Dry Climates: B

What makes a climate “dry”

A dry climate is defined not by heat but by a shortage of water. The book’s core idea: rainfall is so low that it cannot support the growth of plants. That single fact — inadequate precipitation for vegetation — is the defining feature of the entire B group. These climates are not small pockets; they cover a very large area of the planet, stretching across latitudes from 15° to 60° north and south of the equator.

Where dry climates occur and why

The location of dry climates is not random. It follows a clear pattern tied to global pressure belts, ocean currents, and mountain barriers.

At low latitudes, roughly 15° to 30°, dry climates sit in the zone of the subtropical high pressure. Here, descending air causes subsidence and temperature inversion, and both suppress rainfall. The air is sinking, warming, and drying — clouds do not form easily, and precipitation is rare.

On the western margins of continents, the dry zone pushes closer to the equator. This happens where cold currents flow along the coast, especially over the west coast of South America. The cold water chills the air above it, stabilising the atmosphere and further reducing the chance of rain, so the dry climate extends equatorwards and occupies the coastal land.

In middle latitudes, from 35° to 60°, the situation is different. Dry climates here are confined to the interiors of continents, where maritime-humid winds from the oceans cannot reach. These areas are often surrounded by mountains, which block moisture-bearing winds and leave the interior dry.

Classification of dry climates

Dry climates are first divided into two broad types:

• Steppe or semi-arid climate (BS) — slightly wetter, transitional
• Desert climate (BW) — truly arid

Each of these is further subdivided by latitude:

So the letter code tells you two things at once: how dry it is (S for steppe, W for desert) and where it sits (h for subtropical, k for mid-latitude).

Subtropical steppe (BSh) and subtropical desert (BWh)

These two climates share common precipitation and temperature characteristics, so the book treats them together.

The subtropical steppe lies in the transition zone between humid and dry climates. Because of this position, it receives slightly more rainfall than the desert — just enough to support the growth of sparse grasslands. The desert, by contrast, gets too little rain for even that.

Rainfall in both climates is highly variable. This variability is not a minor detail; it has serious consequences. In the steppe, the variability affects human life far more than in the desert, and it more often causes famine. The reason is simple: the steppe’s agriculture and grazing depend on that thin margin of rainfall, so when it fails, the impact is severe. The desert, being already dry, has less to lose. …