Climate zones

Every place has its own usual weather. That is its climate.

Places with the same kind of climate are put together in a group. Each group is a climate zone. Plants and animals are different in each zone.

There are thousands of places on Earth, each with its own climate. To make sense of them, geographers sort them into a few big groups called climate zones. Places in the same zone have similar temperatures and similar amounts of rain, even if they are far apart.

One of the best-known ways of sorting climates was made by Wladimir Köppen, a scientist who studied both plants and climate, at the end of the 1800s. He noticed that plants and climate go together: warm, rainy places grow forests, and places with little rain tend to be deserts. So he used plants as clues and drew a map of the world's climate zones. Scientists still use updated versions of his map today.

National Geographic describes the Köppen system as one of the most common climate classification systems in the world. Wladimir Köppen, a German botanist and climatologist, developed it at the end of the 19th century, building on earlier biome research that showed vegetation and climate are closely linked: the plants that grow in a region depend on its temperature and precipitation.

Köppen kept revising his map until his death in 1940, and later climatologists, including Rudolf Geiger, updated it, which is why it is often called the Köppen-Geiger classification. A 2018 study by Hylke Beck and colleagues in Scientific Data mapped it at about 1-kilometer resolution for 1980–2016. Their version has five main classes and 30 sub-types, defined by thresholds and seasonal patterns of monthly temperature and precipitation. The authors describe vegetation as "crystallized, visible climate": places in the same class share similar plant life.

There are five main climate zones.

Tropical places are hot and often wet. Dry places get little rain. Temperate places are mild. Continental places have cold winters. Polar places are icy. High mountains are cold too.

In the Köppen system, each main zone has a letter:

A is tropical, the zone near the Equator. B is dry: places where little rain falls, such as deserts and dry grasslands. C is temperate, with milder winters. D is continental, with colder winters. E is polar: tundra and lands of snow and ice. Some versions add H for highland: the cold climates high on mountains.

Each zone is split into smaller types. For example, some tropical places have rain all year, and others have a dry season.

National Geographic lists the zones as A, tropical or equatorial; B, arid or dry; C, warm or mild temperate; D, continental; and E, polar. Most are defined by temperature; B is defined by aridity, so it can overlap the others, and Beck and colleagues give B precedence when it does. Each zone is subdivided: A into Af (no dry season), Am (a short dry season) and Aw (a winter dry season); B into hot and cold deserts (BWh, BWk) and hot and cold steppes (BSh, BSk); C and D by when the dry season falls and by how cool the summer or mild the winter is; and E into tundra (ET) and ice cap (EF). Some modern versions add H for highland climates.

The details of the thresholds vary between versions. For example, Beck and colleagues separate temperate (C) from cold (D) climates with a 0 °C threshold for the coldest month, warmer than the boundary Köppen used.

The Sun shines strongest near the middle of the Earth. So places there are hot.

Near the North and South Poles, the sunshine is weak. So those places are cold.

The biggest thing that decides a place's climate is how much sunlight it gets. Earth is round like a ball, so the Sun heats places near the Equator more than places near the poles. Near the Equator, sunlight hits the ground almost straight on. Farther north or south, the same sunlight comes in at a slant and spreads out over more ground, like a flashlight shining at an angle.

NASA has worked out that, on the days in March and September when day and night are equal, the tropics get about 90 percent as much solar energy as the Equator. Halfway to the poles, places get about 70 percent, and near the Arctic and Antarctic Circles only about 40 percent.

Latitude sets the basic pattern. NASA explains that because Earth is a sphere, the Sun heats equatorial regions more than polar ones: from the Equator toward the poles, sunlight meets the surface at smaller and smaller angles and spreads over larger areas. On the equinoxes, the tropics (0 to 23.5°) receive about 90 percent of the solar energy that reaches the Equator, the mid-latitudes (45°) about 70 percent and the Arctic and Antarctic Circles about 40 percent. At middle and high latitudes, the energy also varies a great deal from season to season.

The atmosphere and ocean continually work to even out this imbalance, moving heat poleward through evaporation, convection, rainfall, winds and ocean circulation, which NASA calls Earth's heat engine. Air heated at the Equator rises and moves poleward, forming the circulation cells that NOAA links to belts of low and high pressure, so latitude helps shape rainfall as well as temperature.

The sea changes the climate too. Warm sea water can make a coast warmer.

High mountains are colder than low land. Winds can bring rain, or keep it away.

Other things change a place's climate too. The ocean soaks up lots of the Sun's heat, especially in the tropics. Ocean currents carry warm water toward the poles and cold water back toward the Equator. Without them, NOAA says, the Equator would be much hotter, the poles much colder, and much less of Earth's land would be good to live on.

Height matters too. As you climb a mountain, the air usually gets about 6.5 °C colder for every 1,000 meters. That is why Nairobi, in Kenya, almost on the Equator but about 1,680 meters up, has usual daytime highs of about 21 to 26 °C, while lowland Singapore's are about 30 to 32 °C. And the winds and belts of high and low air pressure decide where rain falls.

The ocean is the second great climate control. NOAA Ocean Exploration explains that the ocean absorbs most of the Sun's radiation, particularly in tropical waters, and redistributes it: evaporation carries heat and moisture into the air, and outside the equatorial zone, weather patterns are driven largely by ocean currents, which carry warm water poleward and cold water back. Without them, regional temperatures would be far more extreme and much less land would be habitable.

Elevation cools climates at the standard lapse rate of about 6.5 °C per 1,000 m (UCAR). Compare two places near the Equator in WMO data: Nairobi, at about 1,680 m, has mean daily maxima of about 20.6 to 25.6 °C, while Singapore, near sea level, has 30.5 to 32.4 °C. Finally, the general circulation sets moisture: NOAA notes low-pressure bands at the Equator and 50° to 60° latitude and high-pressure bands near 30° and at the poles, and many deserts lie along 30° latitude.

Singapore is near the Equator. It is hot every month, and it rains a lot.

Cairo is in a desert. It is hot in summer, and it hardly ever rains. Both places are warm, but one is wet and one is dry.

Singapore, near the Equator, has a tropical climate. Its weather service says it has lots of rain, high and steady temperatures and humid air all year round. Its usual daytime highs stay between about 30 and 32 °C in every month, rain falls on about 171 days a year, and about 2,113 millimeters of rain falls in a year.

Cairo, in Egypt, is in the dry zone. Its usual summer highs are about 35 °C, but only about 25 millimeters of rain falls in a whole year. National Geographic calls land that gets no more than 25 centimeters a year a desert, and Cairo gets only a tenth of that. Both cities are warm, but the amount of rain makes their climates completely different, and that decides what can grow there.

Singapore illustrates a tropical climate. The Meteorological Service Singapore describes it as typically tropical, with abundant rainfall, high and uniform temperatures and high humidity all year, and little month-to-month variation in temperature; its 1991–2020 mean annual rainfall is 2,113.3 mm, with rain on an average of 171 days. It also has two monsoon seasons: the Northeast Monsoon from December to early March and the Southwest Monsoon from June to September.

Cairo illustrates the dry zone, which Köppen defined by aridity rather than temperature. WMO normals for 1981–2010 give it about 25 mm of rain a year, mostly from November to March, and essentially none from June to August, when mean daily maxima are about 35 °C. National Geographic defines a desert as land receiving no more than 25 cm (10 inches) of precipitation a year, so Cairo receives about a tenth of that limit. The B group is subdivided into hot and cold deserts and steppes: cold deserts exist too.

London has mild weather. Winters are cool, and summers are warm.

Chicago has freezing winters and hot summers. In the far north, Iqaluit in Canada is cold almost all year. Only a few months are above freezing.

Moving away from the tropics, climates get cooler, and sunshine changes more from season to season. London has a mild climate: WMO figures show usual daytime highs of about 8 °C in January and about 23 °C in July, with rain in every month. Chicago, far from the ocean in the middle of North America, has a much bigger swing: its usual January high is just below freezing and its usual July high is about 29 °C.

In the polar zone, it is cold almost all year. In Iqaluit, in Arctic Canada, the usual daytime high is about −22.5 °C in January and only about 11.6 °C in July. Polar climates include the tundra and lands of snow and ice.

Temperate (C) and continental or cold (D) climates are both mid-latitude climates, separated by how cold the winter gets. WMO normals (1981–2010) show the contrast: London's mean daily maxima run from about 8.1 °C in January to 23.4 °C in July, with about 558 mm of rain spread over every month, while Chicago's run from −0.6 °C in January to 28.9 °C in July, with January minima of −8.6 °C.

Polar (E) climates are cold all year. Köppen's E group divides into tundra (ET) and snow and ice (EF). Iqaluit's 1971–2000 WMO normals show mean daily maxima of −22.5 °C in January and 11.6 °C in July, with January minima of −30.6 °C and only about 412 mm of precipitation a year. Highland climates (H), added in some versions, cover mountainous elevations, where the lapse rate brings cold even near the Equator.

Climate zones can change over a very long time.

Scientists make maps of the zones. They check how the zones are changing as the world gets warmer.

Climate zones are not fixed forever. They depend on temperature and rain, so when those change, the zones can shift. Scientists study this with careful maps.

In 2018, a team of scientists made a very detailed map of the world's climate zones for 1980 to 2016. They also used computer models to map where the zones might be by the years 2071 to 2100 if the world keeps warming a lot. Maps like these help scientists study where plants and animals may be able to live.

Because the Köppen-Geiger classes are defined by temperature and precipitation thresholds, a changing climate moves their boundaries. Beck and colleagues' 2018 study paired its 1-km present-day map (1980–2016) with a map for 2071–2100 built from 32 climate model projections under a high-emissions scenario (RCP8.5), each with a confidence map showing how much the inputs agree. They note the classification is widely used in ecology and climate-impact studies, for example to model where species and vegetation can live.

Such projections are scenarios, not forecasts: the outcome depends on future emissions, and the study's confidence maps show where the classification is uncertain. The present-day map has confidence levels too; they are generally lower near the borders between climate zones, especially at high latitudes.