The weather outside can change within a few hours. A cool morning may become a hot afternoon, and a week of rain may follow several dry weeks. Yet people also recognise patterns that return over many years. Chennai usually receives much of its important seasonal rain later than Mumbai, western Rajasthan stays much drier than the Western Ghats, and northern India has a stronger winter than the far south.
Those longer patterns are climate. They emerge from thousands of weather events spread across the seasons. When we compare these patterns across the world, broad regions appear: warm and wet equatorial lands, dry subtropical belts, mild west-coast regions, cold continental interiors and polar margins. The boundaries between them are gradual, but a map needs lines and names. Climate classification provides those working boundaries.
This chapter first explains what climate contains and why world climates differ. It then shows how a classification turns continuous variation into comparable regions. The letter codes come later, after the temperature and rainfall pattern behind each region is clear.
Weather changes; climate describes the longer pattern
Weather is the condition of the atmosphere at a place over a short period. Temperature, rain, cloud, wind, pressure and humidity can all change from one hour or day to the next. A thunderstorm, a cold morning and a week-long heatwave are weather events.
Climate describes how such conditions are distributed over a much longer period. It includes the usual temperature and rainfall, but it also includes their seasonal rhythm, year-to-year variation, frequency and extremes. Two places can have the same annual average temperature and still have very different climates. One may remain mild through the year, while the other has a severe winter and a hot summer.
Rainfall shows the same problem with averages. A place that receives moderate rain in most months differs from a place that receives the same annual total in one short wet season. The timing, intensity, reliability and frequency of rain matter along with the total. Climate is therefore a distribution of conditions through time, not merely an average value.
One unusual year does not create or destroy a climate. A desert can experience a damaging flood, and a humid region can suffer a drought. Such events belong inside the range of possible weather. A lasting change in the distribution is different from ordinary variability within it.
Climate normals give us a reference
A long record contains many warm, cool, wet and dry years. To compare one period with another, climatologists calculate a climate normal. A standard normal commonly averages a variable over 30 consecutive years. The period 1991–2020 is one current example.
The word *normal* does not mean that every year should match the average. It is a reference for questions such as whether a month was warmer or wetter than the chosen period. A useful normal must identify the place, the variable, the method and the years covered. A temperature normal cannot stand in for rainfall, and one station cannot automatically describe a whole region.
Thirty years is a standard length for a reference summary; it is not the definition or duration of climate. A normal does not prove that conditions have stopped changing. Older periods may remain useful when a study needs a long, consistent comparison. Recent periods may describe present conditions better. As climate changes, a normal can also change, so the reference must always be stated.
Why climates differ from place to place
Latitude sets the broad starting pattern. Near the equator, the Sun remains relatively high in the sky through the year. Toward the poles, sunlight arrives at a lower angle and the length of day changes more strongly with the seasons. Earth’s axial tilt and revolution around the Sun move the zone of greatest heating north and south during the year.
Unequal heating drives atmospheric circulation. Air rises frequently in some belts and sinks more often in others. The rain belt near the tropics shifts with the seasons, while subtropical sinking air favours dryness over many regions. Farther poleward, westerly winds, fronts and travelling cyclones distribute heat and moisture. These circulation patterns wander and vary, so they form broad tendencies rather than fixed rings.
Oceans reshape the latitude pattern. Water heats and cools more slowly than land, and moving ocean water carries energy between regions. A coast exposed to prevailing sea winds often has a smaller annual temperature range than a continental interior at the same latitude. Warm currents can support mild, moist maritime air. Cool currents can lower coastal temperatures and help maintain stable air that produces fog or low cloud but little deep rain.
Landmasses create their own contrasts. Large interiors lie far from the ocean’s moisture and moderating influence. They can become very hot in summer and intensely cold in winter. The Northern Hemisphere has vast land areas in the middle and high latitudes, while the Southern Hemisphere has much more ocean there. This difference helps explain why severe continental climates spread widely across northern North America and Eurasia but occupy little land in the south.
Relief adds another layer. Air forced up a mountain slope cools and may release rain or snow. Descending air on the other side warms and dries, creating a rain shadow. Temperature usually falls with height, while slope direction changes exposure to sunlight and wind. A tropical mountain can therefore contain warm valleys, mild middle slopes and tundra or permanent ice near its summit.
Latitude, circulation, ocean influence, continentality and relief act together. A cold current does not create every west-coast desert by itself, and a mountain does not guarantee rain on every windward slope. Moisture supply, wind direction, stability, season and local topography decide the result. This interaction produces gradual transitions, local mosaics and many exceptions to simple latitude bands.
Classification draws useful lines through gradual change
A world climate map compresses a great amount of information. It allows us to compare distant places, look for recurring relationships and connect climatic patterns with water, ecosystems and human activity. Without some grouping, every station would remain a separate case.
The gain in simplicity comes with a cost. Temperature and rainfall usually change gradually, but a classification must choose thresholds. Two nearby places on opposite sides of a threshold can receive different labels even when their climates are very similar. A small change in the reference period or input data can move the mapped boundary.
Climate classes are therefore models, not natural boxes sealed by walls. They are useful when we remember what they measure. They become misleading when a code is treated as a complete explanation of weather, vegetation, farming or society.
Different schemes answer different questions
An empirical, or effective, classification groups places by observed climatic results. It may use measured temperature, precipitation or an estimate of water balance. The Köppen–Geiger system is mainly empirical because it applies temperature and precipitation thresholds. It was designed to correspond broadly with major vegetation patterns, but it does not use a forest or grassland at one site as proof of its code.
The Thornthwaite approach is also effective, but it gives greater attention to precipitation in relation to estimated atmospheric water demand. It is useful for thinking about moisture surplus and deficit. Its calculations and classes serve different purposes from the Köppen letters.
A genetic, or process-based, classification begins with causes such as air masses, pressure belts and circulation. It can explain why a region has a particular seasonal pattern, though causes are harder to reduce to one stable world map. Applied classifications are designed for a task such as farming, building design or water planning.
Trewartha modified parts of the Köppen system, especially its treatment of middle-latitude climates. This shows that classification can be revised when a different boundary serves a different purpose. No scheme is the single natural division of Earth.
How the Köppen–Geiger system builds a code
The Köppen–Geiger system uses monthly and annual temperature and precipitation. It asks whether a place is dry relative to its warmth and rainfall season, how cold or warm its limiting months are, and whether precipitation has a marked dry season. The resulting two- or three-letter code is a compact description of that pattern.
The first letter identifies the major group. A refers to tropical climates, B to dry climates, C to temperate climates with comparatively mild winters, D to cold or continental climates, and E to polar climates. These names vary slightly among books and map legends, so the numerical rule matters more than the English label.
The five groups do not form a simple equator-to-pole ladder. Dry B climates can occur in tropical, subtropical or cold continental settings. High mountains near the equator can have C or E climates. A correct decision sequence must account for these overlaps.
Dryness is tested first
In a widely used modern version, the B test comes first. Warmer climates generally face greater atmospheric demand for water, so the dryness threshold increases with annual mean temperature. The test also considers whether most rain falls during the warmer season, the cooler season or throughout the year.
If annual precipitation falls below that calculated threshold, the place enters Group B. This is more informative than one universal rainfall cutoff. A cool steppe and a hot desert may receive similar totals but lose water at different rates. Even so, the Köppen threshold remains a proxy based on climate data, not a full measurement of soil moisture or potential evapotranspiration at every site.
The drier part of Group B receives W for desert, while the less dry part receives S for steppe. A third letter h or k separates hot and cold dry climates using mean annual temperature. Thus BWh describes a hot desert pattern, while BSk describes a cold steppe pattern. The code says nothing by itself about sand cover, rock type or land use.
Temperature separates the remaining major groups
After dry climates have been removed, Group A requires every month to have a mean temperature of at least 18°C. Group E lies at the other thermal end: its warmest month has a mean temperature no higher than 10°C. C and D occupy the broad range between tropical and polar conditions, and both must have a warmest month above 10°C.
The boundary between C and D is not identical on every Köppen map. Some implementations place it at a coldest-month mean of 0°C; others use −3°C. Under the first rule, a place at −1°C in its coldest month enters D, while under the second it can remain C. Both conventions are established, so a careful map reader checks the legend before comparing boundary regions.
The second letter usually describes precipitation seasonality, though its meaning changes by major group. In C and D climates, f indicates that no season meets the formal dry-season test, s indicates dry summer, and w indicates dry winter. These are quantitative labels. An f climate can still have a wetter season, and a w climate can receive some winter rain.
The third letter refines temperature. For C and D, a marks the hottest summer class. The b and c classes have progressively less summer warmth, while d identifies exceptionally cold winters in a few D subtypes. In B climates, h and k use annual temperature instead. The letters become useful only after the seasonal pattern is understood.
Reading a world climate map through causes
Colour is only the last step in reading a climate map. First locate the place by latitude and hemisphere. Then ask when the Sun is highest and how the main rain and pressure belts shift through the year. This establishes the broad seasonal setting.
Next examine atmospheric circulation. Persistent ascent, seasonal convergence, subtropical subsidence, monsoon flow, westerlies, fronts and mobile cyclones create different rainfall rhythms. Prevailing winds are averages, so a region can still experience days or weeks of very different flow.
Then consider whether the place lies on an eastern margin, western margin or deep inside a continent. Check the nearby ocean, current and sea-surface pattern. Finally add elevation, mountain barriers, slope exposure and local basins. The code describes the outcome of these controls; it does not produce them.
This order also explains why the same code can appear in distant regions with somewhat different histories. It reveals why a nearby mountain slope, coast or valley can depart from the broad map. We can now apply this method to the major world climatic regions.
A climates: tropical warmth with different rainfall seasons
Group A remains warm throughout the year. Annual temperature range is usually modest at low elevations because day length and solar angle vary less than in higher latitudes. Rainfall, however, can range from abundant in every month to a strong wet–dry rhythm.
The moving tropical convergence and rain belt provides the main organising idea. Regions close to its frequent influence receive moist rising air and deep convection for much of the year. Regions nearer its seasonal margins experience a wet season when it approaches and a dry season when it moves away or winds change.
Af: rain in every month
Af is the tropical rainforest climate. In the standard test, even the driest month receives at least 60 millimetres of precipitation. Frequent convergence, abundant moisture and local convection support rain through the year, though no rule says it must fall every afternoon or by only one mechanism.
Large Af regions occur in the Amazon Basin, the Congo Basin and parts of Maritime Southeast Asia. Warm seas, broad lowlands and repeated moisture recycling reinforce rainfall in many of these areas. Relief and coast orientation still create local differences.
Warmth and reliable moisture broadly favour evergreen forest, but a climate code cannot predict the exact ecosystem. Soil, flooding, drainage, disturbance and land use all matter. The detailed structure of tropical forests belongs to the study of biomes.
Am: a short dry period within a very wet regime
Am is the tropical monsoon subtype. Its driest month falls below the Af threshold, but annual rainfall remains high enough to pass a second test that relates that dry month to the total. Heavy rain alone cannot establish this class.
The pattern appears on strongly seasonal tropical coasts and monsoon margins. Examples occur across parts of South and Southeast Asia, West Africa and northern South America. Seasonal wind reversal or a major shift in convergence concentrates rain, while maritime moisture keeps the annual regime very wet. The driest season need not occur in the same calendar months everywhere.
Aw and As: a clear wet and dry season
The remaining seasonal tropical climates enter Aw or, in some implementations and regions, As. They have a dry period strong enough to miss both Af and Am. The letter w denotes dry winter, while s denotes dry summer.
These climates spread across large parts of tropical Africa, central and eastern South America, northern Australia, parts of India and mainland Southeast Asia. The annual movement of the rain belt and seasonal wind changes create the wet–dry rhythm. Distance from the sea and mountain barriers can strengthen it.
Savanna grassland, woodland and seasonal deciduous forest often occur within this climatic setting. Their tree cover varies with soil, fire, grazing and rainfall reliability, so “savanna” is neither a single vegetation density nor a cause of the climate. At tropical elevations, lower temperatures may move a place into C or E despite its latitude.
B climates: several routes lead to dryness
A region is arid when water supply remains low relative to atmospheric demand over the long term. This condition can develop under hot or cold temperatures, beside an ocean or deep inside a continent. Drought is different: it is an unusually dry period relative to the local climate.
Subtropical sinking air creates one major dry belt. As air descends, it warms and its relative humidity falls, which suppresses widespread deep cloud. This mechanism helps maintain deserts across North Africa and Southwest Asia and in parts of Australia, southern Africa and the Americas.
Some western coastal deserts lie beside cool ocean currents. The cool sea surface supports a stable lower atmosphere, while large-scale subsidence limits rising motion. Fog and low cloud may be common even when rain remains scarce. The Atacama and Namib illustrate this coastal route, but the current works with circulation and relief rather than acting alone.
Continental interiors form another route. Central Asia lies far from major oceanic moisture sources, and mountain barriers remove or block much incoming moisture. Cold winters can produce a cold desert or steppe. The Gobi therefore differs greatly from a hot subtropical desert despite sharing a B code family.
Rain shadows create dry areas behind mountains. Moist air loses water on the exposed side; descending air then warms on the leeward side. Patagonia east of the Andes and several intermontane basins show this influence. A rain shadow is relative and changes with wind direction and season.
Within the code, BW is drier than BS relative to the local dryness threshold. BS steppe often forms a transition between desert and a more humid climate, although it can occupy broad regions in its own right. The h and k letters separate hot and cold versions.
A climatic desert need not be a sand sea. It may be rocky, gravelly, salty or mountainous. Likewise, steppe is not merely the advancing edge of a desert. Substrate, vegetation and land degradation require additional evidence beyond the climatic code.
C climates: mild winters with different rainfall rhythms
Group C contains temperate climates whose winters remain above the chosen C/D boundary but below tropical warmth. Ocean influence, subtropical pressure systems, monsoons and midlatitude westerlies create several distinct patterns. C climates therefore cannot be understood as one uniform band between two latitudes.
Mediterranean climates have dry summers
Mediterranean climates develop mainly on western continental margins. During summer, the subtropical high-pressure belt shifts poleward and favours sinking, stable air. In winter, that influence weakens or moves equatorward, allowing westerlies, fronts and extratropical cyclones to bring more rain.
The pattern surrounds much of the Mediterranean Basin. Comparable climates also occur on California’s coast, in central Chile, near Africa’s south-western Cape and along parts of southern Australia. These five broad regions share a seasonal rhythm, although temperature, rainfall amount, relief and fire conditions differ.
The Köppen second letter s marks the formal dry-summer condition. Csa has a hot summer, while Csb has a warm summer under the thermal test. Drought-adapted woodland and shrub mosaics are common, but the code does not determine one vegetation type or farming system.
Humid subtropical and dry-winter temperate climates
Warm temperate eastern margins often receive moist onshore flow in summer. Convection, fronts and tropical weather systems can all contribute rain. Winters may remain wet under passing midlatitude systems or become drier where continental high pressure and seasonal wind reversal dominate.
Cfa identifies a hot-summer temperate climate without a classified dry season. Cwa identifies a hot-summer climate with a formal dry winter, while Cwb commonly marks a milder warm-summer version in monsoon-influenced highlands. Eastern Asia contains extensive examples. Related classes also appear in the south-eastern United States and on suitable eastern margins in Australia, South America and southern Africa.
The regions are not interchangeable. East Asian sites may have a powerful monsoon rhythm and winter continental flow. Other eastern margins receive a different mixture of fronts, maritime air and tropical disturbances. “Humid subtropical” does not mean constant heat or equal rain in every month.
Marine west-coast climates
On many middle-latitude western margins, prevailing westerlies carry maritime air onto land. The ocean limits severe winter cold and extreme summer heat. Fronts and mobile cyclones bring precipitation in many months, while mountains make exposed slopes wetter and leeward areas drier.
Cfb commonly describes the milder marine west-coast pattern, and Cfc a cooler version with a shorter warm season. Important regions include western Europe, the Pacific Northwest of North America, southern Chile, New Zealand and Tasmania. These areas differ in latitude, relief and seasonal rainfall despite their related codes.
Not every west coast has this climate. Subtropical west coasts may be Mediterranean or desert, and polarward coasts can be much colder. Latitude, prevailing wind, current, relief and land shape must be considered together.
D climates: cold winters over large northern lands
Group D has a warmest month above 10°C but a coldest month at or below the chosen C/D boundary. It covers broad parts of northern North America and Eurasia. Comparable latitudes in the Southern Hemisphere contain too little large continental land for an equally extensive D belt.
Distance from the ocean allows strong seasonal heating and cooling. Long winter nights, snow cover and cold air outbreaks can produce severe winters, while long summer days can still create warm or hot interiors. Annual temperature range is generally greater than on maritime coasts at the same latitude.
The second letter again describes rainfall seasonality. Df has no season that meets the formal dry test, although summer and winter totals can still differ. Dw has a pronounced dry winter. This pattern is important in eastern and north-eastern Asia, where cold continental winter flow contrasts with summer maritime or monsoon moisture.
Humid continental climates commonly use Dfa or Dfb. They combine cold winters with hot or warm summers and occur in parts of eastern North America, Europe and Asia. Fronts and cyclones bring precipitation, while summer convection adds rain in many regions.
Mixed and temperate forests occur broadly across many of the warmer, moister D regions. Climate creates an important limit, but soil, disturbance and land use still shape the actual vegetation.
Toward colder interiors and higher latitudes, the warm season shortens. Dfc and the more extreme Dfd belong to the subarctic family. Long cold winters and brief cool summers broadly support boreal forest, but moisture, soil, fire and permafrost vary. The code does not require continuous permafrost.
Some textbooks use named regions such as Siberian, Laurentian or taiga climate. These labels can help locate a pattern, but they should not replace the code and mechanism. One named region can cross thresholds, and similar D climates occur outside it.
E climates: even the warmest month stays cold
Group E is defined by summer warmth rather than latitude or permanent snow. The warmest month averages no more than 10°C. This rule allows polar climates on high mountains far from the polar circles and prevents every snowy winter region from being called polar.
ET is tundra climate. Its warmest month rises above 0°C but no higher than 10°C in the selected implementation. A short thaw can support low vegetation in many places, but warmth remains too limited for normal tree growth. ET spreads along many Arctic coasts and islands and appears above the tree line on high mountains.
Permafrost is widespread in many tundra regions, but it is not the ET test. Maritime tundra and alpine settings may differ greatly in ground temperature, snow cover and drainage. Seasonal snow at a lower latitude also does not create an E climate if summers are warm enough.
EF is ice-cap climate. Even its warmest month averages at or below 0°C. The largest areas lie over the interiors of Antarctica and Greenland, with smaller high-altitude occurrences elsewhere.
Very cold air usually carries little water vapour, so polar interiors can have low annual precipitation. Ice persists where accumulation exceeds melting, sublimation and ice flow loss over time. An ice cap is therefore not evidence of heavy snowfall in every year.
Mountains compress climates into vertical mosaics
Many school maps use H for highland climate because one colour is easier to show across a mountain chain. H is useful as an added mapping category, but it is not a universally original sixth main group of the standard Köppen–Geiger system. Mountain stations can still receive A, B, C, D or E codes from their measured temperature and precipitation.
Elevation lowers temperature through the average vertical structure of the atmosphere, but one fixed lapse rate cannot describe every slope and season. A sun-facing slope receives different energy from a shaded slope. Windward and leeward sides differ in moisture, and cold air can collect in a valley.
These controls create rapid change over short horizontal distances. A tropical valley can lie below a mild C climate, with ET or EF near the summit. Mountains in East Africa, the Andes, the Rockies and the Himalayas all contain such vertical mosaics, but their rainfall and slope patterns are not identical.
A coarse global grid can smooth away a narrow valley, ridge or coastal strip. A detailed map may show several classes where a smaller-scale map shows one. Greater pixel detail also does not remove uncertainty if observations remain sparse.
Climate influences life without deciding it alone
Temperature and water availability set broad limits on plant growth. Seasonal warmth affects the growing period, while rainfall amount and timing influence soil moisture. Köppen’s broad correspondence with vegetation makes the system useful for regional study.
The link is never mechanical. Tropical wet climates can contain different forests because of soil, flooding, disturbance and history. Fire and grazing alter tropical seasonal vegetation. Dry climates can support intensive farming where rivers, groundwater and irrigation supply water.
Agriculture also depends on crops, technology, labour, markets and policy. Settlement reflects water, relief, disease, transport, livelihoods and historical choices along with climate. A code can identify opportunities and constraints; it cannot predict a society.
The same caution applies to soils and biomes. Their geography often follows climate broadly, but local parent material, drainage, organisms and time create important departures. Those subjects need their own complete explanations.
India shows several patterns within one monsoon system
India provides a useful application because one large seasonal circulation interacts with distance, relief and altitude. Much of lowland India remains warm enough for Group A, but the length and strength of the dry season divide very wet, monsoon and tropical wet–dry subtypes. The detailed mechanism, onset and variability of the Indian monsoon require a separate lesson.
The windward Western Ghats and parts of the north-east receive abundant rain, while leeward interiors are drier. Much of central and peninsular India has a strong wet–dry rhythm. The north-west includes steppe and desert climates because precipitation remains low relative to warmth and seasonal demand.
Northern plains often appear under warm temperate or dry-winter codes in classifications that use the 18°C coldest-month threshold for Group A. Himalayan climates change rapidly with altitude and exposure, producing temperate, cold and polar patches. A single label such as “monsoon climate” or “mountain climate” hides these contrasts.
The exact Indian boundary changes with the classification version, normal period, station or grid data and topographic treatment. Köppen mapping can describe the broad pattern, while India’s seasons, rain distribution and regional mechanisms need fuller treatment of their own.
Climate-map boundaries depend on data and method
A climate map begins with observations. Stations are unevenly distributed, especially across mountains, deserts, forests and polar regions. Map makers may interpolate between them, combine several data sets or adjust a grid for elevation. Each choice affects the result.
The reference period matters because a different set of years can change monthly averages. Spatial resolution matters because a large grid merges local contrasts. The selected Köppen rules matter too, as the 0°C and −3°C C/D boundary shows. Near any threshold, modest differences can change a code.
Climate change can also alter the temperature and precipitation values used by a classification. A mapped class may shift when a threshold is crossed, but the physical climate has been changing continuously before that line moves. Ecosystems and land use may respond later or in different ways because soil, disturbance and human decisions have their own timescales.
This does not make classification useless. It tells us how to use it honestly. We should state the scheme and reference period, inspect the legend, treat narrow borders cautiously and compare the code with the physical controls of the region.
From a code to an explanation
When you meet an unfamiliar climatic region, first describe its actual rhythm. Ask how temperature changes through the year, when precipitation arrives, how reliable it is and whether a dry season exists. Then locate the region by latitude, hemisphere, continental position and elevation.
Next connect the rhythm to circulation, ocean influence and relief. Only after that should you decode the letters. If the region is dry relative to warmth and rainfall seasonality, test B first. Otherwise examine tropical, temperate, cold and polar thresholds, then the precipitation and temperature modifiers.
The final explanation should return from the code to the place. A climate class is a compact map language. Understanding comes from seeing how sunlight, air circulation, ocean, land and mountains combine to produce the pattern that the letters describe.