Snow Preservation Overlay

Snow Preservation estimates how much direct sun each slope collects over a ski day and ranks the slopes against each other. Green slopes stay in shade longer and should hold cold snow best. Red slopes take the most sun. Your phone calculates it from terrain elevation data and an idealized clear sky. It doesn't use weather observations or forecasts.

Picking a day

Tap the date pill on the map to choose any day. First Sun and Sunlight Simulator use the same date, so changing it in one changes it in all three.

While this layer is on, the compass rim shows the sun's path for that day. The colored arc is the stretch of sky the sun crosses, shading from yellow where the sun is weak to red where it's strongest. The black part of the rim is sky the sun never reaches that day.

How the estimate works 🤓

The model steps through the day from sunrise to sunset every 20 minutes and works out the sun's azimuth (compass direction) and elevation (height above the horizon) at each step. Positions lower than about 3° are skipped.

At each step it checks two things for every point on the map. The first is how squarely the slope faces the sun. The energy a surface receives depends on the incidence angle, which is the angle between the sun's rays and a line perpendicular to the slope. In winter a steep slope facing the low sun can receive about twice the energy of flat ground, while a slope the sun only grazes gets very little.

The second is whether the point is in shadow. The model traces a line from the point toward the sun for about 15 km and checks whether any terrain blocks it, correcting for the curvature of the Earth. With that range, a distant peak can shade a run, the way Mount Yotei can shade slopes at Niseko.

Each sunlit step is then weighted by how much the atmosphere dims the sun at that height. Low sun passes through more air (a larger air mass), so early-morning light counts for little and midday sun counts most. This part uses a standard clear-sky model.

Two adjustments tune the total for skiing. Sun after solar noon counts 1.25 times as much as morning sun, as a stand-in for the air warming through the day. And the tally stops about 3 hours after solar noon (around 3:30 pm), since lifts are closing by then and later shade won't help the snow you ski.

Using it

After a storm, look for green if you're after powder. High, steep north- and east-facing slopes usually hold cold snow longest.

For spring corn, red slopes soften first. They need a solid overnight refreeze to ski well, and if you arrive late they can turn to slush or release wet loose avalanches.

Limits

This is an estimate built on a clear sky and bare terrain. It doesn't model cloud cover, air temperature, wind or the type of snow on the ground. Trees and buildings mostly don't cast shadows here. Lidar data skips them entirely, and the 30 m global data only roughly captures them. The colors rank slopes against each other within the area on screen, and they can't tell you what the snow actually feels like.