# Nebule

From AMS Glossary

## nebule

A unit of atmospheric opacity.

One nebule is the opacity of a screen having such transmissivity 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 where

*T*_{n}that*T*of an optical path*r*km long through an atmosphere of opacity*n*nebules per km is given by*T*_{n}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.