A freak formation of nitric acid clouds will cover Spanish skies for the first time ever this week resulting in an "extremely beautiful" phenomenon, according to experts at the National Institute of Aerospace Technology (INTA).

A fairly common sight in regions no further than 25km away from the poles, the phenomenon is normally only visible at very low temperatures (below minus 78ºC). The best time to view is on a cloudless day when the clouds receive sunlight from below the horizon and reflect it to the ground, shining brightly well before dawn or after dusk.

Polar stratospheric clouds (PSCs), also known as nacreous clouds, are implicated in the formation of ozone holes because they support chemical reactions that produce active chlorine which catalyses ozone destruction, and also because they remove gaseous nitric acid, perturbing nitrogen and chlorine cycles in a way which increases ozone destruction.