The 100-day rule is the snow-reliability benchmark used across the ski industry: a resort is considered economically snow-reliable if, in seven winters out of ten, it has at least a 30 cm snow depth for a minimum of 100 days between roughly 1 December and 15 April. Introduced in Swiss research and popularised by economist Rolf Bürki and colleagues, it is the line most resorts quietly manage their whole operation against.
It sounds like an abstraction. It is not. The 100-day threshold is the reason snowmaking exists at the scale it does, the reason elevation decides a resort's future, and the reason the marginal wet-bulb window is worth fighting for. This guide explains the rule, the elevation line beneath it, and what a warming climate does to both.
Key takeaways
- The 100-day rule: snow-reliable = ≥30 cm snow depth on ≥100 days, in ≥7 of 10 winters (≈1 Dec–15 Apr).
- It maps onto an elevation line — historically around 1,200 m in the Swiss Alps, rising roughly 150 m for every +1 °C of warming (OECD/Abegg).
- Warming pushes that line upward, moving lower resorts below reliability: at +2 °C, naturally reliable Alpine areas fall from ~500 to ~404; at +4 °C, to ~202.
- Snowmaking is the primary adaptation that keeps otherwise-marginal resorts above 100 days — François et al. 2023 show 50% snowmaking coverage roughly halves the share of high-risk resorts.
- The rule reframes the business: every marginal day defended — by elevation, snowmaking, or a wider wet-bulb window — is a day of snow-reliability protected.
What is the 100-day rule?
It is the industry's working definition of snow reliability: a ski area is reliable if it can offer at least 30 cm of snow for at least 100 days per season in seven of every ten winters. The 100 days is the economic threshold — roughly the operating length below which a resort struggles to cover fixed costs and turn a profit across a normal run of seasons.
The rule packs three separate ideas into one line, and each matters:
- Depth (≥30 cm): enough snow to open and groom terrain, not a dusting.
- Duration (≥100 days): a season long enough to be economically viable, not just skiable.
- Frequency (7 of 10 winters): reliability across a decade, so a single good year does not disguise a marginal location.
That frequency clause is what makes the rule demanding. A resort that hits 100 days in four winters out of ten is not "reliable with occasional bad years" — by this definition it is simply unreliable. The rule was designed precisely to strip out the wishful accounting of remembering the good seasons and forgetting the bad ones. The 100-day rule origins are usually traced to Swiss climate-tourism research and the OECD's 2007 Alpine assessment.
What elevation do you need to meet it?
Historically about 1,200 m in the Swiss Alps — but that number is a moving target, rising with temperature. The OECD's Abegg analysis put the natural snow-reliability line near 1,200 m and estimated it climbs roughly 150 m for every 1 °C of warming. So the elevation that guaranteed 100 days a generation ago no longer does, and the line keeps migrating uphill.
That single relationship — reliability line ≈ 1,200 m, +150 m/°C — is the engine behind most resort-risk headlines. It converts an abstract temperature change into a concrete map of who stays above the threshold and who slips below it:
| Warming | Naturally snow-reliable Alpine areas (of ~666) | Share | |---|---|---| | Present / +1 °C | ~500 | ~75% | | +2 °C | ~404 | ~61% | | +4 °C | ~202 | ~30% |
Source: OECD / Abegg 2007. These are warming thresholds, not calendar years, and they describe natural snow reliability — before snowmaking is taken into account. The elevation mechanics are explored further in will ski resorts survive climate change.
How does climate change move the line?
Upward, and faster at the bottom than the top. Because the reliability line rises ~150 m per °C, warming does not shave days evenly off every resort — it pushes a whole band of lower-elevation areas across the threshold from reliable to marginal, while high resorts are barely touched. The losses concentrate where terrain is low and warm.
The peer-reviewed data is stark on this. Spandre et al. 2019 (Scientific Reports) found that +0.9–2.3 °C produces roughly a 44% snow-season loss at 1,500 m versus about 11% at 3,000 m — a fourfold difference driven purely by elevation. François et al. 2023 (Nature Climate Change), covering 2,234 European resorts across 28 countries, put 53% at very high risk of snow scarcity at +2 °C and 98% at +4 °C — without snowmaking.
Two cautions keep this honest. First, "very high risk of snow scarcity" is not the same as "will close" — resorts adapt, and the study models risk, not closure. Second, the IPCC's assessment of low-elevation snow decline is a range of 10–40% (IPCC AR6 WGII CCP5), not a single figure. The direction is unambiguous; the precise magnitude is a band.
Where does snowmaking come in?
It is the main lever that keeps a marginal resort above the 100-day line — and the numbers show how much it moves. The François analysis is the clearest: with 50% snowmaking coverage, the share of European resorts at very high scarcity risk falls from 53% to 27% at +2 °C, and from 98% to 71% at +4 °C. Snowmaking does not repeal the elevation line; it lets a resort defend the days the natural climate no longer guarantees.
This is why the rule reframes the whole operation. If reliability is 100 days in 7 of 10 winters, then the winters that decide a resort's fate are the marginal ones — the near-misses where a few more open days would have crossed the threshold. In those winters the binding constraint is often the warm edge of the season, the marginal wet-bulb window analysed in the wet-bulb temperature operator's guide and in how snowmaking extends the ski season.
That window is where additive chemistry contributes. A wider usable wet-bulb window means snow can be made in hours that would otherwise be lost — the modelled +3 °C wet-bulb advantage and 300–500 extra snowmaking hours per season attributed to SL6733 are, in 100-day-rule terms, a mechanism for converting marginal winters into reliable ones. Those figures are modelled and pre-commercial, but the logic is direct: more makeable hours at the margin is exactly what the frequency clause of the rule rewards. The value of each of those recovered days is quantified in what one extra open day is worth.
Why does the 100-day rule matter for resort economics?
Because it is where the weather turns into the P&L. Falling below 100 days in too many winters means fixed costs — lifts, staff, debt service — are spread over too few operating days, and the early-season shoulder is where the highest-margin revenue sits. Missing a major early opening can cost on the order of 20% of annual revenue, the theme of what missing Christmas costs a resort.
For a mountain operations director, the rule is therefore not a climatology curiosity but the target function:
- Establish the resort's real reliability record against the 100-day/7-of-10 test — not the memory of good years.
- Identify the marginal winters and the specific days that were lost.
- Cost the levers that would have recovered them — elevation is fixed, but snowmaking coverage, timing, and the width of the wet-bulb window are not.
- Weigh each lever against the revenue of the days it protects, using per-day value rather than snow volume.
Framed that way, snow reliability stops being something the weather grants or withholds and becomes something an operator manages — imperfectly, at the margin, but deliberately.
The bottom line
The 100-day rule — 30 cm, 100 days, 7 winters in 10 — is the quiet benchmark the whole industry runs against, and it sits on an elevation line that warming is pushing steadily uphill. Snowmaking is the primary way marginal resorts stay above it, and the marginal wet-bulb window is where that fight is won or lost. Every day recovered at the warm edge of the season is a day of reliability defended, which is why widening the makeable window — by coverage, timing, or chemistry — maps so directly onto the rule that defines a resort's survival.
If you want to model how many marginal days a wider wet-bulb window would defend against your own 100-day record, request a pilot or talk to us.
Climate figures are cited from peer-reviewed and OECD/IPCC sources; warming values are thresholds, not calendar years, and "risk of scarcity" is not a prediction of closure. SL6733 operator outcomes (+3 °C wet-bulb, 300–500 extra hours) are modelled and pre-commercial — EU lab pilots are targeted for 2026/27, commercial deployment for 2027/28. DeepSnow is the platform brand of SnowLabs Limited (Ireland); DeepSnow Srl (Italy) is in formation.