# Albedo: Definition, Effect, Equation & Measurement

Instructor: Betsy Chesnutt

Betsy teaches college physics, biology, and engineering and has a Ph.D. in Biomedical Engineering

Albedo helps determine the temperature of the Earth and also affects the weather. In this lesson, learn about albedo, its effects on the climate and weather, and how to measure it.

## What Is Albedo?

Have you ever touched a black car in the middle of the summer? It's really hot, right?! You probably noticed that it is a lot hotter than a white car, even if both are side by side. Why is this? What is different between the black car and the white car? The difference between the two cars is that the black one absorbs more of the radiation from the sun and holds onto it while the white one reflects more back.

There is actually a way to quantify exactly how much radiation is reflected from a surface, and this is known as albedo. Albedo is defined as the ratio of reflected radiation from a surface to the amount of radiation that hits it.

Because the amount of radiation reflected back can never be more than the amount that hits the surface, the albedo of any surface must always be between zero and one.

Black surfaces have a very low albedo (near zero) because they absorb almost all of the incident light and heat and don't reflect much back. Therefore, they are hotter. White surfaces, on the other hand, reflect back almost all light and heat, so they have a high albedo (near one) and are cooler.

## The Relationship Between Albedo and the Temperature of the Earth

The albedo of the Earth ranges from close to zero for snow-covered areas, to nearly one for dark forests and oceans. The amount of cloud cover also affects albedo because clouds reflect heat and light, and they also have a low albedo.

The albedo of the entire Earth can be estimated by analyzing aerial and satellite pictures of the Earth. Albedo can also be estimated by measuring something called earthshine. Earthshine is sunlight that reflects from the surface of the Earth and then off the moon. We can see it as a faint glow around the shadowed part of the moon at night. Earthshine has been measured at the Big Bear Solar Observatory in California since 1998.

Although albedo can be very different in different locations and can change depending on the season and amount of cloud cover, the average albedo of the entire Earth is consistently about 0.3.

While the average global temperature depends on several factors, including the amount of solar irradiation and the gases in the atmosphere, albedo also plays a large role in determining the temperature of the Earth. For example, if significant amounts of ice in glaciers and at the North and South pole melt, the average albedo of the Earth could decrease as white, icy areas turn into dark areas of ocean and rock. This would mean that even more of the Sun's radiation would be absorbed, further increasing the temperature of the Earth and melting more ice. This is one factor that may be contributing to climate change.

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