Clear air turbulence (CAT)
Clear air turbulence (CAT): sudden, severe turbulence in cloudless air. Violent buffeting, no cloud warning. Normally above 15,000 ft, most often near a jet stream (see jet-stream) (AWH ch.19).
Cause and location (TC AIM AIR 2.10):
- Wind shear (rate of change of wind speed away from the core) causes it, not raw core speed.
- Jet core over 110 kt has significant turbulence zones:
- above the core: sloping tropopause
- below the core: jet front
- low-pressure (poleward) side of the core
- Curving jet, especially around a deep pressure trough: more turbulent edges than a straight one.
- Mountains: worse above and to the lee of ranges. Expect CAT crossing a strong jet stream near mountainous terrain.
- Wind-shift areas near troughs: frequently turbulent. Shift sharpness matters more than location. Ridge lines can also carry rough air.
- Tropopause (where temperature stops decreasing with height): turbulence from penetrating a sloping tropopause is most pronounced in the temperature-change zone on its stratospheric side.
- TC AIM CAT rules of thumb apply to westerly jet streams.
Locating CAT on charts (TC AIM AIR 2.10):
| Indicator | Threshold | Notes |
|---|---|---|
| Horizontal shear, isobaric chart (e.g. 250 mb) | 30 kt isotachs closer than 90 NM | Normally north (low-pressure) side of jet axis |
| Vertical shear, wind-aloft charts or reports | over 5 kt per 1,000 ft | Likely turbulence |
| Upper air chart isotherms | 5°C isotherms closer than 2° of latitude (120 NM) | Usually sufficient vertical shear too: vertical shear tracks horizontal temperature gradient |
Encountering CAT (TC AIM AIR 2.10):
| Situation | Action |
|---|---|
| Direct tailwind or headwind | Change flight level or course (turbulent areas elongated with wind, shallow and narrow). Turning south (Northern Hemisphere) more favourable. If not feasible, climb or descend to next flight level, usually smoother |
| Crosswind | Climb if temperature rising, descend if falling. Temperature constant: either |
| Abrupt wind-shift at sharp pressure trough | Cross the trough, do not fly parallel. Flight-level change less likely to help |
| Significant CAT reported or forecast | Adjust to recommended turbulent air penetration speed on the first ripple (intensity can build rapidly). Moderate or severe CAT expected: adjust airspeed before encountering it |
CAT forecast is only generalized over a large area, so PIREPs are the best real-time source (TC AIM MET 2.2.1).
Trap: blaming raw wind speed for CAT. A fast, smooth core is not necessarily rough. Shear away from the core buffets the aircraft, not speed at the core.
References
- AWH 19.2.3.2.1
- TC AIM MET 2.2.1
- TC AIM AIR 2.10