City lights shining through clouds

July 19th, 2012 |
VIIRS Day/Night Band from Suomi/NPP satellite

VIIRS Day/Night Band from Suomi/NPP satellite

The New Moon on 18 July means that no visible light reflected from the Moon can illuminate clouds, and the day/night band on the Suomi-NPP satellite can therefore view any clouds only with difficulty. However, it still views city lights, and illuminated roads, or gas field flares, among other things. The region above shows data contaminated by stray light. The Suomi/NPP satellite as it took this image was, although on the night side of the planet, nevertheless outside the shadow of the Earth, and radiation from the Sun was able to contaminate the image. Despite this, the lights of Minnesota and Iowa, both the main cities and the smaller towns, are plainly visible. City lights in eastern Wisconsin and Illinois have a more diffuse character because they are being viewed through cirrus clouds from a departing convective complex, as evident in this enhanced 10.7 micrometer imager from 0800 UTC.

VIIRS Day/Night Band from Suomi/NPP satellite

VIIRS Day/Night Band from Suomi/NPP satellite

The GOES-R IFR Proabability image from 0801 UTC, above, shows high probabilities of low clouds over north-central Iowa and south-central Minnesota. Surface observations plotted on the image confirm IFR conditions, with low visibilities and low ceilings. In addition, GOES-R Cloud Thickness values in the region are from 800 and 1000 feet thick. It is interesting that the Day/Night band can view the city lights through a low stratus deck that is hundreds of feet thick.

It is very difficult to distinguish between low stratus and clear regions in the stray light zone. That task is somewhat easier when there is no stray light, as shown in this image over the southern United States: In low light conditions, such as during a New Moon, low clouds have a smudgey appearance, but that signature is overwhelmed in the stray light zone. How thick does a cloud have to be before it is opaque to visible light?

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