Koeppen's classification of world climates

ET is tundra climate: the warmest month has a mean temperature of at least 0°C but below 10°C. This permits some seasonal surface thaw. EF is ice-cap climate, where even the warmest month remains below 0°C, so sustained thaw is generally absent. EF is therefore colder, and neither code describes a hot desert or tropical monsoon climate.

Köppen codes are read hierarchically. The first letter identifies the broad group, such as A for tropical, B for dry, C and D for temperate or continental climates, and E for polar climates. The second letter generally describes seasonal precipitation, while a third letter may indicate summer or winter temperature characteristics.

Köppen’s dry group B is identified by the relationship between precipitation and evaporative demand, not by temperature alone. A place is dry when available precipitation is insufficient compared with the moisture that could be lost through evaporation and transpiration. This principle separates desert climates BW from semi-arid steppe climates BS.

Csb is a Mediterranean-type warm-summer climate. C identifies a temperate climate, s indicates a dry summer, and b indicates a warm or moderate summer rather than an extremely hot one. Thus Csb has mild winters, a distinctly dry summer, and moderate summer temperatures. It is neither polar, desert, nor tropical rainforest climate.

Both Dfa and Dfb have D for continental climate and f for precipitation in all seasons, so winter dryness is not the main distinction. The final letter is thermal: a indicates a hotter summer, while b indicates a warm but less intense summer. Thus the most useful difference is summer temperature intensity, represented by the third letter.

EF is the permanent ice-cap climate, in which even the warmest month remains below 0°C. Because conditions are already close to the freezing threshold, a small rise in temperature can increase surface melting, reduce snow and ice persistence, and alter albedo. These changes can amplify warming and affect glaciers, ice sheets, and sea-level processes.

Mountain regions contain strong environmental contrasts over short distances. Temperature falls with altitude, slopes receive different amounts of solar radiation, and mountains create windward and leeward rain-shadow effects. A single Köppen code based on averaged data may therefore hide several local microclimates. Local elevation, aspect, relief, and precipitation patterns must be considered for a fuller interpretation.

The Köppen Aw type is tropical savanna climate. It remains warm throughout the year but has strongly seasonal rainfall, usually with a wet summer and a comparatively long dry winter. This seasonal moisture regime supports grasses and scattered trees rather than dense evergreen rainforest. Af has no distinct dry season, while Am has a shorter dry interval.

In Köppen notation, B identifies dry climates, S identifies steppe or semi-arid conditions, and h indicates a hot thermal subtype. Therefore, BSh means hot semi-arid steppe. BWk is a cold desert, Csa is a Mediterranean climate, and ET is tundra. Reading each letter prevents confusion between dryness and temperature.

Köppen group A is defined mainly by tropical temperatures and sufficient moisture, while group B is defined by aridity or water deficit. Thus a hot region may belong to B rather than A if evaporation demand exceeds precipitation. B is not a polar group, and A climates can receive abundant rainfall.

In the Köppen system, C climates have mild winters, whereas D climates have severe or cold winters. The distinction is therefore primarily based on winter temperature, especially the temperature of the coldest month and the duration of cold conditions. Summer rainfall, autumn fog, and spring winds do not define the basic C–D boundary.

The Cs climate, especially the Mediterranean type, has dry summers and wetter winters. Crops must therefore be selected and scheduled around winter precipitation, stored soil moisture, and summer irrigation needs. Af is wet throughout the year, BSk is cold semi-arid, and Dfc has a cool continental regime rather than the Mediterranean seasonal pattern.

Cwa is a humid subtropical climate code. C identifies a mild mid-latitude climate, w indicates a dry winter, and a indicates a hot summer in which the warmest month averages at least 22°C. Csb has a dry summer, Cfb has no distinct dry season, and BSk is cold semi-arid.

In Köppen codes, the second letter often describes the seasonal distribution of precipitation. f means there is no distinct dry season, s means the summer is dry, and w means the winter is dry. The letter f has no connection with polar ice; polar conditions are represented by E, with ET and EF as important subtypes.

The third letter b denotes a warm-summer subtype in both Cfb and Dfb. The warmest month has a mean temperature below 22°C, while at least four months generally average above 10°C. By contrast, the letter a indicates a hotter summer with the warmest month averaging 22°C or more. The first letter identifies the major climate group.

ET and EF are the two principal polar Köppen subtypes. ET, tundra, has a warmest-month mean temperature above 0°C but below 10°C. EF, ice cap, remains at or below 0°C even in the warmest month. Thus the thermal condition of the warmest month separates tundra from permanent ice-cap climate.

A shift from ET tundra to EF ice-cap climate requires the warmest-month mean temperature to fall to 0°C or below. That colder condition greatly reduces or eliminates seasonal thaw, preventing tundra vegetation from surviving. Increased rainforest, hot dry summers, or maritime moderation would not produce this polar transition.

A coastal location is not automatically humid. Cold offshore currents cool and stabilize the lower atmosphere, reducing evaporation and cloud growth; persistent subtropical high pressure can also produce descending dry air. Together these factors may create a coastal desert, such as the Atacama, represented by BWh: hot desert climate.

Köppen classification describes climate, not a single year's weather. It uses long-term averages and recurring seasonal patterns of temperature and precipitation. One exceptional year may include drought, extreme rain, or unusual heat, but such an event may not represent normal conditions. Temperature is a central criterion, so option B is also false.

Af is the tropical rainforest climate: temperatures remain high and rainfall is abundant in every month, so dense, multilayered evergreen forest can grow. EF is an ice-cap climate, BWh is hot desert, and Csa is Mediterranean with dry summers; none of those conditions supports the vegetation stated in the other options.

Aw is the tropical savanna climate, characterized by a distinct wet season and dry season. Seasonal drought limits dense forest growth, while the warm wet season supports grasses and allows scattered drought-tolerant trees to survive. Cfb is oceanic, EF is ice cap, and Dfc is a cool continental climate, so they do not fit this vegetation pattern.

Within Köppen group B, the second letter W or S identifies desert or steppe dryness, while the final letter h or k identifies the thermal condition. h means hot, generally with a mean annual temperature of at least 18°C; k means cold, generally below that threshold. Thus BWh and BSh are hot, whereas BWk and BSk are cold.

The five principal Köppen groups are based mainly on temperature and moisture. A represents tropical climates, B dry climates, C temperate or warm mid-latitude climates, D cold or snow climates, and E polar climates. Additional letters then describe precipitation seasonality and thermal details, making the code more specific than the main letter alone.

The second letter separates these two climates: s means a dry summer, whereas f means no distinct dry season. Therefore Csb is more likely where summers are clearly dry and most precipitation falls in the cooler season. The final b indicates a warm but not excessively hot summer; it is not the feature that distinguishes Csb from Cfb.

In the Köppen system, f means that no distinct dry season occurs, so Cfa has precipitation throughout the year. In Cwa, w indicates dry winter; Aw has a dry season in the tropical savanna regime; and BSk is cold semi-arid steppe. The second letter therefore provides the decisive clue about rainfall seasonality.