VFR, MVFR, IFR and LIFR: What Flight Categories Actually Mean

Flight categories provide a convenient summary of whether an aerodrome is comfortable, marginal or genuinely difficult. They appear as colour coding on most weather dashboards, including our own, and they are widely understood. They are also frequently over-interpreted, and I should like to explain both their value and their limits.

The four categories

The categories are determined by two parameters only: the ceiling and the prevailing visibility.

Understanding the ceiling

The ceiling is not simply the lowest cloud reported. It is defined as the height above ground of the lowest layer reported as broken or overcast — that is, covering five oktas or more.

Layers reported as FEW or SCT do not constitute a ceiling regardless of how low they are. An aerodrome reporting scattered cloud at 400 feet with nothing above it has, formally, an unlimited ceiling. This is a point which causes considerable confusion, and it is worth verifying against the actual METAR rather than relying upon the summary figure alone.

A necessary note on Indian operations

The thresholds above originate in United States practice and use statute miles, a unit not employed in Indian aviation weather reporting, where visibility is given in metres. The categories remain a useful shorthand, and they are applied consistently across international weather dashboards for that reason.

They are not, however, regulatory minima in India. Operating minima for any Indian aerodrome are determined by the instrument approach procedure in use, the aircraft category, the available approach and runway lighting, and the operator's approvals, all within the framework established by the DGCA. A particular ILS approach may have a decision height and required visual range entirely unrelated to the boundaries in the table above.

Why the categories remain valuable

Notwithstanding those limitations, I find them genuinely useful for three purposes.

Regional awareness. A map of colour-coded aerodromes conveys the structure of a weather system almost instantly. A line of IFR and LIFR stations across the Indo-Gangetic plain on a December morning describes the extent of a fog event more quickly than reading twenty individual reports.

Alternate selection. When identifying candidate alternates, the categories allow rapid elimination of aerodromes which are clearly unsuitable, leaving a shorter list for detailed assessment against actual minima.

Trend recognition. Watching a station move from VFR through MVFR to IFR over successive observations is a clear indication of deterioration, and often gives earlier warning than a scheduled forecast amendment.

What the categories omit

This is the point I would most wish to emphasise. The categories consider ceiling and visibility, and nothing else. They are entirely silent regarding:

• Crosswind and gusts, which may render a runway unusable in otherwise perfect visual conditions
• Thunderstorms and cumulonimbus, which may be present at an aerodrome reporting VFR
• Windshear and turbulence
• Runway surface condition, contamination and braking action
• Freezing precipitation and airframe icing
• Volcanic ash, dust storms and smoke beyond their effect upon reported visibility

An aerodrome may be comfortably VFR while a mature thunderstorm sits over the final approach. I have seen precisely this situation many times during the pre-monsoon season in northern India, and it is the reason I would never accept a green indicator as a substitute for reading the report itself.

In summary

Flight categories are an excellent index and a poor conclusion. They tell you where to look. The METAR, the TAF, the radar picture and the published minima tell you what to do.

Live flight categories for Indian aerodromes, derived from current METAR data, are shown on our Live Weather Dashboard, and route-level conditions may be assessed using the Route Forecast tool.