What it does
The Köppen climate classification sorts the world’s climates into groups using nothing more than the average temperature and precipitation of each month. It is the system behind the familiar codes on climate maps: Af for tropical rainforest, BWh for hot desert, Csa for a Mediterranean climate, Cfb for an oceanic one, Dfb for a humid continental one, ET for tundra.
Give this tool the 12 monthly means for a place and it works out the class, showing every test it applied and the numbers it compared, so you can see exactly why a place is, say, Csb rather than Cfb. It also draws a Walter-Lieth climate diagram, the compact chart ecologists and geographers use to show the length of the dry season at a glance.
How to use it
- Pick a made-up example, or choose My own data and paste 12 rows (January to December): the month, the mean temperature and the total precipitation. You can also paste two rows of 12 values, or open a CSV or Excel file. A row for the year or annual total is skipped.
- Choose °C and mm or °F and inches.
- Give a station name and elevation for the diagram if you like.
- Optional settings:
- C and D split. Peel and colleagues (2007) put the boundary between temperate (C) and continental (D) climates at a coldest month of 0 °C; Köppen’s original used −3 °C. Places with a coldest month between the two change class.
- Hot or cold dry climates. By default a dry climate is h (hot) when the mean annual temperature is 18 °C or more. Some maps use the coldest month above 0 °C instead.
- Diagram start. Southern hemisphere diagrams usually start in July so the summer sits in the middle.
How the class is found
- E (polar): the warmest month is below 10 °C. ET if it is above 0 °C, EF if not. This is checked first so ice caps are not called deserts.
- B (dry): the annual precipitation is below 10 × Pthreshold, where Pthreshold = 2 × mean annual temperature, plus 28 if 70 % or more of the precipitation falls in the summer half, or plus 14 if neither half gets 70 %. BW (desert) below 5 × Pthreshold, otherwise BS (steppe).
- A (tropical): the coldest month is 18 °C or more. Af if every month has 60 mm; Am if the driest month has at least 100 − annual/25 mm; otherwise Aw (or As when the driest month falls in summer).
- C or D: by the coldest month. Second letter: s if the driest summer month has under 40 mm and under a third of the wettest winter month; w if the driest winter month has under a tenth of the wettest summer month; otherwise f. Third letter: a if the warmest month reaches 22 °C, b if four or more months are above 10 °C, d if the coldest month is below −38 °C, otherwise c.
The summer half is whichever of April–September or October–March is warmer, so the rules work in both hemispheres.
Reading the Walter-Lieth diagram
Temperature (red) and precipitation (blue) share one chart, with 20 mm of precipitation level with 10 °C. Where the precipitation line drops below the temperature line, the month is dry (red dots); above it the month is wet (blue hatching). Months above 100 mm are drawn at a tenth of the scale, in solid blue.
Worked example
The made-up Mediterranean station has a mean annual temperature of 17.5 °C and 550 mm of precipitation, 81 % of it in winter. Pthreshold is 2 × 17.5 = 35 mm, and 550 mm is well above 10 × 35 = 350 mm, so it is not dry. The coldest month is 10 °C (between 0 and 18): C. The driest summer month has 2 mm, below 40 mm and below a third of the wettest winter month (95 / 3 = 31.7 mm): s. The warmest month is 26 °C: a. The result is Csa, Mediterranean with a hot summer, and the diagram shows five dry months, May to September.
FAQ
Where do I get monthly normals? National weather services publish 30-year climate normals for their stations. Use the means for 1991–2020 if you can.
Why does my map show a different class? Maps use gridded data and sometimes different rules (the −3 °C boundary, As versus Aw, h/k by temperature). Try the options above.
Where the numbers come from
- Köppen-Geiger rules: Peel, M. C., Finlayson, B. L. and McMahon, T. A. (2007), Updated world map of the Köppen-Geiger climate classification, Hydrology and Earth System Sciences 11, 1633 to 1644 (the published criteria, written into code by fibub; no maps or data copied).
- Original C/D boundary of −3 °C and the h/k split by the coldest month: Köppen's classification as commonly described in climatology textbooks.
- Climate diagram: the layout of Walter and Lieth (1960 to 1967), 10 °C to 20 mm, with precipitation above 100 mm drawn at a tenth of the scale.
- All example stations are made up by fibub for illustration; they are not real places.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-08.