• (a.) Having the nature of an isothere; indicating the distribution of temperature by means of an isothere; as, an isotheral chart or line.
I·soth'er·al adjective Having the nature of an isothere; indicating the distribution of temperature by means of an isothere; as, an isotheral chart or line. 
isothere, isotheral A line connecting places on the earth’s surface having the same mean summer temperature.