A rough scale
- Under 1 cm: a dusting. Pretty, no difference to the skiing.
- 1 to 5 cm: a top-up. Groomed pistes ski a little softer in the morning; hard-packed slopes stay hard.
- 5 to 15 cm: a good refresh. Off-piste gets a soft surface, pistes are pleasant all day, and tracks fill in.
- 15 to 30 cm: a proper snow day. Off-piste is worth getting up early for; expect queues for the first lifts and avalanche control that may delay openings.
- 30 cm or more in a day: a powder day, the kind people plan trips around. Also the kind of day when high lifts stay shut for wind or avalanche work, and roads to the resort close.
The site’s rankings use similar thresholds: light, moderate and heavy on the map correspond to under 5 cm, 5 to 15 cm and over 15 cm in 24 hours.
Snow is not water
Ten centimetres of snow is not ten centimetres of rain. Fresh snow is mostly air. Cold, dry snow can be 20 times the volume of the water in it; wet, mild snow only 5 or 6 times. Forecast models predict the water and convert it to snow depth with an assumed ratio, and that assumption is one reason totals vary between models and between forecasts and reality. Ten centimetres of Utah powder and ten centimetres of Sierra cement are very different things to ski.
It settles
Fresh snow compacts within hours under its own weight, faster if it is mild or windy. A 30 cm overnight fall can measure 20 cm by mid-morning and 15 cm by the next day, before anyone has skied it. Resort snow reports usually state the depth measured at a fixed time; forecast totals are what falls, not what is left.
Where it lands
Wind moves snow. On a stormy day the windward slopes can be scoured to the old surface while lee slopes and gullies collect two or three times the forecast total. The forecast figure is for open, flat ground at the height of the forecast point.