A coastal desert is an arid environment situated along the western edges of continents, typically near the tropics. These regions are defined by extreme dryness, receiving very low levels of annual rainfall despite their proximity to a major body of water like an ocean. This paradoxical aridity is directly linked to the influence of cold ocean currents that flow parallel to the shore, creating one of the planet’s most distinctive biomes.
The Defining Physical Characteristics
The climate of a coastal desert is moderated by the ocean, resulting in temperatures that are far more stable than those found in interior deserts. The daily temperature range, known as the diurnal range, is small, often fluctuating by only about 10°C or less, which contrasts sharply with the extreme swings experienced in inland deserts. While rainfall is minimal, the atmosphere is often saturated with moisture, manifesting as frequent and persistent fog or low-lying stratus clouds. These “fog deserts” often experience more days blanketed by mist than days with measurable precipitation. This fog acts as a moisture source, sometimes providing the only water available to the local ecosystem.
The Unique Formation Mechanism
The existence of coastal deserts is inextricably linked to the presence of cold ocean currents, often fed by a process called upwelling. Upwelling occurs when deep, cold water is brought to the surface near the coast, drastically cooling the sea surface temperature. As air masses move toward the land, they pass over this cold water, cooling the air just above it.
This cooling leads to the formation of a thermal inversion layer, which explains the lack of rain. Warmer air from the land or higher altitudes flows over the cold, dense air trapped near the ocean surface. This creates a stable atmospheric condition where the cold layer acts like a lid, preventing the air below from rising and cooling enough to form rain-producing clouds. Moisture in the chilled air is instead condensed into the persistent fog that characterizes these areas.
The cold currents also contribute to the land’s desiccation because the air passing over them evaporates less water and holds little moisture to begin with. This combination of atmospheric stability and low moisture content results in environments that are exceptionally dry, even with the ocean nearby.
Biological Adaptations to Fog and Aridity
Life in coastal deserts depends on specialized methods to acquire water from fog and dew, rather than from infrequent rainfall. Plants have developed intricate strategies to capture atmospheric moisture, often possessing specialized leaf structures or surface textures that facilitate the condensation of water droplets. Some species receive up to 60% of their total water intake from this fog-harvesting process.
Many plants exhibit features found in other arid regions, such as extensive, shallow root systems designed to rapidly absorb surface moisture from dew, fog drip, or rare rain events. Other plants feature fleshy stems and leaves that store water for long periods.
Animals have also evolved behavioral and physical adaptations to survive in this fog-rich environment. The darkling beetles of the Namib Desert, for instance, demonstrate a behavior known as fog-basking. These insects climb to the top of dunes and orient their bodies to allow the fog to condense on their backs. They then tilt their heads downward to let the collected water drip directly into their mouths. Other animals obtain moisture from the food they consume, possessing efficient physiological mechanisms to conserve water by producing concentrated urine and dry feces.
Prominent Global Locations
The most well-known examples of this biome are found along the western coasts of South America and Southern Africa. The Atacama Desert, stretching across Chile and Peru, is often cited as the driest desert in the world, with some areas receiving no measurable rainfall for decades. Its aridity is intensified by the influence of the cold Humboldt Current, also known as the Peruvian Current. Similarly, the Namib Desert, which runs along the coasts of Namibia and Angola, is shaped by the cold Benguela Current. These two deserts illustrate the formation mechanism, with their coastal edges frequently shrouded in dense fog. Other coastal deserts include portions of the Baja California Desert in Mexico, which exhibits the fog-dependent ecology characteristic of this environment.