The question of the world’s driest thing is complex, depending on whether one refers to a physical location, a naturally occurring substance, or a manufactured material. Dryness is a complex scientific property requiring precise measurement of residual moisture. To determine the record-holder, one must establish the metric of water absence: is it the moisture content in the air, or the water molecules bound within a solid matrix? This distinction determines if the answer is a desert, a chemical compound, or a specialized synthetic creation.
Defining Extreme Dryness
Scientists quantify the lack of water using different metrics for atmospheric conditions versus physical materials. Atmospheric dryness is commonly measured by Relative Humidity (RH), which expresses the amount of water vapor present in the air as a percentage of the maximum amount it can hold. The dryness of a substance is often measured by Water Activity (\(a_w\)), a thermodynamic property that indicates the “free” or unbound water available for chemical reactions and microbial growth.
Water activity is defined as the ratio of the water vapor pressure above a material to the vapor pressure above pure water at the same temperature. Pure water has an \(a_w\) value of 1.0. A lower \(a_w\) value indicates less available moisture, and materials with extremely low values are considered desiccated or anhydrous. While RH describes the environment, \(a_w\) describes the material itself, representing the Equilibrium Relative Humidity (ERH) when the material is in balance with the surrounding air.
The Driest Known Material
The manufactured substance widely considered the driest known solid material is silica aerogel, sometimes referred to as “frozen smoke.” This synthetic, porous, ultralight foam is derived from a gel in which the liquid component has been replaced by gas. It is a product of specialized engineering designed to achieve extremely low density and high porosity.
Its creation begins with the sol-gel process, which forms a wet gel structure from chemical precursors like silicon alkoxides. The subsequent drying process allows the aerogel to maintain its delicate, highly porous structure. The most effective method is supercritical drying, which removes the liquid solvent by passing it through its critical point of temperature and pressure. This technique eliminates the liquid-gas boundary and surface tension, preventing the fragile, nanoscale silica skeleton from collapsing as it dries.
The resulting material can be up to 99.8% air by volume, composed of a convoluted network of silica nanoparticles. This structure results in a density only three times that of air. Due to the near-total void space and effective moisture removal, the material achieves levels of dryness far beyond any natural desiccant. This gives it an exceptionally low \(a_w\) and makes it a powerful moisture absorber.
The Driest Place on Earth
When considering the environment, the title of the driest location on Earth is contested between two hyper-arid regions: one in the tropics and one in a polar zone. The Atacama Desert in Chile holds the record as the driest non-polar desert, with some central areas recording almost zero rainfall for centuries. Annual precipitation in the hyper-arid core of the Atacama can be as low as 1 to 3 millimeters.
This extreme atmospheric dryness is caused by a potent combination of geographical and meteorological factors. The desert is trapped in a double rain shadow, with the Andes Mountains and the Chilean Coastal Range blocking moisture from both the Pacific and Atlantic Oceans. The cold, north-flowing Humboldt Ocean Current compounds this effect by cooling the air above the Pacific. This cold air holds less moisture, and the resulting stable, high-pressure system prevents the formation of rain clouds, leading to persistent aridity.
Specific spots within the McMurdo Dry Valleys of Antarctica are often cited as the overall driest locations. This group of valleys is the largest ice-free area on the continent. The absence of moisture is maintained by intense katabatic winds, which are high-density air masses that rush down the slopes from the polar plateau. These winds heat up as they descend, instantly evaporating all moisture, ice, and snow, resulting in atmospheric moisture levels that rival the Martian environment.