Patagonia, a vast region at the southern tip of South America spanning Argentina and Chile, is defined by its rugged, expansive landscape. This area, particularly the Patagonian Steppe, presents a stark, windswept environment. With its seemingly endless plains and sparse vegetation, many observers wonder if this cold, arid expanse is accurately defined as a desert.
Defining a Desert: Climatological Criteria
The scientific classification of a desert relies on a strict climatological standard based on aridity, not sand dunes or high temperatures. Under the Köppen climate classification system, arid climates fall into Group B, defined by a severe lack of moisture. Classification depends on the relationship between annual precipitation and potential evapotranspiration.
A region is classified as arid (BW) if precipitation is less than 50% of a calculated threshold. If precipitation falls within 50% to 100% of that threshold, the climate is considered semi-arid (BS). Regions with a mean annual temperature below 18°C receive the “k” designation, indicating a cold arid or semi-arid climate (BWk or BSk).
The Climate Profile of the Patagonian Steppe
The Patagonian Steppe, covering the majority of the region in Argentina, aligns closely with arid conditions. This ecoregion is cool to cold with remarkably low average annual precipitation. In the driest central and eastern parts, rainfall often registers below 200 millimeters (about 8 inches) per year, consistent with desert conditions.
The climate is further defined by persistent, strong westerly winds that sweep across the treeless plains. These high wind speeds contribute to a significant rate of evaporation, exacerbating the low precipitation levels. Mean annual temperatures range from 5°C to 16°C, placing it within the “cold” (k) temperature subgroup. The vegetation is primarily xerophytic, consisting of scrub brush and tufted grasses adapted to survive the drought, frost, and wind.
The Andes’ Influence: Understanding the Rain Shadow Effect
The reason for Patagonia’s intense aridity is the rain shadow effect, created by the Andes Mountains. Prevailing winds are strong westerlies, carrying moisture-rich air across the Pacific Ocean toward the continent. When this moist air encounters the towering Andes, it is forced to rise rapidly up the mountain slopes.
As the air ascends the western, or windward, side, it cools, causing moisture to condense and fall as heavy precipitation, supporting the temperate rainforests of southern Chile. By the time the air crests the mountains and begins its descent down the eastern, or leeward, side, it has lost nearly all of its moisture. This dry air compresses and warms as it sinks, a phenomenon known as a föhn wind, which actively absorbs any remaining surface moisture. This process casts a rain shadow over the Patagonian Steppe.
Final Classification: Is Patagonia a True Desert?
Based on the climate data and the Köppen classification system, large swathes of the Patagonian Steppe meet the technical criteria for a desert. The core of the region functions as a true cold desert (BWk) due to the combination of extremely low precipitation and low average annual temperatures. However, the region is not uniform; many areas closer to the Andes foothills or the Atlantic coast receive slightly more moisture, classifying them as cold semi-arid steppe (BSk). This mix of arid and semi-arid conditions is why the region is often referred to interchangeably as the Patagonian Steppe and the Patagonian Desert.