september weather in Berner Oberland

September is usually the best hiking month in the Alps. The weather is usually dry (although don't count on absolutely no rain) and days are warm but not hot. You probably won't need it during the day, but consider a light-weight fleece if things get on the cooler side at night. For jeans vs. shorts, I say, split the difference and bring a pair of outdoor pants, something like this. If you're doing any kind of half-way strenuous hiking, you'll be warm enough once you start to move uphill.

"I will be coming from Paris and will assume that I will be wearing shorts there"

I don't know what you consider "shorts weather," but I do not think of early September in Paris as being shorts weather. Do check the forecast before you leave, so that you can adjust your packing accordingly.

Thank you Tom!!!
@ Harold.. I am from upstate NY...I do not know how long you have lived here but we wear shorts into October bro!!! If you are a true New Yorker you would know that bro!!

steve,

I've always found the weather to be reasonably good in September, although one must plan for rain at any time of the year. The days are generally warm but the evenings can be on the "cool" side. The cool evenings become more pronounced in October.

I don't wear shorts when travelling, so can't comment on whether they would be appropriate for that time of the year. If the weather is good, shorts should be fine in the daytime but trousers would be more appropriate in the evenings in both the Berner Oberland and Lucerne.

Thanks Ken! I am mid forties and can't stand the heat. So I wear shorts even in the middle of an upstate New York winter (except for the office and church of course). I just want to be comfortable :)

Are you sure about wearing shorts in Paris?

I think jeans will be too warm for you in both countries at that time and not worth the weight to carry. If you're a hiker, you already know the importance of being prepared with adjustable layers. One year, I was preparing to pack the same clothes for Spain in October as I had worn in Bulgaria the previous October. Good think I checked the weather forecast and realized that wouldn't work.

@George,

What is so UNBELIEVABLE about my reply? As it may be considerably cooler in the evenings, that would seem to indicate that trousers would be more appropriate. The OP is certainly welcome to wear whatever he likes.

We will be in Paris, Luzern, and the Berner Oberland in September too. My husband hates the heat and cares nothing for fashion or what anyone else wears. Unless it is in the 40s, I can guarantee you will not be the only guy on shorts. : )