summer and clothing layers

I notice that a lot of Japanese women oftentimes wear 2 layers of clothing (or a shirt/tank plus a sweater) during the hot, humid summer weather. It baffles me! After thinking about it for awhile, I am guessing it must be because they will feel the same degree of hotness whether they wear one layer or two, and so they just dress the way they do to look nice/fashionable. Is that so?

Not really sweaters, but a light cardigan or something? I believe that has more to do with outside being hot and indoors (inside buildings and trains) being quite chilly with air conditioning. And also because of wishing to avoid sunburns on the arms.

I understand what the OP is trying to say and I agree. It would look much more "trendy" to wear layers than to wear one top as you would at a beach. Plus, modern fabric is so well-made that it won't make you any warmer just because you wore an extra Uniclo or Asian-Indian scarf or something.

Of course, I do agree with AK, but most women would keep the extra layer on even when walking under the heat. And then they might need another layer for air-conditioned areas!

maybe, to protect the skin from sunburn.

As a woman who has lived in Japan for a long time, I feel it is both of AK's reasons: one, to protect the skin from the sun, and two, because the air conditioning in some places is too cool and you need that extra layer indoors!

Maybe some women also do it for fashion as well, but I'm not that good at fashion, so its all practical for me!

Fashion I think
But there are many stores, train stations and restaurants with nice air conditioning so as long as your not out long its fine.
Even men in japan wear a suit jacket with dress shirt and shirt underneath. I tried that once and so hot. Thats why they carry handkerchiefs/hand towels. Thats why theres vending machines on every street!

I wear it purely to protect my arm from sunburns, though it works for fashion purposes as well.