Nacreous clouds, sometimes called mother-of-pearl clouds, are rare but once seen are never forgotten. They are mostly visible within two hours after sunset or before dawn when they blaze unbelievably bright with vivid and slowly shifting iridescent colours. They are filmy sheets slowly curling and uncurling, stretching and contracting in the semi-dark sky. Compared with dark scudding low altitude clouds that might be present, nacreous clouds stand majestically in almost the same place - an indicator of their great height.
They need the very frigid regions of the lower stratosphere some 15 - 25 km (9 -16 mile) high and well above tropospheric clouds. They are so bright after sunset and before dawn because at those heights they are still sunlit.
They are seen mostly during winter at high latitudes like Scandinavia, Iceland, Alaska and Northern Canada. Sometimes, however, they occur as far south as England. They can be less rare downwind of mountain ranges. Elsewhere their appearance is often associated with severe tropospheric winds and storms.
Rare nacreous clouds over Belfast, Ireland, recently.
February 10, 2016
By Eric Hand
They need the very frigid regions of the lower stratosphere some 15 - 25 km (9 -16 mile) high and well above tropospheric clouds. They are so bright after sunset and before dawn because at those heights they are still sunlit.
They are seen mostly during winter at high latitudes like Scandinavia, Iceland, Alaska and Northern Canada. Sometimes, however, they occur as far south as England. They can be less rare downwind of mountain ranges. Elsewhere their appearance is often associated with severe tropospheric winds and storms.
February 10, 2016
By Eric Hand
Lingering atmospheric (click here) pollutants and a blast of frigid air have carved an unusually deep hole in Earth’s protective ozone layer over the Arctic, and it threatens to get deeper. Atmospheric scientists are analyzing data from weather balloons and satellites for clues to how the ozone will fare when sunlight—a third factor in ozone loss—returns to the Arctic in the spring. But they are already worrying about how extra ultraviolet light might affect humans and ecosystems below and wondering whether climate change will make such Arctic holes more common or severe.
Record cold temperatures in the Arctic stratospheric ozone layer, 15 to 35 kilometers up, are the proximate cause for this year’s losses, because they help to unleash ozone-destroying chemicals. “This winter has been stunning,” says Markus Rex, an atmospheric chemist at the Alfred Wegener Institute in Potsdam, Germany. By next week, about 25% of the Arctic’s ozone will be destroyed, he says.
This time of year, the stratosphere tends to warm up with the breakdown of the polar vortex, a cyclone that traps cold air. But if a strong vortex persists another month as light returns to the Arctic after the dark winter, ozone losses will get much bigger, Rex says. Conditions are ripe for losses to surpass a record Arctic ozone hole observed in the spring of 2011, he adds.
At Earth’s surface, ozone is a caustic chemical and a health hazard. But in the stratosphere, it shields the planet from ultraviolet light.
