Moonlit nacreous (or polar stratospheric) clouds backlighting Vince's cross, at Hut Point, McMurdo Station, Ross Island, Antarctica

Nacreous clouds are rare clouds formed under the right conditions in the stratosphere. The name is derived from the Greek word Neikre, meaning mother of pearl, in reference to its incredible iridescence when lit at low angles. There are two variants: The first are often comprised of ice and nitric acid crystals at minus 108 degree F, and are often implicated in destruction of the ozone hole. The second is comprised on principally ice crystals, and not thought harmful. The cross at Hut Point is a memorial for George T. Vince, a seaman with 1902 Discovery Expedition. He was the first man to die on that expedition, and the cross was erected that year.