Degree of frost

A degree of frost is a non-standard unit of measure for air temperature meaning degrees below melting point of water. "Degree" in this case can refer to degree Celsius or degree Fahrenheit.
When based on Celsius, 0 degrees of frost is the same as 0 °C, and any other value is simply the negative of the Celsius temperature. When based on Fahrenheit, 0 degrees of frost is equal to 32 °F. Conversion formulas:

T = 32 °F − T
T = 32 °F − T

The term "degrees of frost" was widely used in accounts of the Heroic Age of [Antarctic Exploration] in the early 20th century. The term appears frequently in Ernest Shackleton's books South and Heart of the Antarctic, Apsley Cherry-Garrard's account of his Antarctic adventures in The Worst Journey in the World, in Jack London's "To Build A Fire", as well as Admiral Richard E. Byrd's book Alone.