Snow
Snow (SN) is precipitation consisting of ice crystals falling from clouds, forming when water vapour freezes directly onto ice nuclei at temperatures below 0°C. In Canadian aviation, snow is one of the most common and consequential weather hazards, affecting visibility, runway conditions, and aircraft surfaces. Reported in METARs as SN.
Types of Snow Events
- Light snow (−SN): Visibility generally > 1 SM; runway contamination developing; de-icing required if temperature is at or below 0°C
- Moderate snow (SN): Visibility 5/8 to 1 SM; significant runway contamination; reduced braking action
- Heavy snow (+SN): Visibility < 5/8 SM; severe runway contamination; zero-zero conditions possible; airport operations may be suspended
- Blowing snow (BLSN): Snow raised by wind to reduce visibility; can occur with no precipitation from above
Aviation Hazards
- Contaminated runways — braking action reduced from good to nil depending on depth and compaction
- Aircraft critical surface contamination — departure under the clean aircraft concept
- Engine inlet icing with heavy wet snow
- Navigation equipment accumulating snow and ice
- Lake-enhanced snow squalls — localized intense snow with near-zero visibility
Runway Contamination
Runway condition codes (RwyCC) and braking action reports (Good, Medium-to-Good, Medium, Medium-to-Poor, Poor, Nil) are critical for landing distance planning in snow conditions. Use published contaminated runway performance data — normal dry runway figures do not apply.
On the TC Written Exam
Snow visibility codes, runway contamination effects, and the clean aircraft concept in snow conditions are all tested. Know METAR codes: SN, −SN, +SN, BLSN, DRSN (drifting snow), and the visibility ranges.
Related Terms
See also: Snow Squall, Lake-Enhanced Precipitation, Aircraft Critical Surface Contamination.