The isotherm of -3 °C for the coldest monthly mean temperature, was observed to be the line where the climate was likely cold enough to support a fixed period of continuous snow cover every year.
The southernmost of the three is frequently referred to as the temperate continental climate, and has hot summers — that is to say, at least one month has an average temperature of 22 °C (71.6 °F) or above.
[3][4] In North America, microthermal climates start north of Boston along the Atlantic seaboard then westward to just below the Great Lakes to the Midwest, the line then moves southward below the Dakotas, through the west near 40 latitude at the eastern edge of the Rocky Mountains, then curving northward near the lowlands of the Pacific coast, reaching the Pacific Ocean just south of Juneau, Alaska.
In Asia, the latitude at which these climates begin is several degrees farther south influence of the vast Siberian anticyclone, or high-pressure system, and in continental Europe the line actually runs longitudinally rather than latitudinally, cutting through central Poland after beginning north of the Arctic Circle along the Norwegian coast, thereafter moving diagonally across Scandinavia.
The boundary between the microthermal and polar climate zones is farthest north in western Europe (actually within the Arctic Circle there), and farthest south near the northeast coast of Canada (at about 56° North latitude on the central coast of Labrador); it then trends northward across Canada before dropping south again as it courses through Alaska.