Wayther Snow and passes
Plan the hour, not the day
Air cools as it climbs — on average about 6.5 °C for every 1,000 metres, which is the international standard atmosphere figure. So a pass fifteen hundred metres above a town sitting at 8 °C is not a place with a forecast of 8 °C, and planning it from the valley forecast is the single most common way a mountain crossing goes wrong. The other thing nobody expects: the summit is usually warmest in the afternoon and coldest just before dawn, so the best hour to be up there is often the opposite of the one you would pick.
The mechanics
Each one is enough on its own to change the plan. Together they are the reason a route forecast is worth setting up before you leave rather than checking on the way up.
The average rate of cooling with height is about 6.5 °C per 1,000 metres, which is the temperature gradient of the international standard atmosphere. It is an average, not a rule — the real rate changes with humidity and cloud, and saturated air cools more slowly — but the direction is dependable. Every climb you plan should be costed in degrees, not in kilometres.
The freezing level is the height at which the air is at 0 °C: snow above it, rain below. When a front lifts warm air over cold air the two levels meet and the weather changes with height instead of distance, which is why one part of a route can be rained on and another snowed in the same hour.
Not the cold. Around freezing, precipitation can arrive as liquid and freeze the moment it lands, and the road goes before the sky does. It is the case to treat with the most caution and the one least likely to be visible from inside a vehicle — which is why it is worth reading off a forecast rather than judging from the window.
Falling snow does not make a road dangerous; a frozen road under snow does. A road that is being cleared, that is shaded all morning, that takes sun on a south-facing bend or sits under an overpass can all be worse than the one next to it. None of that is in a forecast, which is the honest limit of this page and of any weather app.
The part you can act on
Mountains have a wide daily temperature range. The summit is usually at its coldest just before dawn and its warmest in the afternoon, because solar heating is the dominant signal at height. That makes “leave early to beat the snow” — the reflex with good reason everywhere else — the wrong instinct on a pass, where the coldest hours are the ones you would be sitting at the summit in.
The trade is real and worth naming: the afternoon is also when convective snow is most likely in many climates, and it is when the road is busiest. Which is exactly why the answer is a comparison rather than a rule. Put two candidate hours into the forecast and look at the temperature at the point you will be on the summit in each. The one furthest above freezing is the better crossing.
And one thing is worth taking regardless of the numbers: cross in daylight. Ice is visible in daylight and not at dusk, and if anything does go wrong you want to be somewhere that can see it.
The same thing, with a deadline
The booking says fourteen hundred and you are not going to be late for it. So the timing is fixed again — but here that is the normal case rather than the exception, because passes are crossed on the way to something else.
This is the single most useful sentence on the page, and getting it wrong is how people end up fitting chains three hundred metres below a summit that never needed them. Read the forecast at the point on the map that is the pass, at the hour you reach it. The valley figure is a different place, a long way up from here.
Whether a summit is below freezing depends on two things: how high it is, and where the freezing level is at that hour. The second is the one that moves — it rises through the morning, which is why the same pass is a different proposition at nine than at four in the afternoon, and why the level is the number to follow rather than the summit temperature alone.
This looks backwards if you assume the mountain gets colder as the day gets older. It does not, in the way you expect: the freezing level follows the sun, so a pass in the morning is often at its mildest of the day just as the air is coldest. Reading a pass as a nightly descent into colder and colder air is the mistake, and it puts the worst of both on the same journey.
There is usually more than one way over, and they are not equivalent. A higher road in better weather is frequently the right answer, and it is the one nobody considers because it is the longer number on the map. Compare them on time and distance together, the way the app compares routes, and the decision is much easier than it looks.
A summit being above freezing says nothing at all about your campsite at eleven that night, and the temperature you arrive at is not the one you spend the night at. This is the step that turns a good day's driving into a bad night, and it is on the arrival rather than the departure, where nobody looks.
Snow and passes FAQ
Air temperature falls with height, and the average rate through the lower atmosphere is about 6.5 °C for every 1,000 metres of climb. That is the international standard atmosphere figure, agreed by the aviation and meteorological bodies, and it is an average rather than a rule: the real rate changes with humidity and cloud. A 1,500-metre climb is therefore typically something like ten degrees colder than the valley floor, which is why a forecast for the town is the wrong number to plan the summit with.
Later than most people expect. Mountains have a large daily temperature range and the summit is usually coldest just before dawn and warmest in the afternoon, because solar heating is the dominant signal up there. The trade is that afternoon is also when convective snow is most likely in many climates and the road is busier, so the answer is not a rule but a comparison: put the two candidate hours in the forecast and pick the one where the summit is furthest above freezing. Daylight is a genuine benefit regardless, because ice is visible in daylight and not at dusk.
No, and this is the honest limit of the page and the app. Wayther forecasts air temperature and precipitation; it does not measure road surface temperature, and it does not know the road material, the shading or whether a curve has been salted. What it gives you is the two things you can reason from: the temperature at the hour you will be at that point, and the precipitation falling then. Where those sit close to freezing, that is exactly the situation to treat with caution rather than calculate your way out of.
Because of the freezing level, which is the height at which the air is at 0 °C. Above it precipitation falls as snow, below it as rain, and where a front lifts warm air over cold air those two levels meet and the weather changes with height rather than with distance. It is the ordinary explanation for a route that is being rained on at one point and snowing at another, and for two roads a few kilometres apart behaving differently in the same hour.
Temperature and precipitation at every point along the route, timed to your arrival, so the climb is a separate number from the valley and the pass is not the weather of the town at its foot.
Forecasts come from Apple Weather and cover wherever Apple Weather is available, in metric or imperial units. Wayther plans a trip; it does not navigate, it does not measure road conditions and it does not report closures. Keep your maps app for directions, and follow official weather warnings and road closures.
