In climatology, the term mesothermal is used to refer to certain forms of climate found typically in the Earth's temperate zones.
The isotherm of −3 °C (27 °F) 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.
In the US, the northern boundary line between mesothermal and microthermal ranges is north of Juneau and Sitka along the margins of the Pacific Ocean.
It then goes southeastward around about 42N latitude across the north-central US (not including mountain regions) then eastward across the lower Midwest to the East Coast near Boston.
In addition to being subdivisible by summer temperature, mesothermal climates can also be subclassified on the basis of precipitation — into humid, seasonal, semiarid and arid subtypes.