dark

1. (Obsolete.) As used in aviation observations prior to 1 September 1956, descriptive of a cloud cover that is composed of predominantly threatening or unusually dark-colored clouds.

It was denoted by the symbol “+” preceding the appropriate sky cover symbol.
See thin.

1. For observing at night, a person needs two to three minutes to adapt to dark (dark adaptation) before a noninstrumental observation, e.g., visibility at night.