Difference between albedo and emissivity?

In summary, albedo is the ratio of radiation reflected off a body compared to the total incident on that body, while emissivity is the ratio of radiation absorbed compared to the total incident radiation. Snow has a high albedo and a high emissivity, which may seem contradictory, but this is due to the different wavelengths of radiation being considered and the variable nature of snow's reflectivity.
  • #1
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This has been asked already, but it was either answered incorrectly or I'm just not understanding it right.

I was under the impression that albedo is the ratio of radiation reflected off a body compared to the total incident on that body. Hence black bodies have an albedo of 0 and more white/shiny surfaces have higher albedos.

I thought emissivity = 1-albedo, so, the ratio of radiation absorbed (and therefore emitted) compared to the total incident radiation, because radiation is either reflected or absorbed then emitted, right?

So, according to values on the internet, snow has an albedo of 0.9 (Wikipedia) and an emissivity of 0.969+ (EngineeringToolbox). How does that make sense? How can the radiation emitted and absorbed add up to more radiation than there was in the first place?

Thanks
 
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  • #2
The references are unclear about the specified wavelength range. For example, snow is highly reflective in the visible but highly absorptive in the infrared. In addition, the reflectivity for snow can be highly variable based on grain size. Basically, the two numbers may not be directly comparable:

http://nsidc.org/data/docs/daac/nsidc0066_avhrr_5km/spectra.jpg
 

What is the definition of albedo?

Albedo refers to the measure of the amount of light or solar radiation reflected by a surface. It is usually expressed as a percentage, with higher percentages indicating a higher reflectivity.

What is the definition of emissivity?

Emissivity refers to the measure of the ability of a surface to emit thermal radiation. It is a unitless value between 0 and 1, with higher values indicating a higher ability to emit thermal radiation.

How are albedo and emissivity related?

Albedo and emissivity are inversely related - a surface with a high albedo will have a low emissivity, and vice versa. This means that a surface that reflects a lot of light will also emit less thermal radiation, and a surface that absorbs a lot of light will emit more thermal radiation.

What is the importance of albedo and emissivity in climate science?

Albedo and emissivity play a significant role in climate science, as they affect the amount of solar radiation and heat absorbed and emitted by the Earth's surface. Changes in albedo and emissivity can contribute to global warming or cooling, and understanding these factors is crucial in predicting and mitigating the effects of climate change.

How are albedo and emissivity measured?

Albedo is typically measured using specialized instruments such as radiometers or spectrometers, which measure the amount of light reflected by a surface. Emissivity is measured using infrared thermometers, which detect the amount of thermal radiation emitted by a surface. Both albedo and emissivity can also be calculated using mathematical models based on surface characteristics such as color, texture, and composition.

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