From AMS Glossary
(Rare.) A line of equal percent deficiency from the mean.
The term was introduced by W. R. Baldwin-Wiseman (1934) for a line showing equal differences of the cumulative rainfalls for each month of a drought period from the long-period average rainfall of the month, expressed as percent of the latter.
Baldwin-Wiseman, W. R. 1934. The cartographic study of drought. Quart. J. Roy. Meteor. Soc.. 60. 523– 532.