Microthermal

In climatology, the term microthermal is used to denote the continental climates of Eurasia and North America.

The word microthermal is derived from two Greek words meaning "small" and "heat". This is misleading, however, since the term is intended to describe only the temperature conditions that prevail during the winter months, rather than those of the entire year.

The characteristic feature of the microthermal climate is cold winters — specifically, winters that are cold enough to ensure that snow will remain on the ground continuously for a fixed period of time every year. Conceptually, an average temperature of 0°C or colder is assumed to be necessary to bring this about; thus the climate of a location where at least one full month is this cold is classified as microthermal (however, at least one month in the summer must average 10°C or higher; otherwise the climate would be reckoned as polar). This definition places all of the world's microthermal climates in the Northern Hemisphere, as the absence of broad land masses at upper-middle latitudes in the Southern Hemisphere precludes the existence of such temperature conditions there.

Microthermal climates are typically subdivided into three categories based on the temperature characteristics of the summer season. 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. The middle zone is often labelled hemiboreal, and no summer month there has an average temperature as warm as 22°C, but at least four months will average 10°C (50°F) or higher. The northernmost of the three microthermal zones is the subarctic, or boreal zone; there only one to three months will have average temperatures of at least 10°C.

In North America, microthermal climates commence north of Boston along the Atlantic seaboard, this line drifting gradually southward further inland, reaching approximately 38° at the eastern edge of the Rocky Mountains, then curving dramatically northward near the Pacific coast, reaching the Pacific Ocean just south of Juneau, Alaska. In Asia, the latitude at which these climates begin is several degrees lower due to the pervasive 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.

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