The colours you see on an aviation weather map, green for VFR through to magenta for LIFR, are a quick way to read the flying conditions across a whole region, and they are built from just two numbers: the ceiling and the visibility.
This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.
What the categories are
A flight category is a single label that summarises how good or bad the conditions are for flying by reference to the ceiling and the visibility. The scheme is defined by the NOAA/NWS Aviation Weather Center and used across FAA products, and it sorts conditions into four bands. The rule that ties them together is simple: whichever of the ceiling or the visibility is more restrictive sets the category.
- VFR (visual flight rules), shown green: ceiling greater than 3000 feet and visibility greater than 5 statute miles.
- MVFR (marginal VFR), shown blue: ceiling 1000 to 3000 feet and/or visibility 3 to 5 statute miles.
- IFR (instrument flight rules), shown red: ceiling 500 to less than 1000 feet and/or visibility 1 to less than 3 statute miles.
- LIFR (low IFR), shown magenta: ceiling below 500 feet and/or visibility less than 1 statute mile.
The "and/or" matters. To be VFR, both the ceiling and the visibility have to be good. To fall into a worse category, only one of them has to drop.
What the ceiling means here
The ceiling is the height above ground level of the lowest cloud layer that is reported as broken or overcast, or the vertical visibility into a surface-based obscuration such as fog. Layers reported as few or scattered do not make a ceiling. So a METAR reading SCT008 BKN025 has its ceiling at 2500 feet, the broken layer, not at the scattered 800 foot layer. The FAA Aeronautical Information Manual weather chapter sets out the cloud-amount definitions this rests on.
A worked example
Take this METAR:
KBOS 191554Z 12010KT 4SM BR BKN009 OVC015 09/08 A2998
Reading the two governing numbers: the visibility is 4 statute miles, and the ceiling is the lowest broken or overcast layer, which is BKN009, a ceiling of 900 feet.
- The visibility of 4 miles, on its own, sits in the MVFR band (3 to 5 miles).
- The ceiling of 900 feet, on its own, sits in the IFR band (500 to below 1000 feet).
- The more restrictive of the two wins, so the flight category is IFR, and on a map this station would show red.
Now change the ceiling to BKN012 (1200 feet) and leave the visibility at 4 miles: the ceiling is now MVFR and the visibility is MVFR, so the category becomes MVFR, shown blue. One layer moving 300 feet changed the colour.
Why this is not the legal minimum
This is the point that catches people out. A flight category is a planning tool, not a regulation. The categories were designed to give a fast, regional picture of risk, not to tell you whether a given flight is legal.
The legal minimums for flying under visual flight rules are set separately and depend on the airspace and the altitude. In the United States they live in 14 CFR 91.155, which lays out the visibility and cloud-clearance requirements class by class. In Europe the equivalent visual meteorological conditions minima are in the SERA rules (Regulation (EU) No 923/2012). Those rules, not the colour on a map, decide whether a VFR flight is legal, and they vary with the class of airspace you are in. If you want the detail on the legal side, see our separate guide on VFR weather minima and cruising levels.
So treat the category as a first read, then check both your personal limits and the legal minima for the airspace you will actually fly in. A green VFR station can still hold conditions that are outside your own comfort, and a blue MVFR station can be perfectly legal for an instrument-rated pilot on an instrument flight plan.
Reading a whole route, and what the colour hides
The categories come into their own when you read a graphical forecast for a whole region, where each station or area is painted with its colour. The NOAA/NWS graphical forecasts let you take in a route at a glance: a line of green stations with a patch of blue and red around some hills tells a story faster than reading twenty METARs. The skill is to read not just the colour now but the trend: a string of stations turning from green to blue to red along your track is a front or a band of weather you will fly into, and the timing matters as much as the present state.
But the colour is a summary of two things only, ceiling and visibility, and it is silent about several hazards that can make a green station unflyable for you:
- Wind. A station can be solid VFR green with a 30-knot crosswind that is well beyond your aircraft or your limits.
- Turbulence and wind shear. Neither shows in the flight category, yet either can make an approach unwise.
- Icing. Freezing conditions in cloud do not change the category but change everything about whether you should be there.
- Thunderstorms. A convective cell can sit in nominally VFR air, and the category will not warn you.
There is also a subtlety in how the ceiling is found when fog or an obscuration hides the sky. When there is no defined cloud layer, the ceiling becomes the vertical visibility, the distance you can see straight up into the obscuration, reported as a VV group in the METAR. A VV002 is treated as a 200-foot ceiling for the category, which is firmly in the LIFR band. So a foggy morning with the sky obscured can show magenta across a whole region even though, strictly, there is no cloud layer at all, just fog the observer cannot see through.
The practical reading, then, is to use the colour as the first cut over a route, watch the trend along your track, and then go behind the colour to the actual reports and forecasts for the wind, ice, turbulence, and convection that the category cannot show.
The European picture
The colour-coded flight-category scheme, with its VFR, MVFR, IFR and LIFR bands, is a product of the United States National Weather Service, and you will see it most often on US-oriented charts and apps. The underlying idea, summarising conditions by ceiling and visibility, is universal, but other regions present it differently.
In Europe, the same low-level decision is supported by route forecasts and area forecasts that describe expected conditions along a corridor, and some countries publish a coded general aviation forecast (often called a GAFOR) that grades route segments by how flyable they are, which serves the same at-a-glance purpose as the US colours. The UK Met Office and other national meteorological services produce the regional versions.
What stays constant everywhere is the gap between a flight category and the legal minima. The category, whatever its local form, is a planning summary; the binding minima for VFR flight come from the airspace rules, 14 CFR 91.155 in the United States and the SERA visual meteorological conditions tables in Europe, and those vary by class of airspace. So a pilot flying internationally should read whichever at-a-glance product the region provides, then go to that region's legal minima, rather than carrying a single set of numbers across borders.
Common pitfalls
- Treating the colour as permission. The category is risk shorthand, not a legal clearance to go.
- Forgetting that the worse number wins. Good visibility does not rescue a low ceiling, or the other way round.
- Counting scattered cloud as a ceiling. Only broken or overcast layers, or vertical visibility, make a ceiling.
- Mixing up units. The boundaries are in statute miles and feet, the US convention; a report in metres needs converting first.
- Reading a single station as the whole route. Conditions change along the way, so read the category at each point that matters.
In Pilot EFB
Pilot EFB works out the flight category from the reported ceiling and visibility and shows it as a colour-coded badge on each airport's briefing, using the same FAA and National Weather Service category boundaries. It is a descriptive emphasis on the decoded weather to help the worst conditions stand out; it does not make a go or no-go decision, advise you whether a flight is legal, or replace the regulatory minima for your airspace. The raw report is always kept alongside the badge so you can check it. A briefing you have pulled stays readable offline; fetching fresh weather needs a connection. Pilot EFB is offline-first and is not a certified Electronic Flight Bag.