Clouds do not screen out all UV radiation. Also, sand, snow and water reflect UV radiation, which means being in the shade under a beach umbrella does not provide complete protection. Wearing a hat with a wide brim is a good idea to protect the eyes, ears and neck. Most clothing, except white, reflects or absorbs the radiation, but wet cotton, no matter what color, can let a large amount through. Sunglasses that block UV rays are vital to protect the eyes from the sun.
Eye doctors can test eyewear for ultraviolet protection.
The U.S. Environmental Protection Agency has a Web Page with links to several documents with a great deal of in-depth information about UV and the UV Index.