The Thornthwaite climate classification is a
climate classification system created by American climatologist
Charles Warren Thornthwaite in 1931 and modified in 1948.
1931 classification
Precipitation effectiveness
Thornthwaite initially divided climates based on five characteristic vegetations:
Rainforest,
forest
A forest is an area of land dominated by trees. Hundreds of definitions of forest are used throughout the world, incorporating factors such as tree density, tree height, land use, legal standing, and ecological function. The United Nations' ...
,
grassland
A grassland is an area where the vegetation is dominated by grasses ( Poaceae). However, sedge ( Cyperaceae) and rush ( Juncaceae) can also be found along with variable proportions of legumes, like clover, and other herbs. Grasslands occur na ...
,
steppe and
desert. One of the main factors for the local vegetation is precipitation, but most importantly, precipitation effectiveness, according to Thornthwaite. Thornthwaite based the effectiveness of precipitation on an index (the P/E index), which is the sum of the 12 monthly P/E ratios. The monthly P/E ratio can be calculated using the formula:
Temperature efficiency
Similarly to precipitation effectiveness, Thornthwaite also developed a T/E index to represent thermal efficiency. Featuring six climate provinces:
Tropical,
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 span of temperature, with winters not cold enough to sustain snow cover. Summers are warm within oc ...
,
microthermal
In climatology, the term microthermal is used to denote the continental climates of Eurasia and North America.
Etymology
The word ''microthermal'' is derived from two Greek words meaning "small" and "heat". This is misleading, however, since the t ...
,
taiga,
tundra
In physical geography, tundra () is a type of biome where tree growth is hindered by frigid temperatures and short growing seasons. The term ''tundra'' comes through Russian (') from the Kildin Sámi word (') meaning "uplands", "treeless mou ...
and
frost.
The T-E index is the sum of the 12 monthly T-E ratios, which can be calculated as:
, where t is the mean monthly temperature in
°F.
1948 modification
After being criticized for making climatic classification complex, Thornthwaite switched vegetation with the concept of
potential evapotranspiration (PET), which represents both precipitation effectiveness and thermal efficiency.
Estimated PET can be calculated using
Thornthwaite's own 1948 equation.
Thornthwaite developed four indices: the Moisture Index (Im), the aridity and humidity indexes (Ia/Ih), the Thermal Efficiency Index (TE) and the Summer Concentration of Thermal Efficiency (SCTE). Each of the four climatic types can be described by an
English alphabet
The alphabet for Modern English is a Latin-script alphabet consisting of 26 letters, each having an upper- and lower-case form. The word ''alphabet'' is a compound of the first two letters of the Greek alphabet, ''alpha'' and '' beta''. ...
letter and are arranged exactly by the order shown previously.
The first two letters are used to describe the precipitation pattern and the last two are used to describe the thermal regime.
As an example, B3s2A’b’4 (
Tracuateua) describes a wet (B3), megathermal (A’) climate with a large summer water deficit (s2) and which more than 48% but less than 52% of the potential evapotranspiration is felt in the summer (b’4).
Moisture Index
The Moisture Index (Im) expresses the global
moisture
Moisture is the presence of a liquid, especially water, often in trace amounts. Small amounts of water may be found, for example, in the air (humidity), in foods, and in some commercial products. Moisture also refers to the amount of water vapo ...
of the environment and is directly related with the
aridity and humidity indexes. The driving factor in this system is the water budget of a region.
Humidity classes range from Arid to Perhumid (Thoroughly Humid).
This index can be calculated as
, where ''Ih'' and ''Ia'' are the humidity and aridity indexes, respectively.
Seasonal Variation of Effective Moisture
The Seasonal Variation of Effective Moisture is described by two indexes: The Aridity Index (''Ia''), used in wet climates to identify and quantify the severity of drought conditions, and the Humidity Index (''Ih''), used in dry climates to identify and quantify the severity of wet conditions.
These indexes are represented by the equations:
,
, where ''D'' is the annual water deficit, ''S'' is the annual water surplus, and ''PET'' is the annual
potential evapotranspiration
Furthermore, these indices are represented by four letters, which indicate the seasonal distribution of precipitation: r (constantly rainy), d (constantly dry), s (summer deficit or surplus) and w (winter deficit or surplus) and two numbers to indicate the severity.
Wet climates (A, B, C2) can be classified as:
*r (Without or with low deficit) : 0 ≤ ''Ia'' < 16.7
*s (Moderate summer deficit) : 16.7 ≤ ''Ia'' < 33.3 and the deficit in the summer is larger than in the winter
*w (Moderate winter deficit) : 16.7 ≤ ''Ia'' < 33.3 and the deficit in the winter is larger than in the summer
*s2 (Large summer deficit) : ''Ia'' ≥ 33.3 and the deficit in the summer is larger than in the winter
*w2 (Large winter deficit) : ''Ia'' ≥ 33.3 and the deficit in the winter is larger than in the summer
Dry climates (C1, D, E) can be classified as:
*d (Without or with low surplus) : 0 ≤ ''Ih'' < 10
*s (Moderate summer surplus) : 10 ≤ ''Ih'' < 20 and the surplus in the summer is larger than in the winter
*w (Moderate winter surplus) : 10 ≤ ''Ih'' < 20 and the surplus in the winter is larger than in the summer
*s2 (Large summer surplus) : ''Ih'' ≥ 33.3 and the surplus in the summer is larger than in the winter
*w2 (Large winter surplus) : ''Ih'' ≥ 33.3 and the surplus in the winter is larger than in the summer
The deficiency of water in the soil is calculated as the difference between the potential evapotranspiration and the
actual evapotranspiration.
Thermal efficiency
The thermal efficiency index (TE) is defined as the annual potential evapotranspiration (PET)
and has five different classifications:
Megathermal In climatology, the term ''megathermal'' (or less commonly, ''macrothermal'') is sometimes used as a synonym for ''tropical''. It comes from the two greek words "mega" and "thermal" meaning "big" and "heat". In order for a particular place to qualif ...
,
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 span of temperature, with winters not cold enough to sustain snow cover. Summers are warm within oc ...
,
microthermal
In climatology, the term microthermal is used to denote the continental climates of Eurasia and North America.
Etymology
The word ''microthermal'' is derived from two Greek words meaning "small" and "heat". This is misleading, however, since the t ...
,
tundra
In physical geography, tundra () is a type of biome where tree growth is hindered by frigid temperatures and short growing seasons. The term ''tundra'' comes through Russian (') from the Kildin Sámi word (') meaning "uplands", "treeless mou ...
and
perpetual ice.
Summer Concentration of Thermal Efficiency
The Summer Concentration of Thermal Efficiency (SCTE) is a measure of the summer's potential evapotranspiration and can be calculated as
, where ''PET1'', ''PET2'' and ''PET3'' are the estimated values of PET for the three hottest consecutive months.
References
{{Reflist
Climate and weather classification systems