The VIIRS Day-Night Band (DNB) is a broad spectrum (about 0.5-0.9 um), high resolution channel on the Suomi NPP satellite that produces visible imagery at night. From this channel, NASA-SPoRT produces radiance and reflectance products. The DNB radiance product is the DNB in a rather raw form; it is a dim visible image dominated by city lights and other bright emission sources. If there is available moonlight, features like clouds will be discernible in radiance images. However, the effect of clouds, smoke, or fog will be apparent in how the clouds obscure the city lights, making them appear diffused or hazy compared to the cloud-free regions (see blog post on VIIRS Low Light Sensor Application – Cloud Mapping – City Lights). For example, in the radiance image shown from October 8th at 0740 UTC, during a first quarter moon, clouds are barely visible in the image. More apparent is the difference in city lights between the Southeast and northern Illinois.
The reflectance product is derived from the radiance images, normalized by the available moonlight to accentuate the clouds and other meteorological features in the image, and this image is dependent on moon phase and angle. As mentioned in previous blogs, certain moon angles and light conditions will cause the reflectance product to appear “over-exposed”, making the city lights and other emission sources too bright. In the example from October 8th, the clouds are clearly visible, the cities are bright but do not wash out the rest of the image, and the area near the aurora is “over-exposed”. In regional-scaled images, some terrain will even be visible on brightly moonlit reflectance images. Recall, however, that the light quality of the reflectance product will change with changes in moon phase, so this image will look different each week.