Corn snow - sometimes called spring corn - is the granular spring snow surface that forms when repeated daily melt-freeze cycles round the snow crystals into loose, ball-bearing-like grains roughly the size of corn kernels. The cycle is straightforward: the surface melts during the warm afternoon, the meltwater percolates a few centimetres into the snowpack, and the cold night refreezes the surface into a hard crust. The next morning's sun softens the top centimetre back into a forgiving carpet of loose grains, and the window between rock-hard and slush is the prime ski-touring window of the day.
Corn-snow timing is the central skill of Norwegian spring topptur. The classic Lyngen or Sunnmøre day starts well before sunrise so that the climb finishes in the firm pre-dawn snow, the summit is reached as the sun first hits the slope, and the descent catches the precise hour-long window between hard crust and soggy afternoon mush. A skilled guide reads the aspect, the elevation, the cloud cover and the previous day's conditions to predict that window within fifteen minutes.
What corn snow gives the skier is one of the most forgiving descent surfaces in the sport. The loose grains roll under the ski, the edges bite cleanly, and the turn radius shapes itself almost without conscious input. A 1,200-meter descent down a perfect corn-snow slope on a clear April morning in the Lyngen Alps is the standard reference for why the trip is worth the journey.