earth science

Thornthwaite climate classification

Thornthwaite climate classification is a earth science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Thornthwaite climate classification rather than just read about it. In short: 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 types of vegetation: rainforest, forest, grassland, steppe, and desert. He posited that one of the main factors for the local vegetation is precipitat…

Key takeaways

Thornthwaite climate classification belongs to earth science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Thornthwaite climate classification to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Thornthwaite climate classification from memory before moving on to harder problems.

Reference excerpt

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 types of vegetation: rainforest, forest, grassland, steppe, and desert. He posited that one of the main factors for the local vegetation is precipitation, but most importantly, precipitation effectiveness—that is to say how much moisture a plant receives relative to what it needs. Thornthwaite based the effectiveness of precipitation on an index, P / E {\displaystyle P/E} , which is the sum of the 12 monthly P/E ratios. The monthly P/E ratios can be calculated using the formula:

P / E m = total monthly precipitation monthly evapotranspiration {\displaystyle P/E_{m}={\frac {\text{total monthly precipitation}}{\text{monthly evapotranspiration}}}}

Temperature efficiency Similarly to precipitation effectiveness, Thornthwaite also developed an index to represent thermal efficiency, featuring six climate provinces: tropical, mesothermal, microthermal, taiga, tundra and frost. The thermal efficiency index, I ′ {\displaystyle I'} , is the sum of the 12 monthly thermal efficiency ratios i m {\displaystyle i_{m}} , which can be calculated as:

i m ′ = t − 32 4 {\displaystyle i'_{m}={\frac {t-32}{4}}} , where t {\displaystyle t} is the mean monthly temperature in °F (set to 32 if below 32).

1948 modification After being criticized for the empirical basis of his previous climate classification on vegetation, making it unnecessarily complex, Thornthwaite drew away from vegetation as a defining criterion and introduced the concept of potential evapotranspiration (PET), which both represents thermal efficiency and is ultimately used for the computation of precipitation effectiveness as indicated by the moisture index. He calculated PET using his own 1948 equation. Thornthwaite developed four indices: the Moisture Index I m {\displaystyle I_{m}} , the aridity and humidity indexes ( I a {\displaystyle I_{a}} and I h {\displaystyle I_{h}} ), the Thermal Efficiency Index ( T E {\displaystyle TE} ) and the Summer Concentration of Thermal Efficiency ( s or S C T E {\displaystyle s{\text{ or }}SCTE} ). Each of the four are ascribed a letter of the English alphabet. The order, in which the class-denoting letters follow, varies. Thornthwaite himself used “moisture type–thermal efficiency type–moisture seasonality–summer concentration type” (e.g., C2B’2rb’2 for Manhattan, KS, a moist subhumid, second mesothermal climate with little water deficiency and a temperature-efficiency regime normal to second mesothermal). In Latin America, where the classification is sometimes employed, the first two letters are used to describe the precipitation pattern and the last two are used to describe the thermal regime. For example, Tracuateua, B3s2A’b’4, features a humid (B3) megathermal (A’) climate with a large summer water deficit (s2) and in which between 48% and 52% of potential evapotranspiration occurs in the summer (b’4).

Moisture Index

The Moisture Index (Im) expresses the overall moisture of an environment and is directly obtained from the aridity and humidity indexes. If there is excess water in one season, it will be stored in the soil and may be used by plants in another when moisture is deficient (provided that the roots are deep enough to reach it), thus offsetting the effects of drought. Thornthwaite found that every six inches of water surplus counteract a water deficiency of ten and devised a composite index that reflects this. This index can be calculated as I m = I h − 0.6 ⋅ I a {\textstyle I_{m}=I_{h}-0.6\cdot I_{a}} , where Ih and Ia are the humidity and aridity indices, 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:

I a = ( D P E T ) ⋅ 100 {\textstyle {\mathit {I_{a}}}=\left({\frac {D}{\mathit {PET}}}\right)\cdot 100} ,

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Thornthwaite climate classification

Start with the simplest possible case. Write down what Thornthwaite climate classification claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In earth science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Thornthwaite climate classification before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Thornthwaite climate classification ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Thornthwaite climate classification

In research: Thornthwaite climate classification appears in earth science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Thornthwaite climate classification in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Thornthwaite climate classification is common in secondary-school and first-year university syllabi. It links to neighbouring topics Climate and weather classification systems, so understanding it makes those chapters shorter.
In everyday life: Look for Thornthwaite climate classification outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Thornthwaite climate classification in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Thornthwaite climate classification means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Thornthwaite climate classification out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Thornthwaite climate classification in simple terms?

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 types of vegetation: rainfores…

Why does Thornthwaite climate classification matter?

Because it connects several earth science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Thornthwaite climate classification?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Thornthwaite climate classification.