An isotherm is a line on a weather or climate map connecting points of equal temperature. Together with isobars (equal pressure), isotherms are among the most fundamental tools for analysing and visualising atmospheric conditions. They reveal temperature gradients, air mass boundaries, frontal zones, and the influence of geography (latitude, altitude, proximity to oceans) on temperature distribution.
On surface temperature maps, isotherms are typically drawn at 5 °C or 10 °C intervals for large-scale charts, and 1–2 °C intervals for detailed regional analysis. Tightly packed isotherms indicate a strong temperature gradient — often marking a frontal zone where contrasting air masses meet. The 0 °C isotherm (freezing line) is particularly important: it determines the rain-snow boundary at the surface and is critical for aviation (icing zones), road maintenance (gritting decisions), and agriculture (frost risk).
At upper levels (e.g., the 850 hPa chart, roughly 1,500 m altitude), isotherms help identify warm and cold air masses, advection patterns (warm air advection where wind crosses isotherms from warm to cold), and frontal zones. The −5 °C isotherm at 850 hPa is a common threshold used in Spain to forecast snow at medium altitudes. On climate maps, annual mean isotherms illustrate how oceans moderate coastal temperatures (isotherms bend poleward along warm currents like the Gulf Stream) and how continentality creates temperature extremes inland. The concept extends to oceanography (isothermal layers) and geology (subsurface temperature mapping).