What is Albedo?

Albedo is a critical concept in meteorology and climate science, referring to the amount of the sun’s energy that is reflected by a surface. Albedo is a property that every object has, like a car, clothing, mountains, snow, and the earth as a whole.

Image: Comparing the albedo of different color cars. The white car absorbs more of the sun's energy and thus has a higher albedo than the black car. Courtesy of Esters L, Wirtz P, Brückner S, Li Y and Kracheletz M (2023) What Is Albedo and What Does It Have to Do With Global Warming?. Front. Young Minds. 11:1113553. doi: 10.3389/frym.2023.1113553.

The energy emitted by the sun is referred to as solar radiation. Sunlight is the visible part of the solar radiation spectrum reaching Earth. In other words, albedo is the amount of solar radiation reflected by a surface. 

Image: The differences in absorption and reflection by surfaces with and without snow. Courtesy of the-m-factory.com.

Albedo quantifies how much sunlight is reflected by the Earth's surface, and is expressed as a percentage or a fraction. The more reflective a surface is the higher the albedo value. An albedo of 0.3 (30%) means that 30% of incoming solar radiation is reflected back into space, while the surface absorbs the remaining 70%.

Planetary albedo deals with the amount of sunlight reflected by the Earth back out to space and is calculated by averaging the albedo of all Earth’s surfaces. ​​The earth’s albedo is influenced by the physical properties of the planet’s various surfaces. Some surfaces, such as ice and snow, reflect more sunlight and absorb less heat than surfaces like oceans and forests. Clouds reflect a large amount of energy out to space as well. 

Why is Albedo important?

Understanding albedo is essential for comprehending various climatic and weather phenomena, as it is a crucial factor in the Earth’s energy balance. Albedo plays a significant role in temperature regulation, weather patterns, and precipitation, urban heat islands, climate, and winter sports.

What factors influence Albedo?

Several factors influence the albedo of a surface, including surface composition, soil moisture, the angle of incidence, surface texture, and vegetation and land use. Different materials have different reflective properties.

Image: Albedo of several different surfaces. Courtesy of Pearson Prentice Hall Inc.

For example, ice, snow, and sand have high albedos, whereas forests, oceans, and asphalt have low albedos. The angle at which sunlight hits the surface affects its reflectivity. Lower angles (morning and evening) result in higher albedo compared to midday when the sun is directly overhead. Rough surfaces scatter light in various directions, increasing absorption and lowering albedo. Smooth surfaces reflect light more uniformly, increasing albedo. Areas with dense vegetation have lower albedo compared to barren lands. Urbanization and deforestation also impact local and global albedo values.

Table: Typical values of mean albedo. Courtesy of Victor M. Ponce, Member, ASCE, Anil K. Lohani, and Patrick T. Huston, Member, ASCE.

Earth's Energy Balance

Albedo has a direct impact on the Earth's energy balance. Surfaces with high albedo, such as ice and snow, reflect a large portion of incoming solar radiation. This reflection helps keep these regions, and consequently the Earth, cooler. Conversely, surfaces with low albedo, like oceans and forests, absorb more solar energy, which leads to warming. Darker surfaces, such as asphalt and dark rooftops in urban areas, can absorb up to 90% of incoming sunlight, contributing to higher local temperatures.

Image: Albedo of several different surfaces on Earth. Courtesy of InsideClimate News Research.

Urban Heat Islands

Cities often have a lower albedo due to the prevalence of dark surfaces like roads and buildings. This leads to the urban heat island effect, where urban areas experience significantly higher temperatures than their rural surroundings.

Image: Schematic of the urban heat island effect. Courtesy of Skeptical Science.

Weather Patterns and Precipitation

Albedo influences regional climates by affecting local temperature and precipitation patterns. Albedo’s role in temperature regulation makes it a key player in the broader climate dynamics. The high albedo of polar ice caps helps maintain cooler temperatures, and along with the varying albedo across the earth, influences atmospheric circulation patterns. 

Winter Sports/Ski Industry

Albedo plays an important role in the ski industry, as it influences snow quality and preservation, snowmaking, resort operations and maintenance, and the skiing experience on the whole. Fresh snow has a high albedo, reflecting about 80-90% of incoming solar radiation. This high reflectivity helps maintain cooler surface temperatures, slowing the melting process and preserving snow cover for longer periods. Regular grooming is crucial to maintain high albedo at ski resorts. Keeping the snow surface smooth and reflective lowers albedo and helps preserve snow cover.

Image: Albedos of different snow, ice, and water surfaces. Courtesy of Hugo Ahlenius, UNEP/GRID-Arendal/https://www.grida.no/resources/5219.

When snow gets contaminated with dirt, dust, or other particulates, its albedo decreases. This causes the snow to absorb more heat, leading to faster melting and deteriorating snow quality, which can shorten the ski season. Recent research has shown that wind-blown dust can significantly speed up the rate at which snow melts.

Climate

As global temperatures rise, ice and snow begin to melt, reducing the Earth's overall albedo.

Image: Generalized schematic of the ice-albedo feedback loop, shown continuing until the Earth is completely ice-free. Courtesy of Esters L, Wirtz P, Brückner S, Li Y and Kracheletz M (2023) What Is Albedo and What Does It Have to Do With Global Warming?. Front. Young Minds. 11:1113553. doi: 10.3389/frym.2023.1113553.

This reduction in ice means that more solar radiation is absorbed, leading to further warming and additional melting. This positive feedback loop accelerates climate change and contributes to rising global temperatures.

Image: Ice-albedo feedback loop. Courtesy of UCLA Center for Climate Science.

How is Albedo Measured?

Albedo is measured as the fraction of solar radiation reflected from the Earth back into space. Instruments aboard NASA’s Terra and Aqua satellites are widely used to measure Earth's albedo. MODIS (Moderate Resolution Imaging Spectroradiometer) is a key instrument used for observing and understanding the Earth's atmosphere, oceans, and land surfaces, providing detailed information about the reflectance of various surfaces on Earth.

The CERES (Clouds and the Earth’s Radiant Energy System) instrument measures the energy at the top of the atmosphere, including the reflected sunlight, and helps understand the radiation budget and the role of clouds and aerosols in the Earth's climate system.​

Pyranometers are instruments used to measure solar radiation flux density from a field of view of 180 degrees. By placing two pyranometers, one facing upwards (measuring incoming radiation) and one facing downwards (measuring reflected radiation), the albedo can be calculated by the ratio of reflected to incoming solar radiation​.

Image: Two pyranometers fitted back to back to measure albedo. Courtesy of https://www.essearth.com/product/albedometers-from-kipp-and-zonen/.

Luke Stone
Forecaster, OpenSnow