TAFFlight rules
VFR · MVFR · IFR · LIFR — the FAA standard, defined by ceiling and visibility.
The US Federal Aviation Administration classifies weather conditions at an airport into four flight-rules categories based on the lowest cloud ceiling and the prevailing visibility. The categories are color-coded for quick reading and appear on tools like aviationweather.gov's Graphical Forecasts for Aviation. The thresholds are codified in 14 CFR Part 91 and the FAA Aeronautical Information Manual (AIM).
A category describes conditions, not a recommendation. A pilot's decision to operate VFR, IFR, or not at all depends on rating, currency, aircraft equipment, alternates, and many factors beyond what the category summarizes.
Categories
Ceiling greater than 3,000 ft AGL and visibility greater than 5 SM.
Ceiling 1,000–3,000 ft AGL and/or visibility 3–5 SM.
Ceiling 500–999 ft AGL and/or visibility 1 to <3 SM.
Ceiling less than 500 ft AGL and/or visibility less than 1 SM.
How the category is computed
Both ceiling and visibility are evaluated; the more restrictive of the two determines the category. A 200-ft ceiling with 10 SM visibility is LIFR (the ceiling drives it). 4 SM visibility with no ceiling is MVFR (the visibility drives it).
"Ceiling" means the height of the lowest broken (BKN), overcast (OVC), or vertical-visibility (VV) layer in the METAR. Few (FEW) and scattered (SCT) layers do not constitute a ceiling.
Source 14 CFR Part 91 · FAA AIM Chapter 7. Real-time categorization at aviationweather.gov. This page is informational only.