What Is The Solar Energy That Gets To The Earth?

Solar energy is radiant light and heat from the Sun. It is the most abundant source of renewable energy available on Earth. Understanding solar energy reaching Earth is important for harnessing it efficiently for electricity and heat.

This article will provide an overview of the basics of solar energy, including:

• The Sun’s energy output
• Radiation reaching Earth from the Sun
• Effects of Earth’s atmosphere and day/night cycle
• Geographic location
• Weather and clouds
• Surface reflectivity
• Measuring solar radiation

With a foundational understanding of solar energy fundamentals, we can better utilize this clean, renewable resource.

The Sun’s Energy Output

The Sun produces energy through nuclear fusion at its core, converting hydrogen into helium. This process releases enormous amounts of energy in the form of electromagnetic radiation. The total solar irradiance (TSI) is a measure of the solar power per unit area received outside the Earth’s atmosphere. TSI has been precisely measured by satellites since 1978 and ranges from 1360-1380 W/m2, with variations of around 0.1% over an 11-year solar cycle

The solar luminosity is a measure of the Sun’s total energy output and is around 3.846×1026 W. This has increased about 30% over the 4.5 billion year lifetime of the Sun as hydrogen fuel has been converted to helium through fusion. The solar constant is the solar irradiance at a distance of one astronomical unit (AU) from the Sun. Its average value is 1361 W/m2 but varies by around 0.1% over the solar cycle. Changes in TSI show no long-term upward or downward trend over the past 40+ years of satellite measurements (NASA).

Radiation Reaching Earth

The Sun produces energy across the electromagnetic spectrum, but most of the radiation that reaches Earth is in the ultraviolet (UV), visible light, and infrared (IR) wavelengths. UV radiation has shorter wavelengths than visible light, while IR has longer wavelengths. Visible light from the Sun provides the energy that allows plants to photosynthesize and makes Earth habitable for humans.

Some solar radiation reaches Earth directly in a straight line from the Sun, while some is scattered by molecules in the atmosphere, reaching Earth’s surface as diffuse sunlight from the entire sky. The amount of direct vs. diffuse radiation depends on factors like cloud cover, air pollution, and the time of day. At noon on a clear day, most sunlight is direct, while early and late in the day, diffuse sky radiation dominates.

References:

[1] https://quizlet.com/699419686/earths-climate-climate-change-geology-exam-iii-flash-cards/

[2] https://books.google.com/books?id=w-cqAAAAMAAJ&pg=PA245&lpg=PA245&dq=%22radiation+reaching+earth+from+sun%22&source=bl&ots=DusqpHbTgu&sig=ACfU3U0OsQpP1S_-7NI2qRhNya-bq0LGlg&hl=en&sa=X&ved=2ahUKEwj1noqi4aOEAxXz_8kDHTK5ATEQ6AF6BAgJEAM

Effects of Earth’s Atmosphere

As sunlight passes through the atmosphere, various gases, dust, and aerosols interact with the incoming solar radiation, absorbing and scattering the photons before they reach the Earth’s surface (Atmospheric Effects). The most significant absorbers of solar radiation are water vapor, carbon dioxide, and ozone. Absorption by these gases reduces the intensity and changes the spectral composition of the sunlight that makes it through the atmosphere.

Scattering, specifically Rayleigh scattering by air molecules, deflects and redirects incoming solar photons before they reach the surface (Chapter 2 Section 1). This scattering effect can reflect up to 25% of incoming sunlight back into space. In addition, particulate matter in the atmosphere like dust, smoke, and pollution can scatter and absorb solar radiation through Mie scattering effects. Overall, absorption and scattering by the atmosphere filters out large parts of the incoming solar spectrum.

Effects of Day/Night Cycle

The amount of sunlight reaching the Earth’s surface varies dramatically between daytime and nighttime due to the planet’s rotation. Daylight hours result from the part of Earth experiencing direct exposure to the Sun’s rays. As the planet rotates once every 24 hours, any given location enters a period of darkness or nighttime when it rotates to the side facing away from the Sun.

The length of daylight hours also changes throughout the year due to the tilt of Earth’s axis of rotation relative to its orbit around the Sun. This tilt is what causes seasonal variations in the amount of incoming sunlight. During summer in the Northern Hemisphere, daylight hours are longer as that part of Earth is tilted towards the Sun. In the winter, daylight hours are shorter as the Northern Hemisphere tilts away from the Sun’s path through the sky.

According to a discussion on Reddit (source), these variations in daylight exposure are described quantitatively by a measurement called solar irradiance. The cyclic changes in irradiance between day and night, as well as between seasons, significantly impact the amount of solar energy available at any given location on Earth.

Geographic Location

Where a location is on the Earth impacts the efficiency of solar panels and the amount of solar energy reaching that location. Places near the equator generally receive more solar radiation and have higher efficiency solar panels than places farther from the equator. The Earth’s tilt in its rotation around the sun also causes the amount of solar radiation to vary between summer and winter seasons (Source).

Latitude plays a major role in solar efficiency, with locations closer to the equator at lower latitudes receiving more direct sunlight year-round. As latitude increases further from the equator, the amount of solar radiation decreases. This is because higher latitude locations experience more angled sunlight, spread out over larger surface areas. Locations at high latitudes may see little to no sunlight in winter months due to the tilt of the Earth. For example, a solar panel in Miami at 26°N latitude will produce 43% more energy over one year than the same panel in Anchorage at 61°N latitude (Source).

Weather and Clouds

Clouds play a significant role in modulating the amount of solar radiation reaching the Earth’s surface. Cloud coverage can vary substantially based on weather patterns and pollution levels. Thicker or more abundant cloud coverage generally reflects more incoming solar radiation back into space, reducing the amount of sunlight reaching the surface. According to a 2001 study, clouds reduce direct solar radiation by 10-25% globally on average (Portmann et al.). However, the extent of cloud reflection depends on the cloud type, thickness, and altitude. Low-level clouds like stratus typically reflect more radiation than high-level clouds like cirrus.

In addition, pollution particles can seed cloud formation and increase cloud albedo. Anthropogenic aerosols from fossil fuel burning lead to more numerous but smaller cloud droplets that scatter more light. This phenomenon is known as the Twomey effect. As a result, pollution indirectly reduces solar radiation receipt (Quizlet). Overall, cloudiness and weather patterns significantly impact the amount of solar energy reaching Earth’s surface in a given location.

Surface Reflectivity

The reflectivity of the Earth’s surface, known as its albedo, has a significant impact on the amount of solar radiation absorbed. Albedo is a measure of how much sunlight is reflected without being absorbed. Surfaces with high albedo reflect more sunlight.

Different surface types have different albedo values. For example, fresh snow has a very high albedo of up to 0.9, meaning it reflects about 90% of sunlight. Forests and oceans have a lower albedo of 0.1-0.3. Land surfaces typically fall between 0.1-0.4. In general, lighter colored surfaces tend to have higher albedo.

Water reflects only about 6% of incoming solar radiation, whereas snow-covered surfaces reflect up to 90%, according to a study on ground reflectivity (Thevenard, 2006). This difference significantly impacts the amount of solar energy absorbed by the Earth’s surface.

Increasing albedo leads to more reflection and less absorption of solar radiation. This in turn impacts climate and weather patterns. For example, a decrease in Arctic sea ice and snow cover has lowered albedo, allowing more radiation to be absorbed and causing additional warming.

Measuring Solar Radiation

There are several methods used to measure solar radiation reaching the Earth’s surface. One of the most accurate instruments is called a pyranometer. Pyranometers measure solar irradiance across a hemisphere of the sky on a flat surface. They can precisely measure both direct beam and diffuse solar radiation in watts per square meter (W/m2). According to Hukseflux, pyranometers are considered the “gold standard” for solar radiation measurement and are widely used in meteorology, climatology, and solar energy applications.

Another technique is spectroscopy, which analyzes the spectrum of solar radiation to determine how much energy is contained within specific wavelengths. This can identify the contributions of ultraviolet, visible, and infrared radiation. Spectroscopy provides more detailed information about the composition of incoming solar radiation.

Solar resource maps also estimate the solar energy potential for regions using geospatial data on average weather patterns and cloud cover. According to the PV Education site, while less accurate than instrumentation, these maps can estimate regional solar resources for planning renewable energy systems.

Conclusion

In summary, understanding Earth’s solar resource is critical as we transition to a more renewable energy-based society. The amount of solar energy reaching the Earth’s surface depends on several factors, including the Sun’s output, Earth’s atmosphere and geographic location, time of day, weather conditions, and surface reflectivity. While the maximum solar irradiance at the top of the atmosphere is about 1,370 watts per square meter, the amount of solar energy available at the Earth’s surface ranges widely based on those factors. Measurement of solar radiation on the ground provides important data for solar energy applications. As solar power continues to expand as an energy source, insights into the solar resource availability and variability will be key to integration into electric grids and effective utilization of this renewable resource.