasonal shift and chart recognition

Both jets migrate with the sun's seasonal heating, moving poleward and weakening in summer and shifting equatorward and strengthening in winter, tracking the same temperature gradients that create them. A stem describing an unusually strong, low-latitude jet is therefore almost always describing winter conditions, not a random encounter.

On an upper wind or significant weather chart, a jet shows up as closely packed contour lines, or isohypses: the tighter the packing, the stronger the implied wind, which is exactly what the thermal wind relationship predicts from a sharp temperature gradient.