isothermality

Isothermality is a term used in different contexts to denote a condition in which temperature remains constant or nearly constant, either in a physical process or across a geographical area and time. In thermodynamics, an isothermal process is one during which the temperature of the system stays constant while heat is exchanged with the surroundings. For an ideal gas, this implies that the pressure and volume change in a way that keeps the temperature fixed, following PV = constant.

In climate science and biogeography, isothermality is a specific climate-index concept used to describe the relative constancy of temperature over the course of a year. One widely used formulation is that isothermality equals 100 times the mean of the monthly temperature ranges divided by the annual temperature range. Here, the monthly temperature range is the difference between the maximum and minimum temperature within each month, averaged over the year, and the annual range is the difference between the warmest month’s maximum temperature and the coldest month’s minimum temperature. The index is dimensionless and often expressed as a percentage.

Interpreting isothermality helps distinguish climates with large day-to-night temperature swings from those with pronounced seasonal changes. Higher isothermality values indicate that diurnal temperature variability dominates relative to seasonal variability, typical of tropical climates, while lower values correspond to climates with stronger seasonal temperature shifts, such as temperate or polar regions. The concept is widely used in ecological niche modeling, climate classification, and environmental-limitation studies.

See also: isotherm, isothermal process.