十万个为什么·科学阅读30篇（2）
19 / 30
正在确认阅读权限…
Why Do Certain Clouds Glow Brightly at Twilight?
为什么某些云层在暮色中会发出明亮光芒？
-
Noctilucent clouds form high in the mesosphere, about 80–85 km above Earth’s surface.
-
They appear only during summer months near polar regions when sunlight reflects off ice crystals.
-
These tiny ice particles grow on meteoric dust that enters our atmosphere from space.
-
Unlike lower clouds, noctilucent clouds remain sunlit long after sunset due to their extreme altitude.
-
Their electric-blue glow results from scattering of blue wavelengths by nanoscale ice grains.
-
Scientists monitor them closely because increasing frequency may signal changes in upper-atmosphere chemistry.
-
They are visible only when the Sun is 6–12 degrees below the horizon and the sky is otherwise dark.
-
Ground observers must face north in the Northern Hemisphere to spot these delicate, wavy structures.
-
Radar and satellite data confirm their composition includes water ice, not frozen carbon dioxide or ammonia.
-
Studying them helps us understand how atmospheric gravity waves influence cloud formation far above weather systems.