Suncups are abutting bowl-shaped depressions in a snow surface. Typically they form high in the mountains during the spring when the wet-bulb temperature is below 0 °C.
I have travelled through suncups three-metres across and a half-metre deep on the névé of Mount Garibaldi — progress was slow. Starting from dimpled snow, suncups grow slowly to the size of salad bowls, then to washbasins, wading pools and beyond.
Imagine being a bug at the bottom of a small depression in a field of snow on a beautiful sunny day. You are warmed both by direct sunlight and by sunlight diffusely reflected off the sides of the depression. You will be warmer than another bug sitting on an adjacent snow ridge which only experiences direct sunlight. Consequently, the snow at the bottom of the depression will be slightly warmer than the snow at the ridge. This means that evaporation at the bottom of the dip is greater than on the ridge, and this causes the depression to deepen.
Growth of a suncup is slow and the process depends upon both low temperatures and copious sunlight. This is probably the reason that it is seen most commonly at colder altitudes where snow has persisted even as lengthening days give more sunlight. While I look for suncups at the valley bottom, I haven’t really expected to see them.
But, there they were: Salad-bowl sized depressions, albeit spread over a flat snow-covered roof (viewed from a higher floor). Something had accelerated the formation of suncups on this roof, and it didn’t take long to guess the cause: heat loss through the roof from the building below. Snow is a good insulator and the thicker the snow, the better it is. Consequently, the thinner snow at the bottom of a bowl will be warmer than the thicker snow at the edges. This additional warming enabled suncups to form in an unexpected location.
Suncups have formed on the flat roof of a building where heating from the building supplements the sunlight. Incidentally, this picture also contains an illusion. The scene actually shows narrow ridges and broad bowl-like valleys. Sometimes when I look at it, everything seems reversed with narrow valleys separating broad mounds of snow. The trick to seeing it correctly is to realize that sunlight is coming from the left as can be seen with the structures on the roof.