What a BWh climate is like
BWh Hot desert. Rain is rare and unreliable, and the year is hot on average. Think of the Sahara, the Arabian Peninsula and the deserts of the American south-west.
The rules behind the code, in plain English:
- Arid, and dry enough to be a desert: yearly rainfall is below five times the threshold.
- Hot: mean annual temperature of 18 °C or more.
Dataset wording: "Arid, desert, hot". In the maps of Beck et al. (2023) the class is written: B: MAP < 10 x Pthreshold; W: MAP < 5 x Pthreshold; h: MAT >= 18.
Hot desert climate is very dry and hot, with little rain in any season and a huge swing between scorching days and cool nights. Plants are sparse and tough, such as cacti, thorny shrubs and date palms near water. Crops need irrigation from rivers or wells. Thick walls, small windows, shade and loose, covering clothes keep people cool, and life moves indoors in the afternoon heat.
How much of the world is BWh?
- 16%of the world's land outside Antarctica, 1991 to 2020
- 21,587,572 km²area, 1991 to 2020
- 1,503cities of 15,000 or more people (GeoNames) have this climate now
In 1901 to 1930 it covered 15% of that land, so its area then was 3.64% smaller than in 1991 to 2020.
How the BWh area is projected to change by 2071 to 2099
These figures compare the area of BWh in each future map with 1991 to 2020. They come from the Beck et al. (2023) model projections under three emissions scenarios. They show what the scenarios imply, not what will happen.
| Map | Share of land | Change in area vs 1991 to 2020 |
|---|---|---|
| 1901 to 1930 (past) | 15% | -3.64% |
| 1991 to 2020 (now) | 16% | - |
| 2071 to 2099, SSP1-2.6 (low emissions) | 17% | +3.83% |
| 2071 to 2099, SSP2-4.5 (middle of the road) | 17% | +7.26% |
| 2071 to 2099, SSP5-8.5 (high emissions) | 18% | +13.65% |
Shares are of land outside Antarctica. A negative change means the zone shrinks.
What happens to today's BWh land
Of the land that is BWh in 1991 to 2020, this is how much stays BWh and where the rest ends up in 2071 to 2099, by scenario:
| Scenario (2071 to 2099) | Stays BWh | Where the rest goes (biggest first) |
|---|---|---|
| SSP1-2.6, low emissions | 99% | BSh (1.3%) |
| SSP2-4.5, middle of the road | 99% | BSh (1.3%) |
| SSP5-8.5, high emissions | 98% | BSh (1.7%) |
Looking back, 94% of today's BWh land was also BWh in 1901 to 1930. The rest was BWk (3.3%), BSh (1.9%).
Where BWh is found
The countries with the most BWh land in 1991 to 2020:
| Country | Area | Share of all BWh | Share of the country |
|---|---|---|---|
| Australia | 3,454,368 km² | 16% | 45% |
| Algeria | 2,056,269 km² | 9.5% | 89% |
| Saudi Arabia | 1,919,996 km² | 8.9% | 100% |
| Libya | 1,590,969 km² | 7.4% | 98% |
| Sudan | 1,386,702 km² | 6.4% | 75% |
| Niger | 1,104,901 km² | 5.1% | 93% |
| Mauritania | 1,030,687 km² | 4.8% | 99% |
| Egypt | 997,328 km² | 4.6% | 100% |
Places with a BWh climate today
The biggest cities in the 1991 to 2020 map that sit in BWh (at most two per country):
Other arid climates (group B)
Frequently asked questions
What does BWh mean in the Köppen climate classification?
Arid, and dry enough to be a desert: yearly rainfall is below five times the threshold. Hot: mean annual temperature of 18 °C or more. The code is a short way of writing that: the first letter is the major group (B, arid) and the letters after it say more about rain and temperature.
What is a hot desert climate like?
Very dry and hot. Rain is rare and unreliable, and the year is hot on average. Think of the Sahara, the Arabian Peninsula and the deserts of the American south-west.
How much of the world has a BWh climate?
16% of the world's land outside Antarctica was BWh in 1991 to 2020, about 21,587,572 km² (Beck et al. 2023, 1 km maps).
Will the BWh climate zone grow or shrink?
Under the middle-of-the-road scenario (SSP2-4.5), the area of BWh in 2071 to 2099 is 7.26% larger than in 1991 to 2020. This is a model projection, not a forecast, and it differs under other scenarios (see the table on this page).
Data: Beck et al. (2023), CC BY 4.0. See data sources and licences. Köppen-Geiger classes describe long-term averages, not weather.