I can't stand to have a gun next to bare skin, so I always wear a T-shirt. You can do IWB in the summer, it just takes planning. One way is to wear a T-shirt and a larger buttoned shirt that is untucked. This is very common today, especially in churches. (Not for me, mind you, I still wear a suit). This allows you to carry a pretty large gun, and spare mag, easily.

I have another method that is VERY comfortable, and surprisingly fast. If you read gun magazines you see the ad for the Kramer Confidant shirt holster. This is a mesh shirt with a pouch under each arm. You can carry a handgun on one side and a spare mag on the other. I can carry an S&W J-Frame revolver or a Glock 23 or 27. Have done so all day many times and I literally forget it's there. I leave the top two buttons unbuttoned on my shirt and reach in and draw straight up. If I wear a suit I leave a button unbuttoned under my tie. You can even wear a sweatshirt and leave a couple of the buttons on your midsection unbuttoned. This costs about $60, and it is well worth it.

Concealed carry can be done in any weather. You just have to experiment and dress accordingly.
Jim