From Wikipedia, the free encyclopedia

In climatology, the term mesothermal is used to refer to certain forms of climate found typically in the Earth's Temperate Zones.

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.

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-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.

In addition to being subdivisible by summer temperature, mesothermal climates can also be subclassified on the basis of precipitation — into humid, semiarid and arid subtypes.

Megathermal
Microthermal Mike Jones

Applied Climatology, John Griffiths (for use of 6°C in the coldest month as poleward limit of subtropical climates)