Passage V

Mystery Clouds

[1]

They glow an electric blue and mysteriously hover on the edge of space. Though the sun is below the horizon their wispy tendrils gleam brightly. [A] However, these curious formations are noctilucent or "night-shining" clouds, so named because they appear before sunrise or after sunset but when sunlight still illuminates Earth's middle atmosphere. (63)

[2]

[1] Volcanic ash spewed as high as 80 km into Earth's atmosphere, creating years of vivid red sunsets around the world. [2] Sightings of noctilucent clouds were first recorded in 1885, two years after a powerful eruption of Krakatoa, a volcano in Indonesia. [3] A few observers in northern latitudes, in places such as Scandinavia and Russia, reported seeing strange, bright blue, threadlike clouds just after these sunsets. [4] The sunsets faded, but night-shining clouds remained. (66)

[3]

Since the early 1980s, noctilucent clouds have been observed more frequently than ever before and as far south as Utah and Colorado. [B] Night-shining clouds form in the mesosphere, a layer of Earth's atmosphere extending about 50 to 85 km above the planet's surface. Typical weather clouds form only 2 to 10 km above Earth. in the troposphere. Noctilucent clouds, like all, consist of ice crystals that form when water molecules freeze around "seed" particles, such as dust. (68)

[4]

Most researchers theorize that Krakatoa seeded the mesosphere, which is a layer of Earth's atmosphere, in the mid-1880s and that space dust (tiny particles of meteoroids ) is seeding those today. Many experts also believe a role being played by global warming. [C] While greenhouse gases warm Earth's surface, these gases cool and humidify the mesosphere. The mesosphere, perhaps, has become cold and moist enough for ice crystals to form around space dust, creating noctilucent clouds more often and farther south. [D] As scientists keep on figuring out the clouds, sky watchers search the predawn and postsunset skies for the elusive silvery blue wisps.