Upper-Air Meteorology: Tropopause, Jet Streams and Clear Air Turbulence
This domain covers the tropopause, jet stream structure, upper winds and clear air turbulence - the material behind flight-planning winds and high-level hazard avoidance.
Tropopause
- The isothermal layer at the top of the troposphere; ICAO standard 36,090 ft at -56.5°C.
- Higher over the tropics and in summer, lower over the poles and in winter, and lower over a cold low than over a warm high.
- Discontinuities and folding occur at jet streams and are associated with turbulence and ozone-rich air from the stratosphere.
Jet streams
- Polar front jet: associated with the polar front, typically 30,000-40,000 ft, core speeds 100-200 kt, meandering with Rossby waves.
- Subtropical jet: higher (often 35,000-45,000 ft), weaker than the polar jet, near 30° latitude.
- Tropical easterly jet: summer only, in the tropical upper troposphere.
- Low-level jets occur near 1,000-3,000 ft and produce significant wind shear.
- Entrance and exit regions of a jet have divergence and convergence, which drive surface cyclogenesis and give the strongest shear.
Upper winds
- Veering with height indicates warm advection, backing indicates cold advection; the thermal wind relationship links shear to the horizontal temperature gradient.
- Wind speed maxima are found just below the tropopause on the cold side of the jet core.
Clear air turbulence (CAT)
- Turbulence above about 20,000 ft with no visible cloud; not detectable by weather radar.
- Most likely near jet streams (especially the cold side and in troughs), in sharp wind shear (roughly 4 kt or more per 1,000 ft), and in mountain wave.
- Report encounters; forecast charts show CAT areas but the boundaries are unreliable.
Worked example
A wind sounding gives 250°/60 kt at FL300 and 250°/90 kt at FL340. The shear is 30 kt over 4,000 ft, or 7.5 kt per 1,000 ft - well above the 4 kt/1,000 ft threshold. Combined with the aircraft being near a jet core on the cold side of a trough, CAT is likely and the seat-belt sign should be on.
Traps that cost marks
- Assuming the tropopause height is fixed, or that a jet stream always marks the surface front.
- Expecting radar to show CAT.
- Reversing the veering/backing interpretation of temperature advection.
- Forgetting that jet stream direction reverses in the southern hemisphere westerlies context.