Alps: a hot summit day, then the break

Chamonix touches 29 degrees today under near-clear skies. Then the weather breaks. Tomorrow the high drops to 21 with almost 30mm of rain, the day after to 16 and still wet. Zermatt runs the same line, from 24 down to 17 in a single day. Heading up high? Today is the window. Pack a proper shell, because the swing lands fast.