3.1°S 60.0°WManaus, Brazil
Weather versus climate
Weather is what happens today: a storm, a hot afternoon, a cold snap. Climate is the average and range of weather over many years. Climatologists usually work with 30-year averages, called climate normals, so one unusual season does not change a place's climate type.
The five main Köppen groups
The system was developed by the climatologist Wladimir Köppen and later refined by others, so thresholds differ slightly between versions. The table gives the general rules.
| Group | Name | Basic rule | Example place |
|---|---|---|---|
| A | Tropical | Every month averages at least 18°C (64°F) | Manaus, Singapore |
| B | Dry | Evaporation can outpace precipitation over the year | Cairo, Phoenix |
| C | Temperate | Coldest month between about 0°C (or -3°C) and 18°C | London, Rome |
| D | Continental | Coldest month below about 0°C (or -3°C), warmest above 10°C (50°F) | Moscow, Chicago |
| E | Polar | Warmest month below 10°C (50°F) | Greenland's ice sheet, Antarctica |
Some maps add a highland group (H) for mountains, where climate changes over short distances with height.
Reading the letter codes
A second and third letter add detail about rain and heat. A few common ones:
- Af: tropical rainforest, wet all year. Am: tropical monsoon. Aw: tropical savanna with a dry season.
- BW: desert. BS: steppe, a semi-arid grassland. A final h or k means hot or cold.
- Cfb: oceanic, mild with rain spread through the year, like much of western Europe. Csa: Mediterranean, with hot, dry summers. Cfa: humid subtropical.
- Dfb: humid continental with warm summers. ET: tundra. EF: ice cap.
What controls a place's climate
- Latitude. Near the equator the sun is high all year and heating is strong. Toward the poles sunlight arrives at a lower angle and is spread thinner.
- Global wind and pressure belts. Warm air rises near the equator and drops its moisture as rain. It sinks again around 30° north and south, which is why many of the great deserts sit near those latitudes.
- Ocean currents. Warm currents like the Gulf Stream keep western Europe milder than places at the same latitude in eastern Canada.
- Distance from the sea. Water warms and cools slowly, so coasts have milder seasons. Places deep inside continents get hotter summers and colder winters.
- Altitude. Air cools with height, by about 6.5°C for every 1,000 m on average (about 3.5°F per 1,000 ft).
- Mountains. Air rising over a range drops rain on the windward side and leaves a dry rain shadow behind. See mountain ranges.
Using climate zones to plan a trip
For a trip, the climate type tells you more than a single average temperature. In tropical savanna climates, the wet and dry seasons matter more than heat. In Mediterranean climates, summers are reliably dry. South of the equator, the seasons are reversed: July is winter in Buenos Aires and Cape Town. Mountain areas inside any zone can be much colder than nearby lowlands.
Climate maps describe averages over a past period. As the climate changes, some boundaries are shifting, so newer maps may place a city in a different type than an older textbook does.
Climate zone FAQ
How many climate zones are there?
The Köppen system has five main groups and around 30 subtypes. Simpler school schemes use three to five broad zones, such as tropical, dry, temperate, continental and polar.
Is a desert always hot?
No. A desert is defined by dryness. The Gobi has very cold winters, and Antarctica's interior is a cold desert because it gets so little precipitation.
What climate zone has the most people?
Temperate and tropical climates hold most of the world's population, because they support farming. Exact shares depend on the dataset and the map version.