Arid climates are defined by their extreme dryness, receiving less than 250 millimeters (about 10 inches) of rainfall per year. This makes water a scarce resource. Aridity is characterized by a significant imbalance between low precipitation and high potential for water loss through evaporation. This severe lack of moisture profoundly shapes landscapes, influencing ecosystems and the types of life that can survive within them.
Characteristics of Arid Environments
Arid environments exhibit distinct climatic features. These regions are characterized by extremely low annual precipitation, often falling below 250 millimeters, and sometimes even less than 100 millimeters in hyper-arid zones. High evaporation rates frequently exceed rainfall, leading to a persistent moisture deficit in the soil and atmosphere. Another characteristic is the significant fluctuation in daily temperatures, with hot days followed by much cooler nights due to the lack of cloud cover. These factors create landscapes with sparse vegetation, consisting of species adapted to drought conditions.
Worldwide Presence of Arid Climates
Arid climates cover approximately one-third of the Earth’s total land area. The most extensive regions are found in belts around 30 degrees north and south of the equator. This global pattern includes vast expanses of North Africa, such as the Sahara, and significant portions of West Asia and Australia. Many arid zones are situated on the western sides of continents or deep within continental interiors. These geographical distributions result in major desert systems spanning multiple continents, including North America, South America, and Southern Africa.
Natural Causes of Aridity
Several natural mechanisms contribute to arid climates. High-pressure systems, particularly in subtropical regions, cause air masses to descend, warm, and dry, suppressing cloud formation and precipitation. This phenomenon, part of the Hadley Cell atmospheric circulation, creates stable, dry conditions.
The rain shadow effect occurs when mountain ranges block moisture-laden air, forcing it to rise and release precipitation on the windward side. The now-dry air then descends on the leeward side, warming and absorbing moisture, creating arid conditions.
Cold ocean currents also contribute to aridity, especially along western coasts. As cold currents flow toward the equator, they cool the overlying air, inhibiting rain-producing clouds. This often leads to coastal deserts where fog may be present but rainfall is minimal. Finally, continentality, or vast distance from large bodies of water, reduces moisture availability. Air masses traveling far inland lose moisture, leading to drier conditions in continental interiors compared to coastal areas.
Notable Arid Regions
The Sahara Desert in North Africa exemplifies aridity caused by a combination of factors. It is largely attributed to the semi-permanent subtropical high-pressure system, which suppresses rainfall. Its immense size also contributes to extreme dryness due to continentality. Additionally, mountain ranges like the Atlas Mountains create a rain shadow effect, further enhancing aridity.
The Atacama Desert in South America, along the Pacific coast of Chile and Peru, is one of the driest places on Earth. Its extreme aridity is primarily influenced by the cold Humboldt Current offshore, which cools the air and prevents precipitation. The towering Andes Mountains to its east also contribute through the rain shadow effect, blocking moist air masses from the Amazon Basin. Some areas have recorded virtually no rainfall for extended periods.