Snow has a high albedo, which is the measure of a surface’s reflectivity. Freshly fallen snow is one of the most reflective natural surfaces on Earth, playing a role in regulating the planet’s temperature. This high reflectivity means that snow sends a large fraction of the sun’s energy directly back into space, preventing the Earth’s surface from absorbing that heat. Fresh, pure snow can reflect up to 90% of incoming solar radiation, establishing it as a major factor in the Earth’s energy balance.
Understanding Reflectivity
Albedo quantifies the fraction of solar radiation reflected by a surface, expressed as a value between 0 and 1. A surface with an albedo of 0 absorbs all incoming sunlight and reflects none, similar to a perfectly black body. Conversely, a surface with an albedo of 1 reflects all incoming sunlight.
Most natural surfaces fall between these two extremes, with dark surfaces having low albedo and light surfaces having high albedo. For instance, a dark surface like the open ocean or fresh asphalt has a low albedo, typically below 0.10, meaning it absorbs over 90% of the sun’s energy. Fresh snow boasts an albedo ranging from 0.8 to 0.9, reflecting 80% to 90% of the sunlight back to the atmosphere.
The Physics Behind Snow’s Brightness
Snow’s high reflectivity is due to the physical structure of the snowpack itself, not the transparent color of the ice. Fresh snow is composed of countless complex, multi-faceted ice crystals with a significant amount of trapped air. When sunlight enters this structure, it encounters the jagged surfaces of these microscopic ice grains.
The light is scattered and refracted multiple times by the crystal surfaces and the air pockets. This repeated scattering across all visible wavelengths prevents the light from penetrating deep into the snowpack or being absorbed. Since all colors of visible light are scattered almost equally, the combined effect is perceived as brilliant white.
Snow Albedo’s Role in Global Energy Balance
Snow cover acts as a natural thermostat for the Earth, and its high albedo regulates global and regional temperatures. By reflecting a large portion of the sun’s energy back into space, snow prevents the surface and the lower atmosphere from heating up. This cooling effect is particularly pronounced in polar and high-latitude regions where snow and ice cover are extensive and persistent.
The most important consequence of snow’s reflectivity is its contribution to the “ice-albedo feedback loop.” When temperatures rise, snow and ice begin to melt, exposing the darker land or ocean surface underneath. These darker surfaces absorb significantly more solar energy, causing additional warming. This extra warmth, in turn, causes even more snow and ice to melt, accelerating the initial warming trend. This positive feedback mechanism is why polar regions are experiencing warming at a much faster rate than the global average.
How Impurities Change Snow’s Reflective Power
The high albedo of snow is not constant and can be reduced by changes in the snowpack or the presence of impurities. As snow ages, repeated melt and freeze cycles cause the original ice crystals to grow larger and become more rounded. These larger, denser grains scatter less light and absorb more solar radiation, which can lower the albedo from 0.8 down to a range of 0.4 to 0.7 for aged or melting snow.
The deposition of light-absorbing impurities also darkens the snow surface and accelerates melting. Particles like black carbon, often referred to as soot, reduce albedo even at very small concentrations. Dust from deserts or pollution also settles on the snow, reducing its reflectivity and causing it to absorb more energy. This darkening effect increases the rate of snowmelt and contributes to the positive ice-albedo feedback loop.