Why do your veins pop out when you lay in the sun?

Dilation: Veins dilate or enlarge with heat and constrict in cold. This is one method of the body's way of regulating heat loss. If you are in the heat, veins will dilate to increase sweating and heat loss. If in the cold, they will constrict to prevent heat loss.

Vein Dilatation: The veins have very thin walls which have capabilities of enlargement or contraction. When we are in situations where the body is heated, like laying the sun or taking a hot shower, the veins in response dilate to allow more blood flow to the surface. Thus allowing the body to radiate body heat to the environment...Pop out.