Black and white imaging records only tonal differences between colored objects, which appear as black, white, or different shades of gray. Proper rendition depends on your own desires, and, for film, the differences between film sensitivity to colors and that of the eye. The latter is due to the fact that most panchromatic emulsions used are more sensitive to blue, violet and ultraviolet than to other colors. Therefore, blue appears as lighter on film than it does to the eye. This can make a blue-sky light enough to appear a similar shade of light gray as the clouds that are in it, making the clouds "disappear."
A more "correct" cloud presence is obtained through the use of a yellow filter, such as a Yellow 8, which can absorb blue light, darkening the sky to more closely match what the eye would see. The Yellow 8 also acts as a general compensator for most subjects, giving a tonal rendition similar to that of the eye.
