1911 Encyclopædia Britannica/Isabnormal Lines
ISABNORMAL (or Isanomalous) LINES, in physical geography, lines upon a map or chart connecting places having an abnormal temperature. Each place has, theoretically, a proper temperature due to its latitude, and modified by its configuration. Its mean temperature for a particular period is decided by observation and called its normal temperature. Isabnormal lines may be used to denote the variations due to warm winds or currents, great altitudes or depressions, or great land masses as compared with sea. Or they may be used to indicate the abnormal result of weather observations made in an area such as the British Isles for a particular period.