If anyone tells you what the weather in Paris will be at any particular time, don't believe them. They don't know, I don't know, you won't know until you get there. Paris is so changeable and unpredictable nobody really knows. The morning weather report for that afternoon in Paris will be wrong as often as right. Many times the morning weather report for that morning is wrong. They say sunny, you look out the window and it's raining. One trip it was sunny and mild, cloudy and windy, cold, rain, snow, hail, then sunny and mild again......and all on the same day. People have been cold in July, warm in October. So bring most of your clothing for the season, but throw in a couple of other pieces for the other stuff. If close to being between seasons, bring a bunch of both. Always have layers to put on or take off.........and always, always, umbrellas.
Now comes the big argument - should you dress fashionably to try not to stick out as a tourist or do you dress in what has become known as "typical tourist" - jeans and sneakers? For during the day, the thing to do is not sweat it - it's really personal preference. We happen to feel better dressing nicely all the time but there will be plenty of people, Americans as well as French, wearing blue jeans, sneakers, and even (ugh) the dreaded sweat suit. Well, sweat suits are pretty much reserved for the American tourist. That's for during the day. At night things change. It's my humble opinion that if you're going for dinner in a nice restaurant (not expensive, not fancy, just nice) you should dress appropriately. It just shows a little respect for the restaurant, the other patrons, and makes you feel like it's an occasion. Meals in France are always an occasion, one of the things that make the place so special. Food is never thrown at you, silverware is changed, the glasses sparkle, the waiters are professional. So I suggest you dress accordingly.
This from Bobbi: For women, Bobbi says for during the day in the colder months black slacks, black sweater, winter coat, scarves - all sizes. For evening, similar but nice - black slacks or skirt and black stockings. Pantyhose fine. Cotton or wool turtlenecks good. With vest or scarf it changes your outfit. Brown is fine - no bright colors except in scarves. She says no sneakers (but like I say we like to dress nicely). Twin sweater set perfect. Red sweater is fine too. She always wears sweater sets. For warmer weather she says to wear silk or cotton. Slacks and a blazer. Take a shawl. Have something warm in case. Cotton tank dress for daytime. No loud colors except in scarf. Plain pants and gap polo shirt with a little sweater over it if necessary. Remember it can get pretty cool also so be prepared.
For men during the day, not being a fan of blue jeans, I would suggest black jeans if you're going to wear jeans, chinos in the warmer weather or regular slacks in colder weather. Then layer with a shirt, sweater, turtleneck shirt or sweater, leather jacket in the colder weather. A scarf is good to have when it's cold. In the evening, you do not have to wear jackets except in the top places, the Michelin starred places. Otherwise slacks and shirt are fine, sweater if it's cool. You can wear a sport jacket but it's not required.
Remember a few things.
You're allowed to wear the same outfit over and over - nobody will be judging you.
Two pairs of slacks and two shirts or blouses gives you four outfits.
The weather is extremely changeable so you always have to be prepared.
If you bring it and don't need it, fine. But if you need it and don't have it, you'll be very unhappy.