Page 333 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 10 Global Climate Systems 297
▲Figure 10.19 South Georgia Island, a polar marine climate. This abandoned whaling station at Grytviken, South Georgia, processed more than 50 000 whales between 1904 and 1964, about a third of all whales processed on the island during this period. The whales of the Southern Ocean were driven almost to extinction. [Bobbé Christopherson.]
Characteristics of Dry Climates
Dry climates are subdivided into deserts and steppes according to moisture—deserts have greater moisture deficits than do steppes, but both have permanent water shortages. Steppe is a regional term referring to the vast semiarid grassland biome of Eastern Europe and Asia (the equivalent biome in North America is shortgrass prairie, and in Africa, the savanna; see Chapter 20). In this chapter, we use steppe in a climatic context; a steppe climate is considered too dry to support forest, but too moist to be a desert.
The timing of precipitation (winter rains with dry summers, summer rains with dry winters, or even distribution throughout the year) affects moisture availability in these dry lands. Winter rains are most effective because they fall at a time of lower moisture demand. Relative to temperature, the lower-latitude deserts and steppes tend to be hotter with less sea- sonal change than the midlatitude deserts and steppes, where mean annual temperatures are below 18°C and freezing winter temperatures are possible.
Earth’s dry climates cover broad regions between 15° and 30° latitude in the Northern and Southern Hemispheres, where subtropical high-pressure cells predominate, with subsiding, stable air and low relative humidity. Under generally cloudless skies, these subtropical deserts extend to western continen- tal margins, where cool, stabilising ocean currents operate offshore and summer advection fog forms. The Atacama Desert of Chile, the Namib Desert of Namibia, the Western Sahara of Morocco, and the Australian Desert each lie adjacent to such a coastline (Figure 10.20).
However, dry regions also extend into higher lati- tudes. Deserts and steppes occur as a result of orographic
lifting over mountain ranges, which intercepts moisture- bearing weather systems to create rain shadows, espe- cially in North and South America (Figure 10.20). The isolated interior of Asia, far distant from any moisture- bearing air masses, also falls within the dry climate classification.
The world’s largest desert, as defined by moisture cri- teria, is the Antarctic region. The largest non-polar des- erts in surface area are the Sahara (9.1 million km2), the Arabian, the Gobi in China and Mongolia, the Patagonian in Argentina, the Great Victoria in Australia, the Kalahari in South Africa, and the Great Basin of the western United States.
tropical, subtropical
hot Desert Climates
Tropical, subtropical hot desert climates are Earth’s true tropical and subtropical deserts and feature annual aver- age temperatures above 18°C. They generally are found on the western sides of continents, although Egypt, Somalia, and Saudi Arabia also fall within this classification. Rain- fall is from local summer convectional showers. Some regions receive almost no rainfall, whereas others may receive up to 35 cm of precipitation a year. A represen- tative subtropical hot desert city is Riyadh, Saudi Arabia (Figure 10.21).
Along the Sahara Desert’s southern margin in Africa is a drought-prone region called the Sahel, where human populations suffer great hardship as desert conditions gradually expand over their homelands. Geosystems Now in Chapter 18 examines the process of desertifica- tion (expanding desert conditions), an ongoing problem in many dry regions of the world.
In California, Death Valley holds the record for highest temperature ever recorded—57°C during July 1913. Extremely hot summer temperatures occur in other hot desert climates, such as around Baghdad, Iraq, where air temperatures regularly reach 50°C and higher in the city. Baghdad records zero precipitation from May to September, as it is dominated by an intense subtropi- cal high-pressure system. In January, averages for Death Valley (11°C) and Baghdad (9.4°C) are comparable. Death Valley is drier, with 5.9 cm of precipitation, compared to 14 cm in Baghdad, both low amounts.
midlatitude Cold Desert Climates
Midlatitude cold desert climates cover only a small area: the countries along the southern border of Russia, the Taklamakan Desert, and Mongolia in Asia; the central third of Nevada and areas of the American Southwest, particularly at high elevations; and Patagonia in Argen- tina. Because of lower temperature and lower moisture- demand criteria, rainfall must be low—in the realm of 15 cm—for a station to qualify as a midlatitude cold desert climate.