A desert is defined as any biome where precipitation is exceedingly low, severely limiting plant and animal life. The subtropical desert is a specific, widespread arid region characterized by consistently high temperatures and extreme dryness. These biomes are not random occurrences but result from large-scale, predictable atmospheric forces. Understanding these deserts requires examining their measurable climatic characteristics and the global air circulation patterns that generate them.
Defining the Subtropical Desert Biome
Subtropical deserts are identified by their geographical location, typically situated between 15° and 30° latitude north and south of the equator. The climate is consistently hot and dry, with annual precipitation rarely exceeding 250 millimeters. The rate of water lost to evaporation significantly surpasses the minimal amount received through rainfall, defining the extreme aridity of the biome.
Temperatures fluctuate drastically between day and night due to the lack of moisture and cloud cover. Daytime soil surface temperatures can soar above 60°C (140°F), while temperatures may drop near 0°C (32°F) after sunset. Major global examples include the Sahara and Arabian Deserts, as well as the Kalahari Desert in Southern Africa. The severity of the climate establishes the subtropical desert as the hottest of all desert types.
How Global Air Patterns Create Subtropical Deserts
The existence of subtropical deserts is directly linked to the massive atmospheric engine known as the Hadley Cell circulation system. This thermally driven circulation begins near the equator, where intense solar heating causes air to warm and rise. As this moist, heated air ascends, it cools and condenses, releasing its moisture as heavy rainfall over the tropical rainforests below.
The now-dry air mass flows poleward high in the atmosphere, eventually cooling and descending back toward the Earth’s surface at approximately 30° latitude. This downward movement creates a persistent band of high atmospheric pressure known as the subtropical high. As the air sinks, it warms adiabatically due to compression.
This warming significantly increases the air’s capacity to absorb moisture from the land below, making cloud formation and precipitation nearly impossible. The resulting clear skies and stable, high-pressure conditions suppress weather systems that might bring rain, creating the characteristic aridity. The trade winds, which are the lower branches of the Hadley circulation, then flow back toward the equator, completing the cell.
Survival Strategies of Desert Life
The extreme heat and lack of water in subtropical deserts have driven the evolution of highly specialized biological adaptations in both plants and animals. Flora employ several strategies to survive long periods between rainfall events:
- Succulence, storing water in thick, fleshy stems and leaves (e.g., cacti and euphorbia).
- Reducing water loss by shedding leaves during dry periods, only growing new ones immediately after rain (e.g., ocotillo).
- Developing extensive, shallow root networks to quickly capture moisture from brief surface rains.
- Growing deep taproots that can reach groundwater sources located more than 20 meters below the surface.
- Surviving the harshest times as dormant seeds, germinating rapidly to complete their life cycle in a matter of weeks after rain.
Animals primarily rely on behavioral strategies to mitigate the intense heat and conserve moisture. Many desert fauna, including rodents and reptiles, are strictly nocturnal, emerging only at night when temperatures are much cooler. During the day, they retreat into burrows or shade, where the air is cooler and more humid, effectively escaping the heat.
Physiological adaptations are also common for minimizing water loss. Mammals like the kangaroo rat can survive without drinking water, obtaining all necessary moisture metabolically from the seeds they consume. Many desert animals possess specialized kidneys that produce highly concentrated urine and very dry feces, allowing them to retain a maximum amount of water within their bodies.