Föhn spells and the false spring
Air that has dropped its moisture on the windward slope warms as it descends. The effect is large enough to be mistaken for the season changing.
The classic sign is not the temperature itself but the speed of the change. An ordinary warm front arrives over many hours, preceded by a lowering deck of cloud. A föhn spell can lift the valley by twelve to fifteen degrees between mid-morning and mid-afternoon, under a sky that clears from the south-west and stays uncomfortably bright.
Dew point is the tell. During a föhn it falls while the temperature rises, and the spread between the two widens past twenty degrees. Under a genuine warm front both rise together.
What the record shows
Counting only episodes where the spread exceeded eighteen degrees for at least four consecutive hours, the ten seasons in this record give the following distribution.
| Season | Episodes | Longest run |
|---|---|---|
| Spring | 31 | 3 days |
| Summer | 6 | 1 day |
| Autumn | 27 | 4 days |
| Winter | 11 | 2 days |
The clustering in spring and autumn follows the pressure gradient across the range, which is steepest when the continental interior and the Atlantic are furthest apart in temperature.
Effects worth recording
Snowmelt accelerates sharply, and small watercourses that are ordinarily clear run turbid within a day. Ground-nesting birds begin calling several weeks before they would otherwise, then fall silent when the spell breaks. Both are worth noting with dates: the gap between the first call and the first confirmed nest is the most useful number in the whole record.