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。
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At least 60 to 70 calories per square centimeter of thermal energy are required to produce surface melting or darkening.