1911 Encyclopædia Britannica/Isotherm

ISOTHERM (Gr. ἴσος, equal, and θέρμη, heat), a line upon a map connecting places where the temperature is the same at sea-level on the earth’s surface. These isothermal lines will be found to vary from month to month over the two hemispheres, or over local areas, during summer and winter, and their position is modified by continental or oceanic conditions.