Sunlight Simulator Overlay

Sunlight Simulator shows which slopes are in sun and which are in shadow at a date and time you choose. It works out where the sun sits in the sky for that moment and place (its azimuth and elevation), then casts shadows across the terrain. First Sun and Snow Preservation use the same date, so changing it in one changes it in all three.

Sun and snow through the day

East-facing slopes get sun first. After a clear, cold night, slopes that saw sun the day before often start out with a hard melt-freeze crust, which skiers call a sun crust.

South- and west-facing slopes warm up later in the day. In spring that can soften a crust into corn. If it gets too warm, the snow turns heavy and wet.

Slopes that stay in shade keep colder, drier snow, and powder often lasts days longer there.

Forecasters call strongly sunlit slopes solar aspects. When those slopes warm quickly, wet loose avalanches become a concern. These start at a single point and fan out as they slide. Snowballs or little wheels of snow rolling down a slope (often called roller balls or pinwheels) mean the surface is getting wet. Cornices also weaken as they warm.

Changing the date

In midwinter the sun stays low, and steep north-facing slopes can go weeks without direct sun. By spring it's higher and stronger, and north aspects start to soften as well. Try moving the date to see how a slope's sun changes over the season.

Limits

The simulator assumes a clear sky. Thin cloud can still warm the snow, so a slope shown in shadow can still get soft. Forest shade is mostly missing, so a slope that looks sunny may be largely shaded under the trees. It also doesn't know the air temperature. Warm air, rain, or a night that never freezes can soften snow on any slope.

Built from a digital elevation model (DEM). Shadows are cast using elevation data. Boulders, cornices and short rock bands may not cast shadows on the map, and shadow edges are approximate.