Climate zones: the Köppen groups

A place where every month averages 18 °C or more is tropical in the Köppen system; one where no month reaches 10 °C is polar.

From sunlight to climate

The first units showed that the amount of sunlight a place receives over the year depends mostly on latitude: steady and strong near the equator, strongly seasonal at mid-latitudes, extreme near the poles. Climate is the long-term pattern of weather, usually measured over 30 years, and it follows that sunlight pattern in broad bands. The bands are far from neat, though. A city on a cold ocean current, behind a mountain range or high on a plateau can have a very different climate from another city at the same latitude.

To compare climates around the world, the climatologist Wladimir Köppen built a classification from two things that are easy to measure and that plants respond to: monthly average temperature and monthly precipitation. Updated by Rudolf Geiger, the Köppen-Geiger system is still in wide use. A 2018 study in the journal Scientific Data mapped it at 1 km resolution and describes it as five main classes and 30 sub-types.

The five main groups

The groups are checked in a fixed order. First ask whether the place is dry enough to be B, because a desert can be hot or cold. If not, look at temperature. If even the coldest month averages 18 °C or more, it is A, tropical. If even the warmest month stays below 10 °C, it is E, polar; 10 °C is roughly the warmth that trees need in summer, so E climates are treeless tundra or ice. Between those, the coldest month decides: above 0 °C is C, temperate, and below 0 °C is D, continental. Some versions use −3 °C instead of 0 °C as the C and D boundary.

Second and third letters

More letters describe when the rain falls and how hot the summer is. In group A, f means rain in every month (rainforest), m means monsoon and w means a dry winter (savanna). In group B, W is desert and S is steppe (semi-arid), with h for hot and k for cold. In C and D, f means no dry season, s a dry summer and w a dry winter, followed by a for a hot summer (a month above 22 °C), b for warm and c for cool. In E, T is tundra and F is ice cap. So Cfb reads "temperate, no dry season, warm summer", the climate of much of western Europe.

The dry test for group B uses a threshold that grows with temperature, because warm air evaporates more water. Multiply the mean annual temperature in °C by 20, then add 280 if at least 70% of the rain falls in the summer half of the year, 140 if 30% to 70% does, or nothing if less than 30% does. If annual precipitation in millimetres is below half of that threshold, the climate is BW, desert; between half and the full threshold, it is BS, steppe. For a place averaging 20 °C with mostly summer rain, the threshold is 20 × 20 + 280 = 680 mm.