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Why does copper develop a green patina outdoors?

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A periodic table element box for copper shows its symbol Cu, atomic number 29, atomic weight 63.546, electron configuration, and name. Icons indicate copper is a solid, weakly basic transition metal with face-centered cubic crystal structure. A key explains the symbols and color coding. — copper Properties of copper.

When copper is exposed to the atmosphere for long periods, it develops a thin green protective coating known as a patina. This layer is a mixture of hydroxocarbonate, hydroxosulfate, and small amounts of other compounds that form as the metal reacts with the air.1

This green coating is sometimes referred to as verdigris, a term derived from the Anglo-French phrase vert de Grece, meaning "green of Greece". While this process occurs naturally on copper and its alloys—such as brass and bronze—when they are exposed to air, the coating can also be produced artificially.2