Mesothermal

In climatology, the term mesothermal is used to refer to certain forms of climate found typically in the Earth's Temperate Zones. It has a moderate amount of heat, with winters not cold enough to sustain snow cover. Summers are warm within oceanic climate regimes, and hot within continental climate regimes.

The term is derived from two Greek words meaning "having a moderate amount of heat." This can be misinterpreted, however, since the term is actually intended to describe only the temperature conditions that prevail during the winter months, rather than those for the year as a whole.

Under the broadest definition, all places with an average temperature in their coldest month that is colder than 18°C, but warmer than −3°C, are said to have a mesothermal climate. In some climate classification schemes, however, this is divided into two segments, with a coldest-month average of 6°C being the line of demarcation between them; then only those locations with a coldest-month temperature of between −3°C and 6°C are reckoned as mesothermal, the label "subtropical" being applied to areas where the average temperature in the coldest month ranges from 6°C to 18°C.

Observing the narrower definition articulated above, the mesothermal locations are those where the winters are too cold to allow year-round photosynthesis, but not cold enough to support a fixed period of continuous snow cover every year.

In the USA, the northern boundary line between mesothermal and microthermal ranges is north of Juneau and Sitka at the Pacific Ocean. It goes sharply south to about 38N latitude in the Rockies, then eastward across the lower Midwest to the East Coast near Boston. The southern boundary line between mesothermal and megathermal (or tropical) is across south Florida just above Palm Beach.

Summers in these places may be hot (that is to say, having an average temperature in their warmest month of 22°C or above) or merely warm (with the warmest month averaging between 10°C and 22°C). The hot-summer, or continental, mesothermal climate is encountered exclusively in the Northern Hemisphere, in the landmass interiors of Asia and North America and along their east coasts, while the most frequently seen example of a warm to cool-summer mesothermal climate is the oceanic climates found along the west coasts of all of the world's continents, roughly equidistant between the geographical tropical and polar zones.

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