

Difference Between Direct and Diffuse Solar Radiation Explained
Solar radiation is often called the solar resource or sun radiation or just sunlight, is a general term for the electromagnetic radiation emitted by the sun which is the only source for natural light in our solar system.
There are different types of solar radiation. Solar radiation or solar rays can be captured and turned into forms of energy that are beneficial to humankind. Solar rays can be converted into heat and electricity, using a variety of technologies. Depending on the availability of the solar resource the technical viability as well as economical operation of these technologies at a specific location is considered.
Pyranometer
A pyranometer is a solar irradiance sensor. Pyranometers can measure global irradiance. Solar Irradiation is defined as the amount of solar energy per unit area per unit time that has been incidented on a surface of specific orientation emanating from a hemispherical field of view (2π sr) and is denoted as Eg↓. The global irradiance encompasses direct sunlight as well as diffuse sunlight.
The contribution from direct sunlight is given by E⋅cos(θ); w
here, θ - the angle between the surface normal and the position of the sun in the sky and E- the maximum amount of direct sunlight. The global irradiance is then presented as:
Eg↓ = E⋅cos(θ) + Ed; where Ed accounts for the diffuse sunlight.
The extraterrestrial radiation is also usually expressed in Irradiance Unit (Watts/m²). Due to Earth's elliptical orbit around the sun, extraterrestrial radiation varies throughout the year. It depends on the distance between the Sun and the Earth.
Incoming Solar Radiation
The incoming solar radiation which is also known as insolation is the amount of solar energy reaching the Earth of about 70 percent.
The three relevant bands, or the ranges, along with the solar radiation spectrum are ultraviolet, visible (PAR), and infrared. Of the light reaching the surface of the earth, infrared radiation makes up 49.4%, while visible light provides about 42.3% 9.
Approximately 23 percent of the incoming solar energy is absorbed in the atmosphere by water vapor, dust, as well as by ozone. About 48 percent passes through the atmosphere and is absorbed by the surface and 23% of incoming energy is absorbed in the atmosphere by atmospheric gases, dust, and also by other particles.
Insolation Means
Sun is the only primary source of energy on this planet. Planets rotation, as well as the revolution, can all be considered based on the sun. So the sun plays a very significant role in the entire solar system.
The sun radiates its powerful energy in and every direction into space in short wavelengths, which are referred to as solar rays. The energy received by the surface of the earth in the form of short waves is termed Incoming Solar Radiation or Insolation. The amount of insolation that has been sustained on the surface of the earth is far less than what has been radiated from the sun because of the small size of the earth and its distance. The temperature of a place is also composed by the insolation received. The insolation differs according to the latitude, which leads to the difference in the temperature.
Solar Ray: Definition
Solar rays or solar radiations are described as the imaginary lines between the sun and an observation point on the earth.
Diffuse Sky Radiation
Diffuse sky radiation is solar radiation that reaches the surface of the earth after having been scattered from the direct solar beam by molecules or particulates in the atmosphere. It is also called sky radiation, or diffuse skylight, or just skylight, which is the determinative process to change the colors of the sky. Approximately 23% of the direct incident radiation of total sunlight is removed from the direct solar beam by scattering into the atmosphere. And of this amount only about two-thirds finally reaches the earth as photon diffused skylight radiation.
Diffuse Radiation and Direct Solar Radiation
As sunlight passes through the atmosphere, some of the sun rays are absorbed, scattered, and reflected by the following:
Air molecules
Water vapor
Clouds
Dust
Pollutants
Forest fires
Volcanoes
This is known as diffuse solar radiation. The solar radiation that reaches the surface of the earth without being diffused is known as the direct beam solar radiation. The sum of the diffuse as well as the direct solar radiation is called to be the global solar radiation. Atmospheric conditions may lead to the reduction of direct beam radiation by 10 percent on clear and dry days whereas it can be reduced by 100 percent during thick and cloudy days.
Basic Principles of Solar Radiation
Every place on the Earth receives sunlight for at least part of the year. The amount of solar radiation or solar rays that reach any spot on the Earth's surface varies according to the following:
Geographic location
Time of day
Season
Local landscape
Local weather
FAQs on Solar Radiation: Meaning, Types & Role in Physics
1. What is solar radiation?
Solar radiation is the electromagnetic energy emitted by the Sun, produced by nuclear fusion reactions in its core. This energy travels through space and reaches Earth, spanning a spectrum that includes ultraviolet (UV), visible light, and infrared (IR) radiation. It is the primary source of energy for most processes on Earth's surface.
2. What are the main types of solar radiation that reach Earth?
The solar radiation that penetrates Earth's atmosphere is primarily composed of three types:
- Infrared (IR) Radiation: Accounts for about 49% of the energy. We feel this as heat.
- Visible Light: Makes up about 44% of the energy and is the portion of the spectrum that human eyes can see.
- Ultraviolet (UV) Radiation: Constitutes the remaining 7%. While it has the shortest wavelength and highest energy, most of it is absorbed by the ozone layer.
3. What factors affect the amount of solar radiation received on Earth's surface?
The amount of solar radiation, or insolation, reaching a specific point on Earth is influenced by several key factors:
- Angle of Incidence: The angle at which the sun's rays strike the Earth. A more direct angle (closer to 90°) concentrates the energy over a smaller area, increasing insolation.
- Duration of Daylight: Longer days result in more hours of sunlight and therefore greater total radiation received.
- Atmospheric Transparency: Clouds, dust, pollution, and certain gases can reflect, scatter, or absorb sunlight, reducing the amount that reaches the surface.
- Earth's Distance from the Sun: Due to its elliptical orbit, Earth receives slightly more radiation when it is closer to the Sun (perihelion) and less when it is farther away (aphelion).
4. How is solar radiation measured, and what are its standard units?
Solar radiation is measured in terms of irradiance, which is the rate at which solar energy falls onto a surface. The standard instrument used for this measurement is a pyranometer. The standard unit for solar irradiance is watts per square meter (W/m²).
5. Why isn't all solar radiation that enters the atmosphere absorbed by the Earth's surface?
Not all solar radiation reaches the surface because the atmosphere interacts with it in three main ways. A significant portion is either reflected back to space by clouds and ice or scattered by air molecules (which is why the sky appears blue). Additionally, some radiation is absorbed by atmospheric gases like ozone, which absorbs harmful UV rays, and water vapour.
6. How does solar radiation relate to the concept of a 'blackbody' in Physics?
In Physics, the Sun is approximated as a near-perfect blackbody radiator. This means it emits thermal radiation based on its temperature. According to the Stefan-Boltzmann Law, the total energy radiated is proportional to the fourth power of its surface temperature. Furthermore, Wien's Displacement Law explains why the Sun's peak emission is in the visible light spectrum, as the peak wavelength is inversely proportional to its high surface temperature.
7. What is the 'solar constant', and why is it not truly constant?
The solar constant is the average amount of solar radiation received per unit area at the top of Earth's atmosphere, measured on a surface perpendicular to the incoming rays. Its value is approximately 1367 W/m². However, it is not truly constant due to two main reasons: variations in the Sun's energy output over its 11-year sunspot cycle and changes in the Earth-Sun distance throughout the year because of Earth's elliptical orbit.
8. What is the difference between solar radiation and terrestrial radiation?
The key difference lies in their wavelength and origin. Solar radiation is incoming energy from the Sun and is primarily shortwave radiation (high-energy, with a peak in the visible spectrum). In contrast, terrestrial radiation is outgoing energy emitted from the Earth's surface after it absorbs solar energy. It is longwave radiation (lower-energy, with a peak in the infrared spectrum). This distinction is fundamental to understanding the Earth's energy balance and the greenhouse effect.
9. What are some practical applications of understanding solar radiation?
Understanding solar radiation is crucial for many fields, including:
- Solar Energy: For designing and placing photovoltaic (PV) panels and solar thermal collectors to maximize energy generation.
- Meteorology and Climatology: It is a primary driver of weather systems and a critical variable in climate models.
- Architecture and Building Design: For creating energy-efficient buildings that utilize passive solar heating and natural lighting.
- Agriculture: To determine crop suitability, planting schedules, and the potential for photosynthesis in different regions.

















