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Isotherm, isohyet, contour — lines joining equal values of anything.
Also known as: Isopleth, Lines joining equal values
The same device applied to different quantities. Once a reader knows that a closed line joins points of equal value and that spacing means gradient, every one of these charts is readable, whatever the quantity.
Lines joining points of equal value: isotherms for temperature, isohyets for rainfall, isobars for pressure, and contours for height.
Contexts: Maps
Spacing means gradient in every case. Lines close together mean the quantity changes rapidly over a short distance, which is why a reader who understands one isoline chart understands all of them.
They cannot cross, because a point cannot have two values of the same quantity. Two isolines appearing to touch on a chart is a plotting artefact or a discontinuity, never a real crossing.
Smooth curved lines across a map, roughly parallel and never crossing, each labelled with a numerical value.
A contour is the height case of the same device.
Isobars are the pressure case of the same device.