Put climographs side by side and patterns appear: places with similar latitudes and settings (what sets a place's climate) have similar climates. In 1900 Wladimir Köppen sorted the world's climates into groups by temperature and precipitation, partly from the plants that grow in each. Versions of his scheme are still the standard.
Key idea
Köppen's five main groups: E, polar (warmest month below 10 °C); B, dry (yearly precipitation below a threshold that rises with temperature); A, tropical (coldest month 18 °C or warmer); C, temperate (coldest month between −3 °C and 18 °C); D, continental (coldest month −3 °C or colder, warmest at least 10 °C). Check them in that order: E, then B, then A, C or D. (Some U.S. books use 0 °C instead of −3 °C between C and D.)
Why it works
Each line marks a change that plants and people notice. Trees need a warmest month of about 10 °C, so polar climates are treeless tundra and ice. Dryness depends on more than rainfall: hot places lose water to evaporation faster, so the dry threshold rises with the average temperature, about $20 \times (\text{mean temperature in °C}) + 280$ mm when most of the precipitation falls in summer (+140 when it is spread out, +0 when it falls mostly in winter). Below half the threshold the climate is desert (BW); between half and the full threshold, steppe (BS).
| Place | Coldest month | Warmest month | Yearly precipitation | Group |
|---|---|---|---|---|
| Singapore | 26.5 °C | 28.5 °C | 2180 mm | A (tropical) |
| Phoenix | 13 °C | 35.5 °C | 190 mm | B (dry) |
| London | 5.5 °C | 19 °C | 605 mm | C (temperate) |
| Moscow | −6 °C | 19 °C | 710 mm | D (continental) |
| Utqiaġvik, Alaska | −27 °C | 5 °C | 112 mm | E (polar) |
Worked example: Classify a town
A town's warmest month is 24 °C and its coldest −8 °C. It averages 9 °C over the year and gets 800 mm of precipitation, about two thirds in summer. Which group?
- Warmest month 24 °C: at least 10 °C, so not E.
- Dry threshold (spread-out precipitation): $20 \times 9 + 140 = 320$ mm. 800 mm is more, so not B.
- Coldest month −8 °C is −3 °C or colder: D, continental.
Answer: D (continental)
Worked example: A dry place
A city averages 22 °C, with 23 mm of precipitation a year, mostly in winter. Which group?
- Warmest month is well above 10 °C, so not E.
- Dry threshold (mostly winter): $20 \times 22 + 0 = 440$ mm. 23 mm is far below half of it (220 mm).
- B: a hot desert (BWh), like Cairo.
Answer: B (desert)
Common mistake
Calling any hot place a desert. A desert is defined by too little precipitation for its temperature, not by heat: Singapore is hot and very wet, while Antarctica is so dry that it is the largest desert on Earth. Check the precipitation against the dry threshold, not the temperature alone.