Clouds play a key role in Earth’s radiation budget, covering more than 50% of the planet. However, the binary delineation of cloudy and clear sky is not clearly defined due to the presence of a transitionary zone, known as the cloud twilight zone, consisting of liquid droplets and humidified to dry aerosols. The twilight zone is an inherent component of cloud fields, yet its influence on longwave-infrared radiation remains unknown. Here we analyse spectral data from global satellite observations of shallow cloud fields over the ocean to estimate a lower bound on the twilight zone’s effect on longwave radiation. We find that the average longwave radiative effect of the twilight zone is ~0.75 W m^–2, which is equivalent to the radiative forcing from increasing atmospheric CO₂ by 75 ppm. We also find that the twilight zone in the longwave occupies over 60% of the apparent clear sky within the analysed low-level cloud fields. As low-level clouds are relatively warm, the overall longwave radiative contribution from the twilight zone is likely to be higher. We suggest that the twilight zone needs to be accounted for to accurately quantify cloud radiative effects and close the global energy budget.

云在地球辐射预算中起着关键作用，覆盖了地球的50％以上。但是，由于存在过渡区（称为云暮区），该过渡区由液滴组成，并由湿润的气溶胶干燥而成，因此，对多云和晴朗的天空的二值化描述尚不明确。暮光区是云场的固有组成部分，但其对长波红外辐射的影响仍然未知。在这里，我们分析了全球卫星对海洋浅层云场观测的光谱数据，以估计暮光区对长波辐射的影响的下限。我们发现，暮光带的平均长波辐射效应约为0.75 W m ^–2，这相当于大气CO ₂含量增加引起的辐射强迫。75 ppm。我们还发现，在分析的低空云场中，长波的暮光区占据了视在晴朗天空的60％以上。由于低层云比较温暖，因此来自暮光带的总长波辐射贡献可能会更高。我们建议需要考虑暮光区，以准确量化云辐射效应并关闭全球能源预算。