An isohyet (also spelled isohyet) is a line on a map connecting points of equal precipitation over a given period (typically annual, monthly, or for a specific storm event). Isohyets serve the same cartographic function for precipitation as isotherms do for temperature and isobars do for pressure — they transform point measurements into spatial fields, revealing patterns and gradients.
Isohyetal maps highlight the dramatic influence of geography on rainfall. In Spain, the contrast is stark: the northwest (Galicia) receives 1,500–2,500 mm/year (comparable to the British Isles), while the southeast (Almería) receives barely 200 mm/year (semi-arid). Mountain ranges create sharp precipitation gradients: the windward slopes of the Cantabrian Mountains, Pyrenees, and Sierra de Grazalema receive several times more rain than valleys and plains just 20–50 km away — orographic enhancement at its most dramatic.
The isohyetal method is one of three standard approaches for estimating areal rainfall from point measurements (the others being the Thiessen polygon and arithmetic mean methods). In this technique, isohyets are drawn from rain gauge data, and the area between consecutive isohyets is multiplied by the average precipitation in that band. It is the most accurate method when a dense gauge network is available and topographic effects are significant. Modern hydrology increasingly combines gauge isohyets with radar and satellite estimates to create high-resolution precipitation maps used for flood forecasting, water resource management, and climate studies.