A pilot is planning to route around an area of thunderstorm activity depicted on radar. According to standard recommendations for thunderstorm avoidance, what minimum lateral clearance should be maintained from a thunderstorm cell that appears to be building rapidly, and from the top of a well-developed storm when overflying it if possible?
- A5 nautical miles laterally from any cell, with no altitude clearance required over the top
- B20 nautical miles laterally from any cell exhibiting strong radar echoes or rapid growth, and at least 1,000 feet of altitude for every knot of wind speed at the storm top if flying over it
- CThere is no standard clearance distance; visual assessment alone is sufficient for avoidance
- D10 nautical miles laterally from any cell, and overflight is never recommended under any circumstances
Show answer & explanationAnswer & explanation
Correct answer: B. 20 nautical miles laterally from any cell exhibiting strong radar echoes or rapid growth, and at least 1,000 feet of altitude for every knot of wind speed at the storm top if flying over it
Standard guidance recommends avoiding severe or rapidly building thunderstorm cells by at least 20 nautical miles, since hazards such as hail and turbulence can extend well beyond the visible cloud boundary. If overflight cannot be avoided, clearing the top by at least 1,000 feet for each knot of wind speed at the storm top is recommended due to potential turbulence extending above the visible cloud.
Why the other options are wrong
- A. 5 nautical miles is insufficient clearance for a rapidly building or severe cell; hazards extend much farther.
- C. Standard numerical guidance exists and should be applied rather than relying solely on visual judgment.
- D. 10 nautical miles is inadequate for severe storms, and overflight, while discouraged, is sometimes unavoidable and has specific altitude guidance.
Thunderstorm Avoidance Distances
Recommended clearance from severe or rapidly building thunderstorms is at least 20 NM laterally, and 1,000 ft of altitude per knot of wind at storm top if overflying.
- 20 NM lateral clearance for severe/building cells
- Hazards like hail can exist well outside the visible cloud
- 1,000 ft per knot of top-level wind speed when overflying
Memory trick: Twenty miles wide, a thousand feet per knot when you ride over the top.