What Is The Solar Energy That Reaches Earth’S Surface Called?

Solar energy refers to the radiant light and heat emitted from the sun. Approximately 93 million miles from Earth, the sun is Earth’s primary source of energy, generating immense amounts of light and heat. The sun emits energy in the form of electromagnetic radiation, which travels at the speed of light and reaches Earth in about 8 minutes. This solar radiation drives Earth’s climate and weather and fuels life through the process of photosynthesis. Earth intercepts only a tiny fraction of the sun’s energy output, yet this amount of solar energy hitting Earth continuously far exceeds humanity’s energy needs.

The amount of solar radiation that reaches Earth’s outer atmosphere per unit time is known as the solar irradiance or solar constant. After passing through the atmosphere, solar radiation that reaches Earth’s surface is called solar irradiance. On average, Earth’s surface absorbs about 1,366 watts per square meter (W/m2) of direct solar irradiance when the sun is directly overhead, though this amount varies based on factors like latitude and local weather.

Solar Irradiance

Solar irradiance is the power per unit area received from the Sun in the form of electromagnetic radiation. It is measured in watts per square meter (W/m2). The solar irradiance that reaches the Earth’s atmosphere is called the extra-terrestrial irradiance, while the irradiance that reaches the Earth’s surface is called the terrestrial irradiance (Source: NREL).

The solar irradiance that is incident on a plane surface perpendicular to the rays, with no absorption or scattering, is called the direct normal irradiance. Diffuse horizontal irradiance is the solar radiation received from the Sun after its direction has been changed by scattering by the atmosphere (Source: TheFreeDictionary).

Components of Solar Irradiance

Solar irradiance reaching the Earth’s surface has two main components: direct and diffuse irradiation. Direct irradiation comes directly from the sun without being scattered by the atmosphere. Diffuse irradiation is solar radiation that has been scattered by the gases and particles in the atmosphere before reaching the Earth’s surface (Source 1).

The spectral composition of solar irradiance refers to how the intensity of irradiation varies across the solar spectrum, from ultraviolet to visible to infrared wavelengths. Solar irradiance contains ultraviolet radiation, visible light, and infrared radiation. The relative contribution of each band of wavelengths varies based on factors like time of day, atmospheric conditions, and location on Earth (Source 2).

Measuring Solar Irradiance

Solar irradiance is measured using specialized devices called pyranometers or solar irradiance meters. Pyranometers contain a sensor that measures the solar radiation flux density across a hemispherical surface. The measurements are taken in watts per square meter (W/m2).

Pyranometers must have a spectral response similar to photovoltaic solar panels in order to accurately assess and predict the performance of PV systems. The standard requires a spectral range of 360 nm to 1120 nm. Pyranometers meeting the ISO 9060 standard provide the most accurate measurements of solar irradiance for PV system analysis. How is solar irradiance measured? (Seaward.com)

Typical readings for solar irradiance range from about 100 W/m2 on cloudy days to 1000 W/m2 on bright sunny days. The data can show how solar irradiance varies throughout the day and year. This helps determine the optimal location and tilt angle for solar panels.

Factors Affecting Irradiance

There are several key factors that affect the amount of solar irradiance that reaches Earth’s surface.

The most significant factor is the atmosphere. As sunlight passes through the atmosphere, particles and gases scatter and absorb some of the radiation, reducing the amount that reaches the surface. The amount of scattering and absorption depends on the depth of atmosphere that the sunlight passes through, which is affected by the solar altitude angle. More atmosphere for sunlight to pass through near sunrise and sunset results in greater scattering and absorption compared to when the sun is high in the sky.

Latitude also plays an important role, as locations nearer to the equator receive more direct beam irradiance from the sun. The tilt of the Earth on its axis causes seasonal variations in the angle of incoming sunlight and duration of daylight, resulting in greater irradiance in summer vs. winter for a given location.

Time of day is another obvious factor, as the position of the sun in the sky determines the intensity of incoming radiation and the amount that is absorbed or scattered. Irradiance is at its maximum when the sun is directly overhead.

Local weather is also impactful, with cloud cover reflecting and absorbing a significant fraction of surface irradiance. Clear sky conditions allow the greatest amount of solar radiation to reach the ground. Other localized atmospheric effects like humidity, dust, and pollution concentrations can also play a role in scattering and absorption.

According to research by J. Cha (2021), “…the amount of middle and low-layer cloud cover is also an important factor affecting solar irradiance.” https://www.mdpi.com/2076-3417/11/18/8533

Applications of Solar Irradiance Data

Solar irradiance data has a wide range of applications across several industries and fields, including:

Solar power generation – Accurate measurements of solar irradiance are critical for effective design and operation of solar photovoltaic systems and concentrating solar power plants. Understanding the available solar resource enables optimization of system sizing, orientation and efficiency (Vignola, 2004).

Architecture/construction – Knowledge of solar irradiance patterns on building surfaces empowers architects and engineers to design structures and facades for optimal daylighting, heating, cooling and energy efficiency. Irradiance data informs building codes and standards for solar design (Thekaekara, 1974).

Agriculture – Solar radiation drives photosynthesis and impacts crop growth and yield. Irradiance data allows for selection of appropriate crops and planting times to maximize agricultural productivity.

Climate science – Long-term solar irradiance data provides insights into natural variations and historical trends useful for climate modeling and detection of anthropogenic climate change signatures (Vignola, 2004).

Solar Constant

The solar constant is defined as the total amount of solar radiant energy received per unit time on a surface perpendicular to the sun’s rays at mean Earth-Sun distance outside the atmosphere (i.e. at the top of the atmosphere). In other words, it is the amount of incoming solar electromagnetic radiation per unit area that would be measured at Earth’s average orbital distance from the Sun.

The currently accepted value of the solar constant is approximately 1,361 watts per square meter (W/m2). This value represents the mean total solar irradiance, accounting for variations in orbital distance over the course of a year.

While the solar constant has an average value, it does vary slightly over different timescales. This variability is caused by changes in solar activity, Earth’s orbital parameters, and terrestrial reflectivity and absorption. Typical variations are around 0.1% over a solar cycle of 11 years.

Absorption and Reflection

The Sun bombards the Earth with solar energy, but not all of it gets absorbed. According to the University of California, Berkeley, about 30% of the sunlight that reaches Earth is reflected back into space, while the remaining 70% is absorbed by the atmosphere and surface (Reflection/absorption of sunlight). The Earth’s atmosphere and surface work together to absorb the majority of the incoming solar irradiation.

NASA provides more details on the absorption and reflection of solar energy. Around 23% of incoming sunlight gets absorbed by atmospheric gases like ozone, oxygen, and water vapor. Clouds reflect around 25% of sunlight back into space. Oceans, soil, rocks, ice, and snow reflect about 4% and absorb the rest. The atmosphere and Earth’s surface together absorb 71% of incoming solar radiation (NASA: Climate and Earth’s Energy Budget).

Historical Research

Early observations and theories about solar irradiance date back centuries. In 1838, Claude Pouillet made some of the first quantitative measurements of solar irradiance, estimating it to be 1,228 watts per square meter (W/m2) at the top of the atmosphere (TOA) [1]. Other early estimates included those by Samuel Langley in 1884 (1,353 W/m2) and Charles Abbot in 1913 (1,322 W/m2) [2].

In the early 20th century, Charles Abbot and others at the Smithsonian Astrophysical Observatory studied solar irradiance extensively. They made regular measurements using pyrheliometers and improved the accuracy of estimates. Their data showed variation in solar irradiance correlates with sunspot activity. In the 1950s, theory emerged about how sunspot cycles impact irradiance [1].

It wasn’t until satellite measurements began in 1978 with NASA’s Nimbus 7 mission that continuous and global measurement of solar irradiance became possible. This allowed for better understanding of variations over different timescales [2].

[1] https://www.aanda.org/articles/aa/full_html/2018/07/aa31199-17/aa31199-17.html
[2] https://www.swsc-journal.org/articles/swsc/full_html/2021/01/swsc200108/swsc200108.html


In summary, the solar energy that reaches the Earth’s surface is called solar irradiance. Solar irradiance consists of direct and diffuse radiation and is measured using pyranometers and pyrheliometers. The factors that impact irradiance include latitude, time of day, season, cloud cover, and atmospheric conditions. Understanding solar irradiance is crucial for solar energy applications, climate research, agriculture, and more. By studying the solar constant and absorption/reflection of radiation, scientists gain insight into Earth’s energy budget.

Measuring and analyzing solar irradiance data enables key scientific advances and innovations. As research continues, our knowledge improves regarding Earth’s intricate relationship with the Sun. With a detailed understanding of the solar energy reaching Earth’s surface, we can maximize its benefits for renewable energy while planning for its impacts on climate.

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