Currently, SPoRT is collaborating with CIRA and our Front Range partners (Albuquerque, Boulder, Cheyenne, and Great Falls) to assess several night products from VIIRS. In particular, the focus is on the day-night band (DNB) instrument, but we are also looking at red-green-blue composites from the DNB (radiance and reflectance) as well as the nighttime microphysics and dust. An interesting feature was observed in the overnight hours on July 16 around 1000 UTC . We were originally looking for potential smoke plumes or fire hot spots with the VIIRS DNB radiance product (Figure 1). The radiance was being used as the moon was just coming out of a new moon, limiting the amount of moonlight for the reflectance products. While we did see a fire hot spot, we did notice a faint feature in the northwest portion of Great Falls’ county warning area. There was some difficulty with the observation with the stray light contaminating the image from the northeast. Still, a faint cloud feature was seen as the city lights under the feature were blurred.
We decided to next look at the traditional spectral difference (11-3.9 micron imagery – Figure 2) to see if this was a fog bank. The region of interest was a strong yellow color, which indicates either fog or low clouds.
We decided to return to the VIIRS day-night band instrument and look at the radiance RGB. The radiance RGB can help distinguish between high clouds and low clouds (Figure 3). All of the clouds here are blue, which would indicate high clouds and contradict Figure 2. However, given the very low contribution of moonlight at this time so soon after a new moon, the RGB product shows all clouds as blue. Therefore the radiance RGB was inconclusive in this case.
Finally, we decided to check out the nighttime microphysics red-green-blue composite image (Figure 4). This managed to definitively answer our question that was suggested by the previous images. The nighttime microphysics image shows the feature as a grey-green color. This is interpreted as low clouds and not fog. If this had been fog, the color would have been more of a milky white or have some purple from the ground bleeding through if the fog were thin. Additionally, the brighter blue clouds from Figure 3 show up as yellows, oranges, and reds indicating thicker water clouds which are much higher aloft than the original feature .