SNOWTAM decoder

1. Code 6, dry A dry runway.. Nothing on the surface. Code 6 is the only code that is not reported as a braking action, because a dry runway is the performance case the aeroplane was certificated on.
2. Code 5, good Frost, or a wet surface, or slush, dry snow or wet snow up to and including 3 mm deep.. Braking is as good as the pilot expects on a wet runway. The 3 mm line is the one to hold: at 3 mm and under these contaminants are code 5, and over 3 mm they are not.
3. Code 4, good to medium Compacted snow at an outside air temperature of minus 15 degrees or colder.. Cold compacted snow grips better than warm compacted snow. Minus 15 is the temperature the code turns on, and above that temperature the same surface is code 3.
4. Code 3, medium A slippery wet runway; dry or wet snow over 3 mm deep; compacted snow warmer than minus 15 degrees; dry or wet snow on top of compacted snow.. Braking is noticeably reduced and directional control needs care. This is where a landing distance assessment stops being a formality.
5. Code 2, medium to poor Standing water or slush more than 3 mm deep.. Depth of water is now the problem rather than friction alone: aquaplaning and drag from the displaced water both matter, and the takeoff case can be limiting as well as the landing case.
6. Code 1, poor Ice.. Braking is poor and directional control is poor with it. Crosswind limits published for a contaminated runway are what govern here.
7. Code 0, less than poor Wet ice; water on top of compacted snow; dry or wet snow on top of ice.. The worst code there is. A code 0 third is a runway an operator's own procedures will normally forbid, and it is reported so that the crew does not have to work out how bad it is.

The matrix is the ICAO Global Reporting Format, in force since November 2021 and published in Annex 14 Volume I and Doc 9981. It is quoted here as the international standard it is, not as a DGCA syllabus claim.