Isopleths and interpolation

Isopleths are lines that join places of equal value. They suit continuous data, which change gradually across a surface.

Kinds of isopleth:

• isotherms (equal temperature), isobars (pressure), isohyets (rainfall), isonephs (cloudiness), isohels (sunshine);
• contours (height), isobaths (sea depth), isohalines (salinity).

Choose an equal interval between lines (5, 10 or 20), and write the values on the lines.

Most lines fall between the known points, so we interpolate: the position of the isopleth = distance between the two points ÷ difference in their values × the difference between the lower value and the isopleth value.

Example: two stations 12 mm apart record 20 °C and 26 °C. The 22 °C isotherm lies 12 ÷ 6 × 2 = 4 mm from the 20 °C station. Interpolate, then join the points.