Clouds are cirrus or altocumulus lenticularis, and show very strong irisation similar to that of mother-of-pearl, especially when the sun is several degrees below the horizon. They occur at heights about 20–30 km above the earth. These clouds are rarely seen, and it would appear that they can be observed only in certain regions. They have been observed mainly over Antarctica due to the colder temperatures present there in the circumpolar vortex. They also form over Scotland and Scandinavia in winter during periods with an intense, broad, deep, and homogeneous westerly to northwesterly flow of air over northern Europe; they are also observed in Alaska. Less frequent sightings have been reported at lower latitudes. The simultaneous occurrence of various colors of the spectrum in more or less irregular patterns strongly suggests diffraction by spherical particles. The exact physical constitution of cloud particles has been determined by aircraft (e.g., the NASA ER2 aircraft) penetration showing the presence of nitric acid hydrates (in particular nitric acid tri-hydrate, type I) with the addition of water ice at temperatures a few degrees colder (type II). Nuclei for clouds are thought to be sulfuric acid aerosol, possibly of volcanic origin. The clouds form in regions where dynamic lifting or radiational cooling lowers the air to temperatures below saturation for these different constituents (about −95°C). PSC are thought to play a major role in the formation of the “ozone hole” because they absorb odd nitrogen from the atmosphere, which allows the catalytic destruction of ozone to occur. Nacreous clouds appear stationary and, by day, often resemble pale cirrus. At sunset, all the colors of the spectrum appear; as the sky darkens after sunset, they increase in brilliance. As the sun drops lower and lower below the horizon and the clouds are lighted by last rays, the various colors are replaced by a general coloration that is first orange and then becomes pink, contrasting vividly with the darkening sky. The clouds next become gray and the colors of the spectrum reappear but very weakly, then fade out rapidly. Later, up until about two hours after sunset, the nacreous clouds can still be distinguished standing out against the starry sky as tenuous and gray clouds. They can even be observed all night if there is moonlight. Before dawn, the same series of aspects appear, but in reverse order.