Clear Air Turbulence (CAT): Why Planes Shake in Clear Skies

Clear Air Turbulence is the invisible challenge of aviation—it occurs without clouds, without warning on radar, and often without any visible indication.

What is Clear Air Turbulence?

CAT occurs when two air masses moving at different speeds or directions meet, creating chaotic air movement. Unlike convective turbulence (which happens in and around clouds), CAT happens in clear skies.

Where CAT Occurs

Jet Streams

The primary cause of CAT:

• Polar jet stream: 30,000-40,000 feet, typically 30-60° latitude
• Subtropical jet stream: Usually around 25-35° latitude
• Strongest in winter months

Boundaries of Jet Streams

The edges of jet streams are most turbulent:

• Wind speeds can change by 100+ knots over short distances
• The "tropopause" (boundary between troposphere and stratosphere)

Mountain Waves

Wind flowing over mountain ranges creates:

• Standing waves in the atmosphere
• Turbulence that can extend hundreds of miles downwind
• Effects at cruise altitude (30,000+ feet)

Why CAT is Hard to Predict

Invisible to Radar

Weather radar detects precipitation (water droplets), not air movement. Since CAT occurs in clear air, radar is ineffective.

Rapidly Changing

CAT can:

• Appear and disappear in minutes
• Vary in intensity quickly
• Move with the jet stream

Small Scale

Turbulent eddies are often smaller than weather model resolution, making precise prediction difficult.

How We Detect and Predict CAT

Pilot Reports (PIREPs)

Real-time reports from aircraft experiencing turbulence are currently the most reliable source.

Atmospheric Calculations

Physics-based methods include:

• Richardson Number: Stability indicator
• Ellrod Index: Shear and deformation
• Wind shear analysis: Multi-level wind comparisons

Satellite Data

Modern satellites can detect:

• Upper-level winds
• Temperature gradients
• Humidity variations

Is CAT Getting Worse?

Research suggests climate change is increasing CAT:

• Stronger jet streams due to temperature differentials
• Studies show 15-40% increase in severe CAT over the North Atlantic
• Winter CAT expected to increase further

How Pilots Handle CAT

1. Flight planning: Review forecasts before departure
2. PIREP monitoring: Watch for reports from other aircraft
3. Altitude changes: Climb or descend to find smoother air
4. Speed adjustment: Slow to turbulence penetration speed
5. Routing: Avoid known CAT areas when possible

CAT and Flight Safety

Important facts:

• CAT has never caused a commercial aircraft to crash
• Modern aircraft easily withstand CAT forces
• The main risk is to unbelted passengers

Conclusion

Clear Air Turbulence remains aviation's invisible challenge, but modern forecasting tools like Turbcast help predict where it's likely to occur. Always keep your seatbelt fastened when seated.