Scientists sort the world’s climates into five main zones: tropical (hot all year, like Singapore), dry (very little rain, like Cairo in Egypt), temperate (mild winters and warm summers, like London), continental (snowy winters and warm summers, like Moscow) and polar (cold all year, like Antarctica). This is the Köppen system, and each zone has a capital letter: A, B, C, D and E. A climate is not the same as the weather. Weather is what happens today, like a rainy Tuesday. Climate is the usual pattern over many years, measured with averages, usually over 30 years. That is why every number below comes from long-term records, most of them for 1991 to 2020.
The system is named after Wladimir Köppen, a German-Russian scientist who lived from 1846 to 1940. He first published it in 1884. Köppen had studied plants, and he noticed that the kinds of plants growing in a place tell you a lot about its climate. So his zones match the plants, too: rainforest trees in zone A, cactuses and bare sand in zone B, and almost no plants at all on the ice of zone E. Each zone has a clear rule, so anyone with a thermometer and a rain gauge can work out which zone a place belongs to. Let’s visit them one by one.
Zone A, tropical: Singapore and Manaus, where every month is warm
The tropical zone has one simple rule. Every single month must have an average temperature of 18 °C or more. That means there is no cold season at all. Most tropical places lie close to the equator, the imaginary line around the middle of the Earth. There, the Sun climbs high in the sky at noon all year round, so the ground keeps heating up.
Singapore, a small island country in Southeast Asia, is a perfect example. Its average afternoon temperature never drops below 30.5 °C or climbs above 32.4 °C. Children there never need a winter coat, because there is no winter. It is also very wet. Singapore gets about 2,113 mm of rain in a year. In December alone it gets about 332 mm. If you collected that in a tall jar, the water would be deeper than a 30 cm school ruler is long!
Manaus, a city in Brazil, sits in the middle of the Amazon rainforest. It gets about 2,362 mm of rain a year. The Amazon rainforest covers about 6 million square kilometres, and about 60 per cent of it is in Brazil. All that warmth and water lets plants grow all year, which is why the tropics hold the world’s biggest rainforests.
Not every tropical place is soaking wet all year. Some tropical places, such as Darwin in Australia, have a wet season and a dry season instead of summer and winter. In Darwin, June, July and August together bring less than 4 mm of rain. But all those places still stay warm, so they are still zone A.
Zone B, dry: Cairo, the Sahara and the Atacama, where rain is a surprise
Zone B is not about heat. It is about water. A place is dry when the Sun and wind could dry up more water than the rain brings. Scientists use a formula with the average temperature and the yearly rain to check this. If a place gets less than half of the rain it “needs,” it is a desert. If it gets between half and all of it, it is semi-arid, which means partly dry, like a grassy steppe.
Cairo, the capital of Egypt, is a desert city. In a whole year it gets only about 25 mm of rain. That is about as deep as your thumb is wide! From May to September there is usually no rain at all. In August, the hottest month, the average temperature is 29.2 °C, and afternoons average about 35 °C. People in Cairo can live there because the River Nile brings water from far away.
Cairo sits on the edge of the Sahara, the largest hot desert in the world. The Sahara covers about 9.2 million square kilometres, which is nearly as big as the whole United States. In its sand seas, called ergs, the wind piles sand into huge dunes.
Deserts can also hold records. The official record for the hottest air temperature, 56.7 °C, was set at Furnace Creek in Death Valley, California, on 10 July 1913. And the Atacama Desert in Chile is the driest desert outside the polar regions. It gets around 15 mm of rain a year on average, and some weather stations there have never recorded rain at all. The Atacama is dry because mountains on both sides block wet air from reaching it.
Zone C, temperate: London’s mild middle ground
Temperate means “not too extreme.” In zone C, the coldest month averages somewhere between 0 °C and 18 °C, so winters are cool but not freezing for long. At least one month must average more than 10 °C, so there is a proper warm season too. Most temperate places lie between the tropics and the cold north or south, often near an ocean.
London, the capital of the United Kingdom, is a classic temperate city. Its type is called oceanic, because winds from the Atlantic Ocean keep it mild. In January, the coldest month, the average temperature is about 5.6 °C. In July, the warmest month, it is about 19 °C. That is a gap of only about 13 degrees between winter and summer. Snow does fall sometimes, but it rarely stays for long.
London is famous for rain, but it actually gets only about 615 mm a year, less than a third of Singapore’s. The trick is that London’s rain is spread out. Its wettest month, November, gets about 67 mm, and its driest, March, still gets about 39 mm. So a London child might need an umbrella in any month of the year. Zone C also includes the sunny Mediterranean climate of places like Rome in Italy, where summers are hot and dry and the rain comes in winter.
Zone D, continental: Moscow and Yakutsk, from snowballs to sunburn
Continental climates are found deep inside big continents, far from the sea. Water warms up and cools down slowly, so the ocean keeps coastal places mild. Inland places do not get that help. They freeze hard in winter and warm up fast in summer. In zone D, at least one month averages below 0 °C and at least one month averages above 10 °C.
Moscow, the capital of Russia, is a good example. In January the average temperature is about −6.2 °C. In July it is about 19.7 °C, a little warmer than London. Snow usually starts falling in mid-October, the ground stays white from late November, and the snow melts at the end of March. That is about five months of snowball season every year.
Yakutsk, a city in eastern Siberia, shows how extreme zone D can be. Its January average is −36.9 °C, but its July average is +19.9 °C. On the hottest summer days the temperature can pass 30 °C. The difference between the coldest and hottest temperatures ever recorded there is more than 100 degrees! The ground under Yakutsk is permanently frozen. This frozen soil is called permafrost, and Yakutsk is the largest city built on it. Many houses stand on concrete legs, called piles, so their heat does not melt the ground beneath them.
Here is a puzzle for you. Zone D is almost all in the Northern Hemisphere. Why? Continental climates need big areas of land far from the sea, at the right distance from the equator. In the south, there is very little land between 40° and 60° south, just the narrow tip of South America, Tasmania and New Zealand. It is mostly ocean there, so the Southern Hemisphere has only a few high mountain spots with zone D climates.
Zone E, polar: Longyearbyen and Vostok Station, never warmer than 10 °C
The polar zone has the strictest rule of all. No month, not even in summer, may average more than 10 °C. Near the North and South Poles, the Sun stays low in the sky, so its light arrives at a slant and spreads out. Snow and ice also bounce much of that light back into space. Zone E includes the icy tundra, where only mosses, small flowers and low shrubs can grow, and the ice caps, where nothing grows. High mountains can be zone E too, even near the equator, because the air gets colder the higher you climb.
Longyearbyen, on the island of Spitsbergen in Norway’s Svalbard islands, is one of the world’s most northerly towns. Its warmest month, July, averages only about 7 °C. In February, even the afternoon average is −8.3 °C. In winter the Sun does not rise at all. This is called polar night, and in Longyearbyen it lasts 111 days, from 27 October to 15 February. In summer the opposite happens: the midnight sun shines for 128 days, from 18 April to 24 August, so it is light even at bedtime.
Life there has some unusual rules. People are advised to carry a rifle when they leave town, in case they meet a polar bear. And every year, around 8 March, the town holds a Sun Festival Week to celebrate when sunlight first reaches most of the town again, after the long dark winter.
Antarctica is colder still. At Vostok Station, a research base built by the Soviet Union in 1957, scientists measured −89.2 °C on 21 July 1983. That is the lowest reliably measured natural temperature on Earth. Yet some animals thrive in the cold. Emperor penguins breed in the coldest place of any bird, in air as cold as −40 °C, with winds up to 144 km/h. They huddle together in big groups to share warmth.
Now you know all five zones, A to E. Next time you look at a map, try to guess each place’s letter from how far it is from the equator, how close it is to the sea and how high it is. Then dive deeper into the wettest zone in our guide to rainforests for kids, the driest in deserts for kids, and the coldest in our tour of the Arctic for kids.