Workshop: twelve typical days

The essentials in two minutes

Most mountain days look like one of twelve models. Three are stable: settled anticyclonic fine weather (blue sky, low hard ceiling, haze), the low inversion (sea of clouds in the valley, sunshine above) and the pure breeze day (blue sky, good thermals, strong valley wind in the afternoon). One is the ideal: the classic thermal day, with flat, well-behaved cumulus. Four are unstable: overdevelopment (the sky closes in during the afternoon), the air-mass thunderstorm (cumulonimbus around 15:00), the slack pressure gradient (heat haze, late isolated thunderstorms) and the cut-off low (early thunderstorms, anywhere). Four are driven by the wind or by fronts: the warm front (veil, then rain), the cold front and its post-frontal air (showers, north-westerly wind, cold), the foehn (lenticulars, rotors) and the bise (a cold, dry north-easterly: crystal-clear blue sky, strong wind).

To recognise the day, you only need to look at three things: the chart (where the anticyclones, lows and fronts are, and therefore the wind), the sounding (stable or unstable, dry or humid, where the lids are) and how fast the sky changes in the morning. The verdict follows from two questions: “Does the wind let you fly, or walk along the ridges?” and “At what time does the instability become dangerous?” Figure 11.14 places the twelve days on these two axes; the final table sums up each one in a single line.