You should pose this question in the Road cycling forum. First do a Search, this topic has been covered before.

Some people will wear long sleeve baselayers in the summer to keep the sun off their arms. The fit is snug so it'll wick moisture but the fabric is lighter than regular jerseys. Not sure how effective they are in blocking UV rays.

I have one long sleeve jersey that is the same fabric as my short sleeved ones. I don't wear it anymore as it didn't block the wind off my arms(I wear arm warmers instead).