The Isoline of Ninety Frost-Free Days in Canada

Abstract

T nHIS paper seeks to present a generalized demarcation of the land area of Canada wherein the average frost-free period, when frost is taken as occurring at the temperature of 320 F., is less than 90 days. The reasons for attempting the demarcation are two. (1) Nearly 40 years ago, Reed wrote, though with specific reference to the United States, There is very little agriculture, except that based upon wild hay and grazing, where the average season between killing frost is less than 90 days.' He was thus specifying an observed limit, whether imposed by low temperature alone or by some combination of low temperature with economic, social, and historical influences, upon the cultivation of land-at least in the United States as of the early twentieth century. While other descriptions of limits or barriers to land cultivation associated with low temperature have been advanced, notablythough with specific reference to grain production-that of availability of an accumulation of daily mean summer temperatures above 100 C. in the amount of 15000-16000 C., the world seems not to have been mapped on the basis of any except that of 90 days free of frost.