Isoterm

Isoterm is a term used in several scientific fields to denote a line or surface of constant temperature. It is often treated as a variant spelling of isotherm, a word derived from the Greek roots iso- meaning equal and therme meaning heat. In some languages and technical traditions, isoterm remains the standard form, while in English isotherm is more common.

In thermodynamics, an isotherm commonly refers to a curve on a pressure–volume diagram (P–V diagram) that represents a process occurring at constant temperature. For an ideal gas, the relationship PV = nRT holds, so with T fixed the product PV remains constant, producing a hyperbolic isothermal curve. Isothermal processes are characterized by heat transfer balancing the work done by or on the system, and the internal energy of an ideal gas changing with temperature rather than with volume.

In meteorology and climatology, isotherms are lines of equal temperature plotted on weather maps. These lines help visualize temperature distribution across a region and identify temperature gradients, fronts, and patterns over time. Isotherms are often drawn for specific elevations, such as sea level, and are labeled with temperature values.

In geology and earth sciences, subsurface isotherms map the distribution of temperature within the Earth. Such maps relate to geothermal gradients, heat flow, and the thermal structure of crust and mantle, contributing to studies of hydrocarbon formation, groundwater flow, and geotechnical engineering.

Overall, isoterm or isotherm denotes the general concept of sameness of temperature across a curve or surface, used across physics, chemistry, meteorology, and geosciences.