World climates are organized using a letter-based system that sorts every location on Earth into one of five major groups based on temperature and precipitation. The most widely used framework is the Köppen-Geiger classification, developed in the late 1800s and still the global standard. It assigns each region a two- or three-letter code that tells you, at a glance, how hot or cold it gets and how much rain falls there.
The Five Main Climate Groups
The system starts with five broad categories, each labeled with a capital letter from A through E. These groups move roughly from the equator toward the poles, though geography and elevation create plenty of exceptions.
- A: Tropical. Hot and humid year-round, with average temperatures above 64°F (18°C) every month and more than 59 inches of precipitation per year. Think rainforests, monsoon coasts, and tropical savannas.
- B: Dry. Evaporation outpaces rainfall. This group covers both scorching low-latitude deserts and cooler mid-latitude steppes. It’s the only group defined primarily by lack of moisture rather than temperature.
- C: Temperate. Warm, humid summers (often with thunderstorms) and mild winters. This covers much of the southeastern United States, western Europe, and parts of eastern Asia.
- D: Continental. Warm to cool summers paired with harsh winters that can plunge below -22°F (-30°C). These climates dominate interior North America and northern Eurasia, where distance from the ocean allows extreme seasonal swings.
- E: Polar. Every month averages below 50°F (10°C). Even the warmest summer days stay cool, and large areas remain locked under ice year-round.
How the Letter Codes Work
A single capital letter only tells part of the story. The system adds a second (and sometimes third) lowercase letter to capture precipitation patterns and temperature details. The second letter describes when and how much rain a place receives:
- f = wet year-round
- s = dry summer season
- w = dry winter season
- m = monsoon (a pronounced wet season with a short dry spell)
So “Af” means a tropical climate with rain spread evenly through the year, which is an equatorial rainforest. “Aw” means a tropical climate with a dry winter, which is a savanna. The code is compact, but it carries a lot of information once you know the pattern.
For dry climates (Group B), the second letter indicates severity: “W” for true desert (where evaporation is more than double the precipitation) and “S” for steppe (drier than normal but not full desert). A third letter, “h” or “k,” tells you whether it’s a hot, low-latitude version or a cooler, mid-latitude one. BWh is a subtropical desert like the Sahara. BSk is a mid-latitude steppe like the grasslands of Montana, where winters drop below freezing.
Tropical Climates Up Close
All tropical climates share one rule: every month averages above 64°F (18°C). What separates the subtypes is rainfall. Equatorial rainforests (Af) have no dry season at all, with the driest month still receiving at least 2.36 inches (60 mm) of rain. Monsoon climates (Am) get drenched during a pronounced wet season but have one or more months that dip below that 2.36-inch threshold. Savanna climates (Aw or As) have longer dry stretches, with more than two months below 2.36 inches.
If you’ve visited both Singapore and central Tanzania, you’ve felt the difference between Af and Aw firsthand: constant humidity versus a landscape that alternates between lush green and parched brown.
Dry Climates: Desert Versus Steppe
Group B is the only category defined by moisture balance rather than temperature. The dividing line between desert and steppe comes down to how severely evaporation exceeds rainfall. In a true desert (BW), evaporation runs more than double what falls as rain. In a steppe (BS), it still exceeds precipitation, but by a smaller margin, leaving enough moisture for grasses and shrubs to survive.
Temperature then splits each type in two. Subtropical deserts (BWh) like the Sahara average above 64°F and rarely see frost. Mid-latitude deserts (BWk) like the Gobi sit below that 64°F average and freeze in winter. The same hot/cool distinction applies to steppes, giving four dry subtypes in all.
Temperate and Continental Climates
Groups C and D together cover most of the world’s densely populated regions. The boundary between them hinges on winter cold. Temperate (C) climates have coldest-month averages that stay above roughly -3°C (about 27°F), while continental (D) climates drop below that line. Both require at least one month averaging above 50°F (10°C), which ensures a growing season exists.
Within each group, the same lowercase letters apply. A “Cfa” climate is temperate, with no dry season and a hot summer. That describes cities like Atlanta, Buenos Aires, and Shanghai. A “Dfb” climate is continental, wet year-round, with a warm (but not hot) summer. That’s the climate of Moscow, Minneapolis, and much of Scandinavia. The third letter (a, b, c, or d) ranks summer warmth, from “a” (hottest) down to “d” (extremely cold winters found only in Group D places like interior Siberia).
Polar Climates
Group E has the simplest threshold: no month averages above 50°F (10°C). Within that, two subtypes matter most. Tundra climates (ET) have at least one month between 32°F and 50°F, warm enough for mosses, lichens, and low shrubs but not trees. Ice cap climates (EF) never rise above freezing at all. Every month averages below 32°F (0°C), and the landscape is permanent ice and snow, as seen in interior Greenland and Antarctica.
Some versions of the system add an “H” designation for highland climates, where altitude creates conditions similar to polar zones even at lower latitudes. A mountain peak near the equator can have ice cap conditions just miles from a tropical rainforest in the valley below.
Alternative Classification Systems
The Köppen-Geiger system is dominant, but it isn’t the only option. The Trewartha classification is a well-known revision that redraws the boundaries between temperate and continental climates. In the original Köppen system, Group C (temperate) and Group D (continental) are separated by whether the coldest month falls above or below -3°C. Trewartha instead counts how many months average above 10°C (50°F). A subtropical climate (Trewartha’s C) needs 8 to 12 such months, while a temperate climate (Trewartha’s D) needs only 4 to 7. Trewartha also adds a distinct group for boreal climates with just 1 to 3 warm months, a zone that Köppen lumps into Group D.
The practical difference: some regions that Köppen labels “temperate” get reclassified under Trewartha in ways that better match what people actually experience on the ground. Neither system is wrong. They just draw the lines in different places depending on which temperature thresholds they prioritize.
Climate Zones Are Shifting
These classification boundaries aren’t fixed. As global temperatures rise, climate zones are migrating toward the poles. Modeling studies project that by 2099, roughly 76% of the boreal region (the cold northern forests) could reach growing conditions warm enough for cereal crops, compared to just 32% today. The leading edge of that agricultural climate zone is projected to shift northward by up to 1,200 kilometers (about 750 miles) by the end of the century.
Latitudinal shifts, moving north or south across the map, are far outpacing altitudinal shifts up mountainsides. That means climate maps drawn today will look noticeably different within a few decades, with dry zones expanding and temperate zones pushing into what are currently continental and boreal regions. The letter codes stay the same, but the areas they cover on the map are in motion.