This summer here in the Chicago area we've had quite a few weeks of cooler weather.
I was by the lake one time and this couple walked by in sweatshirts. One of their dogs sniffed me and I petted it and since it was warm I asked if they were?
He didn't say anything but she said, "I'm a bit warm." I asked if she had anything underneath that sweatshirt and she didn't.
I can never get why anyone do that. After all it's summer and dispite being on the chilly side it's supposed to be warm and if I go out, I put on a short sleeve shirt anyhow and wear a jacket.
Most importantly, listen to the weather.