What is causing white patches to appear on my skin? (Photo)

Every time I go out into the sun a few days later my arms and chest are developing white blotches. My skin isn't peeling at all I just wake up with them there and the only way to get rid of them is through going into the sun and tanning then again where the process happens agin. Avoiding the sun altogether is hard as I live in a hot climate. What can be causing this? And does anyone know of any treatments that can stop this from happening?