Unit sometimes used in the description of the total ozone in a column of air overhead.
It is given as the thickness (in units of

or

) of that layer which would be formed by the total ozone in a column reduced to the
pressure of

and temperature of

.
Source:
PAC, 1990, 62, 2167
(Glossary of atmospheric chemistry terms (Recommendations 1990))
on page 2185