The DNB on VIIRS detects reflected moonlight from clouds, fog, and some reflective atmospheric and surface features and emission from city lights, small towns, and local communities (see other VIIRS blog posts on this page). SPoRT Disaster Response Team member Andrew Molthan has processed VIIRS DNB data from before and after Hurricane Sandy to detect where the power is still out in regions affected by Hurricane Sandy. He has focused recent analysis on comparing imagery from before and after the event, where as we showed before, you can identify outages as light sources literally “turned off” in cloud-free imagery. Using newly implemented image processing techniques, Andrew has taken the day-night image from before the event (using a clear period from August 31 as seen on other NASA and NOAA sites) and combined it with November 1’s morning imagery (which was relatively cloud free). By assigning “before” to the red and green colors and “after” to blue, the resulting image has the characteristics of producing bright white lights where lights appear in all images, and yellow where lights are missing in the “after” image. From the technical side, this occurs because if lights are present in both the before and after image, the resulting color of the pixel is white. In regions where the lights are present in the beginning, but not after the storm, the image combination results in a yellow color. Some care must be taken with image interpretation, however, as changes in cloud cover also produce impacts. Movement of clouds changes the intensity of light (partial obscuration, etc.) and thus an analyst must look for other clues to confirm that the “yellows” are actually decreases in light. However, as a quick look product, this combination seems to highlight in yellow many of the coastal areas where power outages are likely or confirmed.
SPoRT receives real-time VIIRS data from the University of Wisconsin through a collaborative partnership under the NOAA GOES-R and JPSS programs. Processing of level 1 and higher data is performed by SPoRT at MSFC.