Well it could be a couple of things. It could either be a superficial fungal infection called pityriasis versicolor or it could be an allergic reaction to the sun called 'Polymorphic light eruption'. You need to be examined by a dermatologist for the same and treated accordingly.

White patches can be because of multiple reasons. The history and examination plays an important role as there are numerous disorders ranging from autoimmune to allergic causes that fall under white or light skin colored patches. If the skin is dry only sundress would not help, an emollient is to be applied and minimal soap use is recommended.