Sorry for the what late reply ( only saw your message now ) , but september is indeed my favorite month , and more specific , the French Alps in sept . Mountain weather is unpredictable , but 8 times out of 10 , I had exellent weather. Lots of climate info on the web , like here :http://meteo05.sepcs.fr/climat/hautes-alpes.pdf
September days are of course shorter and nights can be cool , but the light and the colours are so much nicer .
When the sky is clear , it is cristal clear , without that typical summer haze .
Traffic (France ) , away from the major routes is almost non existent .
With a little luck , you can do a less frequented col , say, Col de la Cayolle , from bottom to top without seeing ANY other vehicle .
Sometimes , you really have the road for yourself .
Compared to the French Alps in september , the Dolomites are like Paris at rush hour .
Typical for september is morning fog in the valleys , and if you camp , pay attention where you put your tent , to catch the rising sun .
Also : the combination of frost at night and the morning sun causes rockfall . Mostly small stuff , but still someting to watch out for , and unlike the Swiss , the French are not in a real hurry to clear the roads ..
In the afternoon , the low sun can be a nuisance too .
Boy , I wish it was september already ..