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.
Low clouds and fog show up as the darkest shades of gray in this IR image. Low-top clouds with warmer temperatures appear darker in IR images than taller, colder cloud tops. In Figure 2, the highest clouds, probably cirrus, off the mid-Atlantic and New England coasts appear bright white. Middle clouds over the central U.S. have warmer cloud tops and appear in shades of gray. Low clouds and fog show up as the darkest shades of gray in this IR image, such as over east Texas and Louisiana.