Why do I get bumps on my hand when it is sunny?

Sunny & Bumpy. You may be referring to photodermatitis that can be precipitated by oral medications. If you take an oral medication check with your doctor as it could be sun sensitizing. The rash or bumps can occur anywhere the sun shines on your skin; out of the sun it usually resolves. If you wash with certain soaps or use certain lotions on your hands this could explain why only the hands are involved.