Snow (SN) is defined as snowflakes—ice crystals that are mostly branched in six-pointed star shapes

Snow is a weather moment that matters in aviation more than most people realize. When you’re piloting, planning, or dispatching, the difference between a light dusting and a snowy runway can change a route, a fuel plan, and even your braking strategy. So let’s break down what SN means in aviation weather reporting and why that tiny letter matters so much.

What SN actually stands for in aviation weather

Here’s the thing: among the possible definitions you’ll encounter, the one that best matches how snow behaves in the atmosphere is this—snow crystals mostly branched in the form of six-pointed stars. In other words, snowflakes are the ice crystals that grow into those delicate, star-like shapes as water vapor freezes in cold air. This is not just a pretty image; it’s a clue about how snow forms, how it falls, and how it can pile up on runways and in the airspace you fly through.

Let me explain why this matters. Snow isn’t just “white stuff.” It’s made up of countless ice crystals, each with a six-fold symmetry because of the way water molecules arrange themselves when they freeze. That structure influences the snow’s texture, how it accumulates, and how it behaves when it’s blown by wind or warmed by sunshine. It also helps meteorologists distinguish snow from other precip types that can look similar on the surface.

Snow vs. other precip: a quick comparison

• Snow crystals in the clouds form from water vapor freezing and growing into ice crystals. When conditions are just right, those crystals branch into intricate, six-pointed shapes that we recognize as snowflakes.

• Small white pellets are not snow. They are typically sleet or ice pellets—precipitation that started as ice and didn’t reach the ground as soft flakes.

• Ice crystals in a cloud describe the outset of snow formation, but until those crystals reach the ground in a snowflake, they aren’t yet SN in the runway sense.

• Water droplets frozen in the air can become freezing rain, which behaves differently on surfaces than snow does. It’s a separate hazard with its own reporting nuances.

So, the hallmark of SN is the presence of snowflakes formed from hexagonal ice crystals, often with those branching, six-pointed star-like patterns.

From sky to runway: why this matters for aviation

Snow is more than a pretty meteorological label. It directly affects flight operations in several practical ways:

• Visibility and weather conditions: Snow can reduce visibility, especially when fresh falls mix with wind. Pilots need to know not just that it’s snowing but how much snow is accumulating and how fast it’s blowing around.

• Surface conditions: Snow on braking surfaces changes friction. White powder can be slippery when light, but a crusty layer made by melt-freeze cycles can surprise you with reduced braking efficiency.

• Runway contamination: Snow accumulation on runways and taxiways is a key factor in NOTAMs and operational decisions. Snow depth, compaction, and moisture content can all dictate whether a runway is usable, partially usable, or closed.

• Icing potential: Snow can transition into ice on aircraft surfaces in certain temperatures and humidity conditions. That icing risk is a separate concern that pilots must monitor, especially during takeoff and landing.

• Route planning: Snow is a telltale sign of a broader weather system that can affect headwinds, tailwinds, and turbulence. Cold air masses with snow can also imply potential icing farther aloft.

In LAWRS-style weather reporting, SN serves as a weather type that signals credible, ground-truth observations about snowfall. It informs ground crews for deicing, affects air traffic control decisions, and guides pilots about what to expect at their destination or along their route.

How snow forms in the atmosphere (a quick science detour)

Snowflakes begin as water vapor in clouds. When temperatures are cold enough and humidity is just right, water vapor deposits directly as ice onto a frozen nucleus, building a tiny crystal. As these crystals climb and drift in the cloud, they collide and merge, creating those familiar dendritic (branching) shapes. The exact look of a snowflake—how many branches, how symmetrical it is—depends on temperature, humidity, and air currents. The famous six-pointed star pattern isn’t a random flourish; it’s a natural consequence of the hexagonal lattice structure of ice.

For the observer, that means a fresh snowfall often brings uniform, fluffy snow, while more complex patterns can create denser, wetter snow that behaves differently on runways. That variability is why aviation weather reports separate snow type and intensity rather than just saying “it’s snowy.”

How observers translate SN into useful aviation data

Observations of SN come through weather reporting systems that feed METARs, TAFs, and real-time weather feeds used by pilots and dispatchers. When SN is present, weather observers note the type and, importantly, the runway and surface conditions when possible. Here’s how that translates on the ground:

• Precipitation type SN signals snowfall rather than other forms like rain or sleet.

• Intensity indicators (which can be added in reports) tell you whether snowfall is light, moderate, or heavy, which matters for visibility and planning.

• Accumulation notes, when available, alert crews to how much snow has collected on airfields. This affects braking action assessments and snow removal priorities.

• Runway contamination considerations come into play as snow depth and compaction can degrade braking performance. In some cases, airports will declare braking action and provide remediation guidance.

Pilots use this information to decide whether to land, divert, or delay. Ground crews use it to schedule plowing, deicing, and snow management. And air traffic control uses it to set safe spacing and routing. Snow is a team effort in aviation, and SN is the banner term that brings everyone’s attention to the same weather story.

A mental model you can carry into every snowfall

• Snowflakes are unique, but they come from a consistent ice-crystal process. The six-pointed star shapes are a telltale sign of ice crystal growth in cold, crisp air.

• Snow can sting visibility and icing conditions at the same time, especially when temperatures hover around the freezing mark and winds shift.

• Snow depth and settlement matter as much as the rate of snowfall. A light snow that quickly moves into a grayer, wetter state can still create slick surfaces if it compacts or melts and refreezes.

A few practical tips for understanding and communicating SN

• If you’re involved in weather interpretation for aviation, keep an eye on both precipitation type and surface conditions. A growing snow cover on the runway is a different signal than a snow shower that’s already passed.

• When writing or interpreting SN notes, pair the snowfall description with runway contamination status and braking action if available. That provides a fuller picture for decision-making.

• Use METAR literacy: SN as a precipitation type is a common shorthand you’ll see in aviation weather reports. It’s a cue to look deeper into visibility, ceiling, wind, and temperature for a complete operational picture.

A small, human touch: why snow is a shared experience

Snow is one of those weather phenomena that feels universal yet personal. It changes how a city sounds (less traffic, more hush), how airplanes look as they land against a white backdrop, and how people plan their day. In aviation, that shared experience turns technical data into actionable decisions. The six-pointed star becomes not just a scientific fact, but a signal that pilots and crews are moving through a weather system together.

If you’ve ever watched snow fall and noticed how it blankets the ground, you’ve seen a simple version of what meteorologists chase with SN: a consistent, credible signal about how the air is behaving near the surface and aloft. In aviation, that signal helps ensure that everyone—from the person operating the radar to the technician on the deicing truck—keeps safety front and center while the plane finds its way to the runway and beyond.

A closing thought

The correct definition—snow crystals mostly branched in the form of six-pointed stars—captures more than a shape. It reflects the physics of the atmosphere, the way we read weather data, and the practical steps we take to fly safely when the world is white with snow. So next time you hear SN in an aviation weather briefing, you’re not just hearing a code—you’re hearing a story about ice crystallizing in the cold, about visibility shifting, and about the careful choreography that keeps air travel moving even when snow decides to fall.

If you’re curious, you can explore more about snow’s role in aviation by looking into METAR codes, how runway condition reports are issued, and the way airports prioritize snow removal and deicing operations during winter. It’s a real-world reminder that weather literacy isn’t just for meteorologists; it’s for anyone who wants to understand how flight works in harmony with the sky.