We'll focus on GOES imagery because it's easier to understand and is much easier to find. The image in Figure 1 is a visible GOES image. You can use such images as if they were photographs. Visible images depict cloud cover by measuring the amount of solar radiation the clouds reflect. As Table 1 shows, thick clouds and fresh snow reflect more light than thin clouds and appear brighter in the images.
Thin clouds appear gray and areas with no shading are generally clear.
But, there's some danger in relying upon visible images alone to determine cloud cover. Because low, middle, and high clouds reflect about the same amount of sunlight, a visible image can be misleading, often not telling pilots anything about cloud heights. Areas that appear clear on a visible image might be covered by a thin veil of clouds or fog that could obstruct your view of an airport, making a VFR landing impossible. To get a truer reading of clouds, you should also look at infrared images. Middle clouds have warmertops and appear in shades of gray.