Under moonlight conditions, the VIIRS low light sensor (or day-night-band (DNB)) canbe used by forecasters to better map the distribution of low clouds and fog than with just infrared data. The left image above shows the VIIRS thermal infrared imagery from October 9, 2012 at 0721UTC (221am local time) for the eastern portion of the U.S. Higher (cold) clouds are readily apparent over the Great Lakes region and off the East Coast with some indication of low level (warm) clouds extending down into the Southeast U.S. The corresponding DNB and derived reflectance product detect both emitted light from cities and reflected moonlight, confirming the extension of low clouds and fog into the Carolinas, eastern Tennessee, Georgia, and northern Florida.
In the high resolution DNB “reflectance” image above (on October 9, 2012 at 0721UTC (0221 local time)), the light emitted by cities and small towns dominates the image. The reflectance image is created by normalizing the radiance data by the available moonlight (which is dependent on phase of the moon and its position above the horizon) at each pixel. Clouds and low fog can be seen in the eastern half of the image as intermediate gray shades. Since light emitted from even modest-sized cities and towns exceeds the magnitude of the moonlight reflected from clouds, city lights appear as bright white saturated pixels in this reflectance product. This pixel saturation also occurs in the presence of low clouds as the light emitted upward from cities is scattered by clouds and transmitted as diffuse light to the VIIRS sensor. This effect is readily apparent in the image above where sharp, crisp, and small white town features are observed in cloud-free regions and, in contrast, only smooth features of larger towns and cities are apparent in cloudy regions. Light from the smaller towns is scattered by the clouds but little of it reaches the satellite.