Behind the Forecast: Albedo: reflecting on reflecting sunlight

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Science Behind the Forecast: Albedo
This photo shows a man using an Angstrom pyranometer to measure albedo circa 1930.
This photo shows a man using an Angstrom pyranometer to measure albedo circa 1930. (Source: NOAA)

LOUISVILLE, Ky. (WAVE) - The sun is always shining bright, regardless if clouds obstruct our view or not. The Earth’s surface sometimes absorbs sunlight, while other times it reflects it.

The National Weather Service defines albedo as “the fraction of radiation striking a surface that is reflected by that surface.” In other words, it is the amount of light that a surface reflects relative to the total amount of sunlight hitting it.

The term albedo comes from the Latin word albus, which means white. Albedo generally refers to visible light. However, albedo may sometimes involve infrared radiation (think heat).

Albedo is a unitless, non-dimensional value that ranges on a scale from 0 to 1. A value of 1 means that an object or surface reflects all incoming solar radiation. Surfaces that absorb all of the sunlight have a value of 0.

Surfaces like snow that reflect more light have a high albedo. Black asphalt has a low albedo because it absorbs much of the sunlight.

Snow reflects more light than sea ice. Sea ice has an albedo ranging from 0.5 to 0.7, meaning that it reflects 50 to 70% of incoming light. Add snow on top of sea ice, and it can reflect nearly 90% of solar radiation. The ocean only reflects about six percent of solar radiation, giving it an albedo of 0.06. The entire planet reflects 30% of the sun’s energy, so the Earth has an average albedo of .30. Clouds have a higher albedo than the darker land. Even aerosols in our atmosphere can affect the albedo over a particular location, think Saharan Dust.

An Angstrom pyranometer, used to measure albedo circa 1930.
An Angstrom pyranometer, used to measure albedo circa 1930. (Source: NOAA)

Albedo is valuable for day to day forecasts. Snowy ground reflects more light, which can keep air temperatures lower. Black asphalt and concrete absorb more sunlight and, in turn, warm very quickly. The lower albedo can lead to much warmer temperatures in places like cities.

Different parts of the planet absorbing and reflecting different amounts of solar radiation create temperature gradients. The temperature gradients help to drive our weather, impacting precipitation, wind, atmospheric pressure, etc.

Satellites now measure the planet’s albedo, which scientists input into various climate and weather models.

Actinometers measure “the intensity of radiant energy,” according to the American Meteorological Society. There are a few types of actinometers, which are classified by the quantities they measure.

  • Pyranometer: measures the combined intensity of diffused sky radiation and direct solar radiation
  • Pyrheliometer: measures the intensity of direct solar radiation
  • Pyrgeometer: measures terrestrial radiation

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