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):5704-11. doi: 10.1364/ao.44.005704.
Overcasts seen from below seldom are uniform, unchanging cloud shields, yet little is known about their short-term photometric variability (periods < or = 2 h). Visible-wavelength spectra of daytime and twilight overcast skies measured at 30-s intervals reveal unexpected temporal variability in horizontal illuminance E(v) and zenith luminance L(v). Fourier analysis of these time series shows peak fluctuations at periods of 2-40 min. Factors such as cloud type and optical depth, presence of fog or snow, and instrument field of view can affect overcast brightness variability. Surprisingly, under some circumstances overcast twilight E(v) exceeds clear-sky E(v) at the same Sun elevation.
从下方看到的阴天很少是均匀、不变的云层覆盖，然而对于它们的短期光度变化（周期≤2小时）却知之甚少。以30秒的间隔测量的白天和黄昏阴天天空的可见波长光谱显示，水平照度E(v)和天顶亮度L(v)存在意想不到的时间变化。对这些时间序列的傅里叶分析表明，在2 - 40分钟的周期内存在峰值波动。云类型和光学深度、雾或雪的存在以及仪器视场等因素会影响阴天亮度变化。令人惊讶的是，在某些情况下，相同太阳高度下的阴天黄昏E(v)超过了晴空E(v)。
