Climates are classified primarily by measuring long-term temperature and precipitation patterns at a location, then sorting those measurements into defined groups. The most widely used system, Köppen-Geiger, divides Earth’s land climates into five major groups using letter codes A through E. But it’s not the only approach. Scientists also classify climates by the air masses that drive weather patterns, by native vegetation, or by ecological variables like how much water evaporates relative to how much rain falls.
Two Fundamentally Different Approaches
Climate classification systems fall into two broad categories. Empirical systems start with observed, measurable data: temperature readings, rainfall totals, what plants grow where. The Köppen-Geiger system is the best-known example. Genetic systems, by contrast, classify climates based on what causes the weather patterns, such as the types of air masses that dominate a region. Class names in genetic systems often reference geography: polar, tropical, continental, marine.
Both approaches have trade-offs. Empirical systems are straightforward to apply anywhere you have weather station data, but they can lump together places that feel very different in daily life. Genetic systems capture the “why” behind a climate but are harder to map consistently across the globe. In practice, the empirical approach dominates modern climate science and education.
The Köppen-Geiger System
Developed in the late 1800s and refined multiple times since, Köppen-Geiger remains the standard. It assigns every location a two- or three-letter code based on temperature and precipitation thresholds. The five major groups are:
- Group A (Tropical): Every month of the year averages 18 °C (64 °F) or higher, with significant precipitation. This group covers rainforests, monsoon regions, and tropical savannas.
- Group B (Dry): The only group defined by dryness rather than temperature. Whether a place qualifies depends on a formula that weighs annual precipitation against temperature and seasonal timing. If rainfall falls below a calculated threshold, it’s classified as desert (BW) or steppe (BS).
- Group C (Temperate): The coldest month averages between 0 °C (32 °F) and 18 °C (64 °F), and at least one month averages above 10 °C (50 °F). This covers a huge swath of the inhabited world, from humid subtropical regions to oceanic climates.
- Group D (Continental): Similar to Group C but with colder winters. The coldest month drops below 0 °C, while the warmest month still reaches above 10 °C. Much of the northern United States, Canada, Russia, and Scandinavia falls here.
- Group E (Polar): No month averages above 10 °C. This includes both tundra (where the warmest month stays between 0 °C and 10 °C) and ice cap climates (where no month rises above freezing).
How the Letter Codes Work
The first capital letter identifies the major group. A second lowercase letter describes precipitation patterns: “f” means wet year-round, “s” means a dry summer, “w” means a dry winter, and “m” means monsoon conditions. A third lowercase letter captures temperature nuances: “a” for a hot summer, “b” for a warm summer, “c” for a cool summer, and “d” for extremely cold winters.
So a place coded “Cfa” has a temperate climate (C), with year-round precipitation (f) and hot summers (a). That describes cities like Atlanta, Buenos Aires, and Shanghai. A code of “Dfc” means continental (D), wet year-round (f), with cool summers (c), which fits much of interior Scandinavia and central Canada.
For Group A, the second letter distinguishes tropical rainforest (Af), which gets at least 60 mm (about 2.4 inches) of rain every single month, from monsoon (Am) and savanna (Aw/As) climates, where at least one month dips below that threshold. The distinction between Am and Aw comes down to whether the driest month still receives more than 4% of the year’s total rainfall.
Group B uses “W” for true desert and “S” for semiarid steppe, then adds “h” (hot) or “k” (cold) to indicate whether average temperatures stay above or below freezing in the coldest month. The Sahara is BWh. The Gobi is BWk.
Where Köppen Falls Short
The system’s biggest weakness is that its temperature boundaries sometimes group places together that feel nothing alike. Under standard Köppen rules, Washington state and Southern California both land in the same “Csb” zone, even though their weather, vegetation, and daily experience are strikingly different. London gets classified in the same C group as Brisbane and New Orleans, despite enormous differences in seasonal warmth and native plant life.
These quirks prompted Glenn Trewartha to modify the system in the mid-20th century. His version splits the middle latitudes into three groups based on how many months average above 10 °C: subtropical (8 or more months), temperate (4 to 7 months), and boreal (1 to 3 months). This better captures the practical differences between, say, the U.S. Gulf Coast and the Pacific Northwest. The Trewartha system keeps the same desert and steppe definitions as Köppen but drops the thermal subcategories within the dry group.
Classification by Air Masses
A completely different tradition classifies climates by the types of air masses that dominate a region. This approach, rooted in work by the meteorologist Tor Bergeron in 1930, identifies air masses by their source region (polar, tropical, or temperate) and their moisture content (continental/dry or maritime/moist). The result is categories like Dry Polar, Moist Tropical, or Dry Temperate.
Modern versions of this idea, like the Spatial Synoptic Classification used in the United States, identify six distinct air mass types by analyzing temperature and humidity data from weather stations. This kind of classification is especially useful for health and environmental research, where the combined effect of heat and humidity matters more than either variable alone.
Classification by Ecology
The Holdridge Life Zones system takes a biological angle. Instead of asking “how hot and wet is it?” the way Köppen does, Holdridge asks what kind of ecosystem the climate supports. It uses three variables: mean annual biotemperature (which only counts temperatures between 0 °C and 30 °C, the range where plants actively grow), total annual precipitation, and the ratio of potential evapotranspiration to precipitation. That ratio captures whether a landscape loses more water to evaporation than it gains from rain.
By plotting these three variables against each other, the system maps locations into life zones like “subtropical dry forest” or “boreal wet forest.” Ecologists and conservation scientists favor this approach because it connects climate directly to the biological communities it supports, rather than relying on arbitrary temperature cutoffs.
Modern High-Resolution Mapping
Climate classification has gotten dramatically more precise in recent years. A 2023 study published in Scientific Data produced global Köppen-Geiger maps at 1 km resolution for the period 1901 through 2020, using seven independent climate datasets for temperature and precipitation. These maps also project classifications forward to 2099 using climate model outputs, letting researchers see not just where climate zones are now but where they’re shifting.
The maps reveal something the original system was never designed to show: climate zones are moving. Tropical zones are expanding poleward, temperate zones are encroaching on continental territory, and some polar classifications are shrinking. A location’s three-letter code in 1930 may no longer match its code in 2020, making these updated maps essential for agriculture, urban planning, and ecosystem management.