十万个为什么·科学阅读30篇（2）
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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？
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Noctilucent clouds form high in the mesosphere, about 80–85 km above Earth’s surface.夜光云形成于中间层高空，距地球表面约80–85公里。
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They appear only during summer months near polar regions when sunlight reflects off ice crystals.它们仅在夏季极地附近出现，阳光照射到冰晶时才可见。
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These tiny ice particles grow on meteoric dust that enters our atmosphere from space.这些微小冰粒以进入大气的流星尘为凝结核生长而成。
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Unlike lower clouds, noctilucent clouds remain sunlit long after sunset due to their extreme altitude.与低层云不同，夜光云因所处高度极高，日落后仍长时间被阳光照亮。
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Their electric-blue glow results from scattering of blue wavelengths by nanoscale ice grains.其电蓝色辉光源于纳米级冰粒对蓝光波长的散射。
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Scientists monitor them closely because increasing frequency may signal changes in upper-atmosphere chemistry.科学家密切监测它们，因其出现频率上升可能预示高层大气化学变化。
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They are visible only when the Sun is 6–12 degrees below the horizon and the sky is otherwise dark.只有当太阳位于地平线下6–12度且天空其他区域已变暗时，才能看见它们。
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Ground observers must face north in the Northern Hemisphere to spot these delicate, wavy structures.北半球地面观测者需面朝北方，才能发现这些纤细、波状的云结构。
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Radar and satellite data confirm their composition includes water ice, not frozen carbon dioxide or ammonia.雷达和卫星数据证实其成分为水冰，而非固态二氧化碳或氨。
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Studying them helps us understand how atmospheric gravity waves influence cloud formation far above weather systems.研究它们有助于理解大气重力波如何影响天气系统上方的云形成。