line connecting points of equal summer temperature
• (n.) A line connecting points on the earth`s surface having the same mean summer temperature.
I'so·there noun [ Iso- + Greek ... summer, ... to heat.] (Physics Geology) A line connecting points on the earth's surface having the same mean summer temperature. 
isothere, isotheral A line connecting places on the earth’s surface having the same mean summer temperature.
a line on a weather map or chart connecting points that have the same mean summer temperature.