line connecting points of equal winter temperature
• (n.) A line connecting points on the earth`s surface having the same mean temperature in the coldest month of the year.
I'so·cryme noun [ Iso- + Greek ... cold.] (Physics Geology) A line connecting points on the earth's surface having the same mean temperature in the coldest month of the year. 
isocryme A line connecting points on the earth’s surface at which the temperature is the same during a specified coldest period of the year. Related cold, frost; freezing units of words: algid-; cheimo-, chimo-; cryo-, kryo-; frigo-; gel-; pago-; psychro-; rhigo-.