isabnormal

[ ahy-sab-nawr-muh l ]
/ ˌaɪ sæbˈnɔr məl /

noun Meteorology.

a line on a map or chart connecting points having an equal deviation from the normal value of some meteorological quantity, as temperature.
Also isoabnormal.
Compare isanomal.

Origin of isabnormal

First recorded in 1850–55; is- + abnormal