From AMS Glossary
A unit of atmospheric opacity.
One nebule is the opacity of a screen having such transmissivity Tn that
that is, such that 100 of the screens placed in optical series would transmit only one-thousandth of the incident light. One speaks of a given atmosphere or atmospheric stratum as possessing an opacity of a certain number of nebules per kilometer. The transmissivity T of an optical path r km long through an atmosphere of opacity n nebules per km is given by
where Tn is defined above. An opacity of one nebule per km is equivalent to an extinction coefficient of 0.069 per km.
Middleton, W. E. K. 1952. Vision through the Atmosphere. p. 14.