Lee Raymond L, Hernández-Andrés Javier
Mathematics and Science Division, United States Naval Academy, Annapolis, Maryland 21402, USA.
Appl Opt. 2005 Sep 20;44(27):5712-22. doi: 10.1364/ao.44.005712.
Time-series measurements of daylight (skylight plus direct sunlight) spectra beneath overcast skies reveal an unexpectedly wide gamut of pastel colors. Analyses of these spectra indicate that at visible wavelengths, overcasts are far from spectrally neutral transmitters of the daylight incident on their tops. Colorimetric analyses show that overcasts make daylight bluer and that the amount of bluing increases with cloud optical depth. Simulations using the radiative-transfer model MODTRAN4 help explain the observed bluing: multiple scattering within optically thick clouds greatly enhances spectrally selective absorption by water droplets. However, other factors affecting overcast colors seen from below range from minimal (cloud-top heights) to moot (surface colors).
对阴天天空下日光（天光加直射阳光）光谱进行的时间序列测量揭示了一系列出人意料的丰富柔和色彩。对这些光谱的分析表明，在可见光波长范围内，阴天远非其顶部入射日光的光谱中性传输体。色度分析表明，阴天会使日光更偏蓝色，且变蓝程度随云光学厚度增加。使用辐射传输模型MODTRAN4进行的模拟有助于解释观测到的变蓝现象：光学厚度较大的云层内的多次散射极大地增强了水滴的光谱选择性吸收。然而，从下方观察时影响阴天颜色的其他因素范围从极小（云顶高度）到无实际影响（地面颜色）。
