I'm invited to an open air concert tomorrow evening, weather says it should be 18 degrees C with clouds and possibly slight rain. Would a T-shirt be fine? Im assuming there will be enough of a crowd t

I'm invited to an open air concert tomorrow evening, weather says it should be 18 degrees C with clouds and possibly slight rain. Would a T-shirt be fine? Im assuming there will be enough of a crowd to warm it up quite a bit (and i probably wont be sitting around...)