A crustose species with a white to grey, cracked thallus and a black prothallus that is visible between the areoles. The black apothecia are blue-grey pruinose when young. They often remain partially immersed.
Medulla: C-, K-
In Scotland, Porpidia cinereoatra is widespread on exposed, siliceous rocks, especially in upland areas. BLS distribution map
scottishlichens.co.uk 