On a weather map, lines joining places with equal temperature are called isotherms.
The word combines the Greek roots for “equal” and “heat.” Like contour lines on a topographic map, isotherms connect points sharing a measured value, but the value here is air temperature. Closely spaced isotherms indicate a strong temperature gradient, often associated with a front or a sharp change in air mass.
Weather forecasters use isotherms to identify warm and cold regions, track the movement of air masses, and estimate where freezing temperatures may occur. Maps may be drawn for surface temperature or for a particular pressure level in the atmosphere.
Isotherms are easily confused with isobars, which connect equal air pressure, and isohyets, which connect equal precipitation. Isotachs instead connect points with equal wind speed, making each term useful for a different map variable.