A couple weeks ago I rolled out the replacement satellite/radar pages, touting that visible satellite loops would now remove a healthy chunk of "night-time" images. Following that I realized that a handful of west coast regions for 1km, 2km and 6km resolution were not properly removing night-time images.
This removal of night-time images was accomplished by taking advantage of the fact that when our images get generated, the file size directly correlates to the average brightness of the image. This means that with visible satellite images, there should be a very natural drop-off in file size for night-time images. Within the code that grabs the most recent set of images, I simply added a file size check for visible images and filtered out the images that fell below a certain file size. This effectively removed night-time images from mix without using any information on sunrise, sunset, time of year, or latitude. I was quite happy with that approach. It meant I didn't have to over think the scripting or tailor the code to each region. Basically; less work, more reward.
Getting back to those west coast regions not removing night-time images; It turns out that GOES-West (GOES-15) changes how it operates when taking images of the dark side of the Earth. It attempts to bring in as much light as possible, which gradually increases the brightness of an otherwise dark image. This results in the minimum file size occurring right around sunrise and sunset, with a relative maximum in file size overnight. Here are some highly sophisticated graphs to help further describe this situation.
GOES-15 on the other hand creates a graph similar to the one below.