Ask an experienced skier what makes a great day and they’ll talk about snow quality before snow quantity. Twenty fresh centimetres that fell wet ski like mashed potato; five that fell cold ski like silk. The variable behind almost all of it is one number: the freezing level — the altitude where the air crosses 0 °C — and the story of a ski day is largely the story of where that line sits and how it moves.
Alpy draws this as the freeze curve: freezing-level altitude, hour by hour, across three days, with your resort’s base and summit marked and a rule at “now.” Here’s how to read it.
Freeze line below the base. The whole mountain is in sub-freezing air. New snow falls dry everywhere; existing snow stays wintry. On a storm day this is the jackpot reading — flakes at the valley floor.
Freeze line between base and summit — the interesting and most common case. The mountain is split: above the line, winter; below it, spring. Rain at the bottom can be snow up top (the saddest lift queue in skiing is the one standing in drizzle below a snowline you can see). This reading decides where to ski: upper-mountain laps hold quality while lower runs go heavy.
Freeze line above the summit. The whole hill is thawing. Surfaces soften progressively through the day — which is not automatically bad news, and that’s where timing comes in.
In settled spring weather the freeze curve breathes daily: up through the afternoon, down overnight. The snowpack follows with a lag, melting by day and refreezing by night, and that cycle manufactures both the best and worst pistes of the season.
The sequence on a clear spring day: dawn surfaces are boilerplate — yesterday’s melt frozen into chattery armor. As the sun and the rising freeze line work on it, the top centimetre loosens into corn: hero snow, edge-friendly and forgiving. Stay past midday and corn collapses into slush that grabs your skis and ruins your knees’ afternoon.
The freeze curve is how you time it. Overnight dip well below the summit → a proper refreeze → expect boilerplate early, corn mid-morning, slush after lunch; aspect shifts the schedule (east faces ripen first, west last). No overnight dip at all → no refreeze → the snowpack never reset, and the day starts where yesterday ended: heavy. One glance at whether last night’s curve dipped under your resort’s base tells you which movie you’re in.
During a storm, the freeze curve is the rain–snow line, give or take a couple hundred metres (falling snow survives a little below the 0 °C altitude before melting). A storm that arrives warm and cools — curve stepping down through the hours — lays wet snow first and dry snow on top: right-side-up powder, the good kind. A storm that warms as it passes builds the inverse: dense over light, upside-down snow that skis strangely and, on steep terrain, is exactly the loading pattern avalanche bulletins get animated about. The curve’s direction during the storm hours tells you which you’re getting tomorrow.
Alpy’s curve is built from Open-Meteo forecast data — model output, refreshed through the day, for your resort’s exact coordinates. Model freezing levels are good and not gospel: local effects (cold air pooling in valleys, föhn winds) can shift reality a hundred metres or more from the model, and the app does not pretend otherwise — the curve is labeled with its source, and it’s a planning instrument, not a promise. The summit temperature shown beside it is the same model sampled at your summit’s elevation, which is why it routinely reads 8–10 °C colder than the village forecast your phone’s weather app shows. (Standard lapse rate is roughly 6.5 °C per 1,000 m — over Zermatt’s 2,263 m of lift-served vertical, that’s a different climate, not a different forecast.)
The night before: check whether the curve dips below base overnight (refreeze?) and where it sits at 09:00 (which half of the mountain gets winter?). Storm inbound: watch the curve’s trend through the storm hours (right-side-up or upside-down?). Spring day: plan runs down the mountain as the day warms, following the corn window from high east faces to low west ones, and be on the deck before the slush wins.
One line on a chart, and most of a ski day’s decisions fall out of it. That’s the best kind of instrument.
The freeze curve is free in Alpy for every resort, next to summit weather and official avalanche bulletins. The app · More from the blog.