Which weather conditions define LIFR restrictions?

• A. < 500 ft ceiling or < 1 SM vis
• B. < 1000 ft ceiling or < 3 SM vis
• C. 1000 - 3000 ft ceiling or 3 - 5 SM vis
• D. > 3000 ft ceiling and 5 SM vis

Answer: (A)

LIFR restrictions come from the most restrictive weather category for aviation, defined by very low ceiling or very poor visibility. Specifically, you’re in LIFR when the ceiling is below 500 feet or the visibility is below 1 mile. Because of the “or” relationship, any one of these conditions dropping below its threshold puts you in LIFR, not requiring the other to be worse.

So, for example, a sky with a 450-foot ceiling but 3 miles of visibility is LIFR, and a sky with 800-foot ceilings but 0.8 statute mile visibility is also LIFR. This is the strictest category, and it typically means instrument flight rules are required.

By contrast, other categories have higher thresholds: MVFR is roughly 1,000–3,000-foot ceilings or 3–5 miles visibility; IFR is about 500–999-foot ceilings or 1–3 miles visibility; VFR requires ceilings above about 3,000 feet and visibility above 5 miles. The option describing less restrictive thresholds corresponds to those other categories, not LIFR.