Is The Energy Transferred From The Sun To Earth Called Radiation?

The sun radiates an enormous amount of energy in the form of electromagnetic waves across space to Earth. This solar radiation is the primary source of energy that drives Earth’s climate and weather patterns. But is the energy transferred from the sun to Earth technically called “radiation”? In this article, we will examine the characteristics of solar radiation, how it interacts with Earth’s atmosphere and surface, and whether “radiation” accurately describes this transfer of energy.

Table of Contents

Definition of Radiation

Radiation refers to the emission of energy as electromagnetic waves or subatomic particles. The term encompasses both energy that we can see with the human eye, such as visible light, as well as energy that cannot be seen such as ultraviolet radiation, infrared radiation, microwaves, radio waves, and X-rays.

The Electromagnetic Spectrum

The electromagnetic spectrum refers to the entire range of electromagnetic waves. These waves carry energy and travel through space at the speed of light. The electromagnetic waves with different frequencies exhibit different types of energy known as radiation:

Radio waves – At the low frequency end are radio waves. They have wavelengths larger than a meter and carry very little energy.
Microwaves – Microwaves have a higher frequency than radio waves. They have wavelengths from 30 centimeters down to one millimeter and are utilized for communications and cooking.
Infrared waves – Infrared waves have an even higher frequency and are utilized in heat lamps, night vision devices, and to probe molecular structure.

Visible light – In the middle of the spectrum is visible light that humans perceive as colors ranging from red to violet. It enables plants to go through photosynthesis.
Ultraviolet – Just beyond violet in frequency comes ultraviolet light, which holds enough energy to break molecular bonds. It causes sunburn but also enables vitamin D production in skin.
X-rays and Gamma rays – At the highest frequency end, x-rays and gamma rays have the smallest wavelengths but carry tremendous amounts of energy. These extremely high-energy rays can be hazardous but also used carefully in medical imaging.

Different types of electromagnetic radiation, categorized by their frequencies and wavelengths across the spectrum, interact with matter in unique ways from radio wave communications to gamma ray medical treatments.

The Sun’s Energy Output

The sun produces vast amounts of energy in the form of electromagnetic radiation. The different types of radiation that our sun outputs include:

Infrared radiation – Longer wavelength, lower frequency radiation that we perceive as heat energy. While the sun does produce some infrared energy, most does not reach the Earth due to its longer wavelength, which is absorbed into space before reaching the Earth’s atmosphere.

Visible light – This is the range of electromagnetic radiation that we perceive as light with the human eye – a range from violet light with short wavelengths, to red light with longer wavelengths. It is the peak range of the sun’s energy output that reaches the Earth’s surface.

Ultraviolet radiation – Shorter wavelength, higher frequency radiation with more energy per photon than visible light. While a lot of this high-energy ultraviolet radiation is blocked by the Earth’s atmosphere, some still makes its way through to Earth’s surface.

Radiation Reaching Earth

As the sun’s radiation travels through space, only a small fraction is able to reach the Earth’s atmosphere. Some of this radiation is absorbed or scattered by gases and particles in the atmosphere before it makes it to the planet’s surface. Visible light from the sun is mostly able to pass directly through the atmosphere and reach Earth’s surface to provide light and warmth. Ultraviolet rays, with shorter wavelengths, are partially absorbed by the oxygen and ozone in the upper atmosphere. Still, a significant amount reaches the surface where it plays an important role in plant growth and health but can also cause sunburns and skin cancer. Infrared radiation is largely transmitted through the atmosphere as heat, although some is absorbed by greenhouse gases like water vapor, carbon dioxide, and methane. The greenhouse effect that these gases create helps regulate Earth’s temperature within a habitable range.

Earth’s Absorption of Solar Radiation

The Earth and its atmosphere absorb over 70% of the solar radiation that reaches it. Different surfaces on Earth absorb and retain heat differently based on their material properties.

The oceans, which cover over 70% of the planet’s surface, readily absorb solar radiation. Water has a very high heat capacity, meaning it can absorb large amounts of heat energy while only experiencing a modest rise in temperature. The absorbed radiation causes the top layer of the oceans to become warmer.

Landmasses also absorb solar radiation, heating the land. Soil and rocks have lower heat capacities than water, so they experience greater temperature rises. Absorption also varies based on factors like color and moisture content – dry, light-colored soil reflects more radiation than wet, dark soil.

The atmosphere – comprised of various gases like nitrogen, oxygen, argon, and others – also absorbs heat. Solar energy excites and accelerates gaseous molecules in the atmosphere through conduction and convection, leading to temperature rises. Different gases have differing molecular structures and heat capacities, impacting absorption levels.

In total, about half the solar energy reaches the surface, heating the land and oceans. The other half directly heats the atmosphere or is reflected back into space by clouds, aerosols, or surface reflectivity.

Earth’s Reflection of Solar Radiation

Not all incoming radiation from the Sun is absorbed by the Earth. Some of the incoming sunlight, around 30% of the total, is reflected back into space. This reflected sunlight never reaches or impacts the surface of the Earth.

There are a few key surfaces on Earth that reflect radiation very effectively:

Clouds – Cloud tops are very reflective in visible and near-infrared wavelengths. Tiny water and ice particles in clouds scatter and reflect radiation like mirrors.

Ice and snow – Ice sheets, glaciers, sea ice, and snow have a very high albedo (reflectivity). Over 90% of sunlight striking these icy surfaces gets reflected back into the atmosphere and space.
Sand and deserts – Sandy deserts reflect up to 20-30% of incoming sunlight due to the light color and grainy nature of sand particles.

Earth’s Re-emission of Infrared Radiation

When solar radiation reaches Earth, some of it is absorbed by the land, water, and atmosphere. As Earth absorbs this incoming radiation, the land and oceans warm up. To maintain equilibrium, Earth must release as much heat energy back to space as it absorbs from the Sun. It does this by re-emitting the absorbed heat as a different form of electromagnetic radiation—infrared radiation. The infrared radiation or heat radiated back from Earth’s surface has longer wavelengths compared to the incoming visible light and ultraviolet radiation from the Sun. This is because Earth’s surface and atmosphere are much cooler than the Sun’s surface temperature of around 10,000°F (5,500°C). The average surface temperature on Earth is only around 60°F (15.5°C). Therefore, according to Wien’s displacement law, the terrestrial radiation emitted by Earth peaks at a longer infrared wavelength compared to the peak wavelength of the solar radiation absorbed.

The Greenhouse Effect

The greenhouse effect refers to the process of greenhouse gases (such as carbon dioxide and methane) trapping infrared radiation in Earth’s atmosphere. While visible light from the sun freely passes through the atmosphere, about 30% of the infrared radiation emitted by Earth’s surface gets absorbed by these greenhouse gases.

This absorption and re-emission of infrared radiation leads to additional warming of Earth’s surface and lower atmosphere. Greenhouse gases emit the absorbed infrared radiation back toward Earth, rather than allowing it to escape directly out to space. So while not actually trapping “heat,” per se, greenhouse gases effectively trap infrared radiation that would have otherwise radiated out to space, causing increased temperatures near the surface.

Conclusion

As we have seen, the sun constantly emits energy in the form of electromagnetic radiation across a broad spectrum of wavelengths, from radio waves to x-rays and gamma rays. Visible light that we can see with our eyes is only one small part of this spectrum.

When this radiation from the sun reaches Earth, our planet absorbs some of it, reflects some of it back into space, and re-emits most of the absorbed energy as longer-wavelength infrared radiation. Some of this infrared radiation gets trapped by greenhouse gases like carbon dioxide and methane, warming Earth’s surface in what is known as the greenhouse effect.

In summary, yes – the energy transferred from the sun to Earth is a form of electromagnetic radiation. This solar radiation across the spectrum is what powers virtually every natural process on our planet, from photosynthesis in plants to heating ocean and air currents that drive our climate and weather. So while the word “radiation” sometimes carries ominous connotations, Earth’s radiation budget and balance is critical to supporting all life as we know it.