Group B

Dry climates

Too little rain for the heat. Deserts, steppes, and some very cold nights.

Tall orange sand dunes under a deep blue sky in the Namib Desert — Dunes near Sossusvlei in the Namib Desert, Namibia.Picture: Vicartb, CC0, Commons

The dry belt

Dry and semi-dry climates cover about a quarter of Earth’s land, mostly in the belt between 15° and 30° latitude. Encyclopaedia Britannica: Köppen climate classification

Group B is the odd one out: it is defined by dryness, while every other group is defined by temperature. Encyclopaedia Britannica: Köppen climate classification Dry climates are split into arid (BW) and semi-arid (BS), with h added for warm and k for cold. Encyclopaedia Britannica: Köppen climate classification

Why deserts form

Sinking air on either side of the tropics makes near-permanent high pressure over the subtropics. Skies stay almost cloud-free all year, and deserts form. Met Office: Climate zones

Most of Earth’s true tropical deserts lie between 15° and 30° latitude, where the Hadley cell’s air comes down. Encyclopaedia Britannica: tropical and subtropical desert climate The driest deserts are best developed on the western sides of continents. Encyclopaedia Britannica: tropical and subtropical desert climate

Look at a globe and you’ll see lots of deserts along 30° north and south, including the American Southwest, northern Africa and Australia. NOAA JetStream: Global atmospheric circulations

Diagram of Earth with lines at 0, 30 and 60 degrees north and south and three loops of air in each hemisphere, labelled Hadley, Ferrel and Polar, with high pressure and deserts at 30 degrees and low pressure at the equator and 60 degrees — Our drawing of the three circulation cells: air sinks around 30°, where many deserts lie.Drawing: our own

Desert weather

Low-latitude deserts mostly get 0–25 cm of rain a year, but how unreliable it is matters more than the small totals. Encyclopaedia Britannica: tropical and subtropical desert climate A desert averaging 5 cm of rain over ten years might have had 50 cm in one wet year and almost none in the other nine. Encyclopaedia Britannica: tropical and subtropical desert climate

In some low-latitude deserts, fewer than 30 days a year have clouds. Encyclopaedia Britannica: tropical and subtropical desert climate

With no clouds to hold the heat in, deserts swing hugely between day and night: some of the hottest places on Earth drop below freezing at night. Met Office: Climate zones Desert days and nights can differ by 35 °C when daytime highs over 40 °C are followed by a fast night-time drop. Encyclopaedia Britannica: tropical and subtropical desert climate

Cold deserts

Not all deserts are hot. In the Gobi Desert of east Asia, the yearly average temperature can be below 0 °C. Met Office: Climate zones

Two-humped camels resting on sandy ground in the Gobi Desert, with low hills behind — Camels in the Gobi Desert, Mongolia.Picture: Bernard Gagnon, CC0, Commons

Steppes (BS)

A steppe is a vast, treeless grassland that gets roughly 25–30 cm of rain a year. Encyclopaedia Britannica: steppe The word steppe comes from the Russian for “flat grassy plain”. Encyclopaedia Britannica: steppe

The great Eurasian Steppe stretches some 8,000 km, from Hungary through Ukraine and Central Asia to Manchuria. Encyclopaedia Britannica: steppe

Rain decides it: a bit less and a steppe turns to desert, a bit more and it becomes savanna or forest. Encyclopaedia Britannica: steppe

Wide, flat green grassland under a big sky in Mongolia, with a small round house in the distance — The Mongolian steppe.Picture: Orgio89, CC0, Commons

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