Key takeaways
- Sun cups are bowl-shaped hollows that form on snow under intense sunlight.
- They create a honeycomb of depressions separated by thin ridges.
- They form by uneven melting/sublimation — hollows trap heat and melt faster than ridges.
- Common on high-altitude and spring/summer snowfields; deep ones make travel awkward and tiring.
From their formation by the sun.
What sun cups are
Sun cups are bowl- or cup-shaped hollows that form on a snow surface under intense sunlight, creating a honeycomb of depressions separated by thin ridges — somewhat like an egg carton of snow. They range from shallow dimples to deep, foot-plus hollows.
How they form
Through uneven melting and sublimation driven by the sun. Once small irregularities appear, the low points (cups) trap and absorb more heat and melt or sublimate faster, while the ridges are more exposed and persist — so the differences amplify into a regular pattern. Strong direct sun, high altitude, and dry air favor them.
Crossing a high snowfield in late summer, a mountaineer hits a field of deep sun cups — stepping in and out of hollow after hollow over the sharp ridges between them, an ankle-twisting, energy-sapping slog that slows the whole party.
How they affect travel
Deep sun cups make travel awkward and tiring: constantly stepping in and out of hollows over the ridges is uneven, ankle-twisting, and energy-sapping, and unpleasant to ski. Small ones are a minor nuisance; large ones on a long snowfield can really slow a party. They’re a sun-and-snow feature alongside corn snow and firn (and relate to spiky ‘penitentes’), and softening cups can also lead to postholing.
The bottom line
Sun cups are bowl-shaped hollows that form on snow under intense sun, creating a honeycomb of depressions and thin ridges as the hollows melt faster than the ridges. Common on high-altitude and late-season snowfields, they range from minor dimples to deep, ankle-twisting hollows that make foot, ski, and snowshoe travel awkward and tiring across a long field.
Frequently asked questions
What are sun cups?
Sun cups are bowl- or cup-shaped hollows that form on a snow surface under strong sunlight, leaving a field of depressions separated by thin, often sharp ridges — somewhat like a honeycomb or egg carton of snow. They range from shallow dimples to deep hollows over a foot deep, and are a common feature on sunny high-altitude and late-season snowfields.
How do sun cups form?
Through uneven melting and sublimation driven by the sun. Once small irregularities appear, the low points (the cups) tend to trap and absorb more heat and melt or sublimate faster, while the high points (the ridges) are more exposed and persist, so the differences amplify into a regular pattern of cups and ridges. Strong, direct sun, high altitude, dry air, and certain snow conditions favor their development.
How do sun cups affect travel?
Deep sun cups make travel awkward and tiring. Walking, skiing, or snowshoeing across a field of cups means constantly stepping in and out of hollows over the ridges, which is uneven, ankle-twisting, and energy-sapping, and unpleasant to ski. Small sun cups are a minor nuisance, but large, deep ones on a long snowfield can significantly slow and tire a party. They're related to penitentes, an extreme, spiky form of sun-melted snow.
Sources
- Snow surface & terrain — The Mountaineers
- Snow science — National Snow and Ice Data Center