It gets colder as you climb
Air cools by roughly 6.5 °C for every 1,000 m of height on average. A 20 °C morning in a valley at 700 m can mean about 7 °C at a 2,700 m summit such as the Stelvio — before wind chill at riding speed. Pack for the summit, not the start.
Snow can linger near high summits well into summer, and meltwater running across the road in the afternoon sun can refreeze on a cold night.
Afternoon storms
On warm summer days in the Alps and Dolomites, cloud often builds over the peaks from late morning and turns into thunderstorms in the afternoon. The usual advice holds: cross the highest passes early, and be down in the valley by mid-afternoon when storms are forecast.
A storm on a pass brings everything at once — heavy rain, hail, sudden gusts, lightning on an exposed road and a road that goes from dry to streaming in minutes.
Wind and cloud at the top
Summits and cols are exposed, so the wind there is often much stronger than below. Cloud sitting on the pass cuts visibility to a few tens of metres while the valley is clear. Both change quickly.
Planning a pass day with RideWindow
Every mountain pass page is scored at the summit, the coldest and most exposed point of the ride, with the week's ride windows and a day-by-day table. For a route over several passes, score the whole route: each stretch is scored for the hour you reach it, and one No-Go stretch makes the whole ride No-Go, because you cannot average your way over a pass in a storm.
The score is weather only. Many high passes close in winter or overnight, and some have tolls or time limits, so check the road's own status before you set off.