One way to make an objective determination of skin fairness is to use the Fitzpatrick scale. The Fitzpatrick scale is a classification of skin tone ranging from I, very fair, to VI, black. Most people who identify as white fall into skin types I, II or III and possibly IV. The purpose of the scale is to determine a person's vulnerability to sunlight and chances of getting skin cancer.
While there is a strong association between white skin and European nations, it is impossible to determine whiteness based on heritage because the movement of people means that some European populations have darker skin while some African and Asian people have fair skin. White, as a racial descriptor, most often applies to people of European heritage. However, some people living in or with ancestry in parts of Africa and Asia also have light skin.
Barring the migration of people between geographic regions, skin pigmentation is a function of proximity. Melanin, the brown pigment of skin, acts as a natural sunscreen. For people living in tropical regions, high levels of melanin protect the skin from excess ultraviolet radiation that robs the body of nutrients, such as folic acid. However, moderate amounts of sunlight help the body produce vitamin D. People living farther from the tropics, where there is less direct sunlight, benefit from lighter skin. There is less harmful sun exposure, and lower levels of melanin allow for better vitamin D production.