On the other hand, the sun plays no role whatsoever in the development of varicose veins, which mainly have heredity, pregnancy and hormonal changes as top causes. However, varicose veins may become worse in the summer due to the heat. When our body temperature rises, blood is directed to the skin veins to help radiate heat from our bodies. This extra flow of blood stretches veins; and, those that are diseased will likely get bigger. In hot weather (or in a sauna, or during exercise), the body tries to cool down by dilating (enlarging) the veins under the skin. In people that have venous reflux (also called venous insufficiency), blood pools abnormally in the leg veins. So the combination of the body being hot and venous reflux causes the veins to really bulge.