Why are light surfaces colder than dark surfaces?

Light surfaces are colder than dark surfaces due to an object's albedo. Albedo is the amount of incoming radiation (energy) that an object either absorbs or reflects. Light surfaces have a high albedo, thus reflecting more of the incoming radiation. Dark surfaces have a low albedo and will absorb more radiation. Because it is absorbing more radiation, the object heats up due to an increase in sensible heat (i.e. more energy).

This theory explains the urban heat island effect, identifying why urban (city) areas are typically warmer than surrounding areas. The black pavement and rooftops in a city give the city a lower albedo than surrounding vegetated areas and thereby increase the temperature of urban areas by 2-3 degrees Celsius.