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Extremely Rare Rainbow Clouds Lit Up Arctic Skies For 3 Days: The Photos Are Breathtaking
Arctic observers were granted a rare spectacle when rare rainbow-colored clouds streaked the skies ahead of the holiday season.
A cold wave sweeping through the Arctic brought the rare polar stratospheric clouds within view for at least three days between December 18 and December 20, per the website spaceweather.com.
"The clouds were visible in the sky all day, but the colors really exploded just before sunset," said Norway-based photographer Ramune Sapailaite, per spaceweather.com.
Observers shared pictures with spaceweather.com from Norway, Sweden, Finland, Alaska, and even as far south as Scotland, per the news outlet Live Science.
The clouds, which also go by the name of nacreous clouds, appear only in extremely cold weather. They are created by tiny ice crystals that scatter sunlight creating little rainbows in the sky. The crystals are suspended in the sky at an extremely high altitude, about 9.3 and 15.5 miles above the surface.
That's higher than clouds usually form, but at extremely cold temperatures, water vapor can start to coalesce, creating the crystals, per Live Science.
They are usually only spotted a handful of times a year, most typically in January.
So seeing them for several days in a row in December is a very rare treat, per spaceweather.com.
注释：
Stratospheric: adj
表示"平流层的；同温层的；"，如：Can stratospheric temperature trends be attributed to ozone depletion?平流层温度趋势能被归因于臭氧减少吗?
nacreous: adj
表示" 含有珍珠母的；有珍珠光泽的"，如：nacreous clouds 珠母云 nacreous pigments 珠光颜色
coalesce: v
表示" 合并；联合"，means ""，如：I hope patriotism can coalesce under this kind of idea and express itself. 愿爱国主义在这样的理念下凝聚起来，表达出来。
中文简要说明：
提到北极，一般人都会想到夜晚壮丽的极光，不过在这个高纬度的国度，还有另一个奇幻景象，那就是贝母云（nacreous cloud，也称为珍珠云或珠母云）。根据科学新闻网站「Science Alert」报导，本月中下旬，贝母云频繁出现在北极的天空，让不少游客惊喜连连，也由于1月才是这种现象的爆发期，在此时出现可说是相当罕见。
贝母云的正式名称为「极地平流层云」（polar stratospheric cloud），顾名思义只出现在两极地区，分布在距离地面15至25公里的高空。这种云虽然美丽，但却十分罕见，主要由于地球的平流层非常干燥，只有当温度降到摄氏零下85度左右时，水分子才会开始凝结成微小的冰晶，折射阳光后呈现珍珠般的闪亮光泽。
来自挪威的摄影师萨帕莱特（Ramune Sapailaite）表示，「抬头一整天都能看到贝母云，但到了傍晚日落时分，缤纷色彩才真正爆发出来。」
12月中下旬期间，适逢一波寒流通过北极地区，在瑞典、挪威、芬兰、阿拉斯加等地，甚至连苏格兰都有民众观测到彩虹云，纷纷拍下梦幻美景放上网络。由于贝母云通常出现在每年1月，民众要想亲眼目睹，未来一个月仍有机会。
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