Thornthwaite

Thornthwaite

See also: Microthermal, Mesothermal, and Megathermal

Precipitation by month

Devised by the American climatologist and geographer C. W. Thornthwaite, this climate classification method monitors the soil water budget using evapotranspiration.^[11] It monitors the portion of total precipitation used to nourish vegetation over a certain area.^[30] It uses indices such as a humidity index and an aridity index to determine an area's moisture regime based upon its average temperature, average rainfall, and average vegetation type.^[31] The lower the value of the index in any given area, the drier the area is.
The moisture classification includes climatic classes with descriptors such as hyperhumid, humid, subhumid, subarid, semi-arid (values of −20 to −40), and arid (values below −40).^[32] Humid regions experience more precipitation than evaporation each year, while arid regions experience greater evaporation than precipitation on an annual basis. A total of 33 percent of the Earth's landmass is considered either arid of semi-arid, including southwest North America, southwest South America, most of northern and a small part of southern Africa, southwest and portions of eastern Asia, as well as much of Australia.^[33] Studies suggest that precipitation effectiveness (PE) within the Thornthwaite moisture index is overestimated in the summer and underestimated in the winter.^[34] This index can be effectively used to determine the number of herbivore and mammal species numbers within a given area.^[35] The index is also used in studies of climate change.^[34]