From AMS Glossary
The line connecting all places at which the mean temperature (°C) of the warmest month is equal to 9 - 0.1k, where k is the mean temperature of the coldest month. (In degrees Fahrenheit the relationship becomes 51.4 - 0.1k.)
Hare, F. K. 1951. Compendium of Meteorology. p. 955.