Suggested attire to wear for European trip in September

Such questions are usually answered with historical climate data – which became quite useless with recent years climate change. You might have days as cold as 5°c and as hot as 30°c. So bring a multitude of options.

Something like a breathable rain jacket for general wear. These are typically light enough to be worn all the time and if you get something loose fitting you can put a layer or two under them. The ones sold in shops with hiking gear are a good choice and they often have plenty of zipped pockets ! Some of them also come with a detachable inner flannel lining, almost like a second jacket, that attaches to internal zips. Can be very nice in colder weather.

Can be quite windy and wind chill can be a shock if you’re not used to it, so consider gloves. A woolen scarf is a useful extra in a backpack.

Water-resistant foot wear if possible, but many soft shoes now are already "good enough" unless you’re hiking or in seriously stormy weather.

You might want gloves. There can be quite a "bite" in the wind (wind chill) and you might find it a shock to the system given your home.

Avoid umbrellas. They’re a nuisance to other people and won’t stop you getting wet, while also being a nuisance to carry. The wind can also make them an absolute pain to use. More trouble than they’re worth. A good jacket will do much better.

You can get breathable tops which are a good choice for walk around. I’ve generally found jeans fine for walking around Ireland (similar to the UK) all year, unless you’re doing serious hiking. They’ll manage OK with rain, but may get a little wet. Cheap jeans are also things you don’t need to worry about getting dirty or damaged.

Make sure your backpack or over-the-shoulder bag is a good water resistant one and/or make sure you have some plastic bags to wrap the contents in. I typically fold up a couple of plastic carry bags and a couple of large zip-lock bags inside my own bag – very handy.

There is no such thing as "enough" zipped pockets. I speak as a person who liked to do street photography.

Enjoy.

I’ve been amused watching F1 this year and listening to the commentators complaining about "scorching hot temperatures" around Europe. They’re no worse than they were when I was there 3 years ago, and were far more comfortable (especially when coupled with the lower humidity) than we were used to at home (Midwest USA).

Look at the expected temps during your trip and prepare for how you would dress for those temps. Then, prepare for hotter and colder days as well.

The general advice is to plan to wear layers. A t-shirt can be topped with a long-sleeve shirt, then a sweatshirt or sweater added over that. You can add a rain coat to any of those layers to keep the rain off (of course), but to also add a layer of warmth and wind resistance.

While it’s awkward to layer up over a pair of shorts (short trousers), bringing a pair of long pants/trousers is a good idea, just in case the weather turns chilly. There are options for "convertible" pants, where the legs will zip off leaving you with shorts, if you really want all the options.

With planning, one extra "warm" layer will suffice for the entire trip – you can wear the same sweater/sweatshirt several times with something clean underneath it, so it shouldn’t add significantly to the bulk you’re packing for your trip. Also, you’ll be in different cities, so who cares if you wear the same outfit more than once – odds are good nobody will ever know.

Use layers to have more flexibility and expect rain. Even if both places are quite diverse, Northern Europe is on the whole colder than India so you will need warm clothes.

At the same time, you chose a time when you might still encounter nice days (that means close to 20°C, not 30°C) but cannot rule out cold nights or bad weather you wouldn’t expect in the heart of summer. Freezing or snow are very unlikely. That’s why a rain jacket on top of 2-3 layers (say a shirt and a fleece or wool sweater) is better than a thick winter coat.

Even when it is hot for the locals, for you it will feel cold.

Bring what you wear in places with the airconditioning set too cold and expect to layer up over that.

A friend from a similar climate takes thin silk pyamas and wears those under normal street clothes.

Also bring a waterproof jacket or buy one locally as rain with wind is likely in most of your destinations and umbrellas do not work well in that weather.

As for styles, anything you like and is decent is acceptable.
If you think it is warm enough for T-shirt and casual shorts, you are welcome to wear those as a tourist. On the other hand, if you are a shirt, tie and jacket guy, those are also acceptable for a tourist.
Most will be somewhere in between, T-shirt, sweater and jeans or shirt and slacks but without tie and jacket.
Do not feel you need to wear any style or fashion, be yourself in your clothing, all is acceptable.

That depends on the actual places you visit (Mt Blanc vs. Corsica), the time of the day and your own sensitivity. How can one find out how many days per year a city x experiences a temperature of less than y degrees at time z of the day? addresses the first two dependencies.

Wikipedia also has a nice summary for many cities and areas, e.g. :