Definition
Snow grains are very small, white, opaque ice particles that fall from the sky, typically less than 1 mm across. They are the frozen equivalent of drizzle — they fall in small amounts, do not bounce or shatter when they hit a hard surface, and usually come from low stratus clouds in cold, stable air. In a METAR, they are reported with the code SG.
Plain English
Tiny, soft snow specks — smaller than snowflakes — that drift down quietly without bouncing. METARs note them as SG.
Context Anchor
Seen in the present weather section of a METAR, where short codes describe what kind of precipitation or weather is occurring at the airport.
Derivation
‘Grain’ comes from the Latin granum, meaning a small seed or particle. The name reflects the appearance — fine, seed-sized bits of ice rather than the larger flakes of ordinary snow.
Why Pilots Care
Signals possible reductions in visibility and the need to check for slippery runway or aircraft surfaces before flight.
Grounding Statement
Picture tiny white specks of ice falling steadily instead of large, soft snowflakes.
Intuition Check
Do not read snow grains as just any small snow. In METAR use, SG is a specific weather code for very small opaque ice particles falling as precipitation.
Example Sentence 1
The METAR included -SG in the present-weather group, so the pilot expected light snow grains and reduced visibility on the approach.
Example Sentence 2
With SG reported, the crew inspected the taxiway for reduced traction during rollout.