Polar mesospheric, or noctilucent ("night shining"), clouds usually are seen at twilight, following the setting of the sun below the horizon and darkening of Earth's surface.
极地气层云，或者说晚霞，通常是那些在黎明前看到的云朵，伴随着地平线以下落幕的太阳和黑暗的地球表面。
At least 60 to 70 calories per square centimeter of thermal energy are required to produce surface melting or darkening.
要使这些物质表面熔化或变黑，至少需要每平方厘米有60到70卡的热能。
At least 60 to 70 calories per square centimeter of thermal energy are required to produce surface melting or darkening.
要使这些物质表面熔化或变黑，至少需要每平方厘米有60到70卡的热能。
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