During summer, when high temperatures arrive, people think they can wear whatever, but wait! You can be fresh and presentable at the same time!
In town the best choice is to wear light and comfortable shirts and trousers (and not undershirts and bermuda shorts!), made of linen or very light cotton to let your skin breathe and permit free movement.
You will discover that long clothes are not warm as you may have thought, and they also protect you from the sunlight!
Linen shirts can solve your problems if you work in a office and you are supposed to wear quite formal, but in this case choose only solid and classic colors. When you need a more casual look instead, dare with stripes and patterns and roll up the sleeves, you will look very relaxed and self-confident!