Technical Talk: Defining Albedo

albedoIt is common knowledge that darker colors tend to absorb light and lighter colors reflect light. When the light comes from the sun, heat and energy are absorbed by the darker colors.

While this is an easily understood phenomenon, the effects can be complicated.

Solar radiation reaches the Earth, providing the light and heat necessary for life; the unabsorbed solar energy reflected back to the atmosphere by light-colored surfaces is called albedo.

Surfaces that appear translucent or white, such as ice and snow, have the highest levels of albedo, thereby reflecting the greatest amount of solar energy. This fact explains why we can easily get sunburnt while skiing. Conversely, bare, wet soil has one of the lowest albedos, thus absorbing the solar energy.

Albedo is a ratio expressed on a scale from 0 to 1, where 0 represents total absorption and 1 represents complete reflectance of the solar energy. The albedo of surface can change over time due to weathering, precipitation, and other factors that affect surface hue.

In the built environment, the albedo of pavements and roofs has been identified as a possible factor in making an urban area warmer than its surrounding rural areas, a phenomenon referred to as the urban heat island effect (UHI).

However, it is important to consider what happens to the reflected solar energy. Just as the albedo of snow means a skier is wise to put on sunscreen, reflective surfaces redirect energy in unaccounted ways. In urban areas, for example, heat and light reflected from a pavement can lead to warmer building façades, which causes warmer building interiors, increased demand for air conditioning, and ultimately greater energy demand.

For more information about albedo, reflective pavements and the built environment, visit https://ncesmart.asu.edu/news/unintended-consequences.

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