The wavy lines on weather maps, topo maps, and others.

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A temperature and pressure map depicting isobars (lines of equal barometric pressure) and isotherms (which are the boundaries of the colored areas depicting temperatures). From this map we can infor the structure of the weather systems over the United States at the time the map was generated. (Map courtesy NOAA) (click image to enlarge)

An isoline is a line drawn connecting points of equal value, the root, iso, from the Greek for for equal.

Isolines (also called by some isorithms find wide use in cartography. On topographic relief maps, isolines of elevation are called contours, presumably because the shape of the land can be inferred from them. Isobaths, by contrast, depict the contours of underwater surfaces. Isobars connect points of equal barometric pressure, which on weather maps suggest the structure of weather systems. Isotherms depict areas of warmth and cold; isohyets depict areas of relative high and low rainfall.