Jet Streams and Upper Air
Jet Streams and Upper Air covers the location and structure of jet streams, the thermal wind relationship, and the link between upper-air patterns and surface weather.
Jet streams
A narrow band of wind of at least 60 kt near the tropopause. Two main types:
- Polar-front jet: linked to the polar front, meandering with Rossby waves, strongest in winter; its core lies just below the tropopause and slightly poleward of the surface front.
- Subtropical jet: further equatorward and higher, above the Hadley cell, generally weaker. A tropical easterly jet also occurs in summer over South Asia.
Thermal wind
The vertical wind shear is related to the horizontal temperature gradient. In the Northern Hemisphere the wind veers with height when the air becomes warmer downwind (warm advection) and backs with height in cold advection. The thermal wind blows parallel to the thickness lines with cold air to the left in the Northern Hemisphere.
Why it matters
The jet core edges and the tropopause break are clear-air turbulence zones. Jet streaks - local speed maxima - produce divergence aloft, supporting surface cyclogenesis, with the strongest lift near the left exit and right entrance regions.
Worked example
A 500 hPa wind of 250°/120 kt and a 300 hPa wind of 250°/150 kt give 30 kt of speed shear between the levels - modest. If the 300 hPa wind were instead 230°/170 kt, the shear would be 50 kt with 20° of directional change, and clear-air turbulence would be more likely, especially near the jet edge.
Traps
- Placing the polar-front jet at the surface front; it is displaced poleward and sits near the tropopause.
- Treating 60 kt as arbitrary - it is the definition threshold.
- Forgetting that tropopause breaks occur at jets.
- Reversing the thermal-wind relationship between hemispheres.