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isotherm,line drawn on a map of a particular region of the earth's surface connecting points of equal temperature; each point reflects one temperature reading or an average of several readings over a period of time. The relative spacing of the isothermal lines indicates a temperature gradient, i.e., the amount of temperature change over a given distance.
an isogram of the temperature of air, water, or soil. Most commonly drawn are isothermal maps showing mean monthly air temperature over a number of years, mean temperature for any period of time, or the temperature at a definite moment of time. To eliminate the influence of elevation when drawing isotherms, temperature values are sometimes reduced to sea level, assuming that air temperature drops an average of 0.6 °C for each 100 m of increase in elevation.
a line on a phase diagram depicting a process occurring at constant temperature (an isothermic process). The equation of the isotherm of an ideal gas is pV = const, where p is the pressure and V is the volume of the gas. For a real gas the equation of an isotherm has a more complex character and becomes the equation for the isotherm of an ideal gas only at low pressures or high temperatures. On a pV diagram, at the point of intersection of the isotherm and the adiabatic curve the latter is steeper than the isotherm. The isotherms of ferromagnets in J, H, coordinates, where J is the magnetization and H is the magnetic field intensity, have a similar character.