Solar irradiance

The sun sends light to our world.

This light is very strong. It hits the top of our air first.

Some of the light passes through the air. This helps plants grow and live. It also helps animals. The light can change how animals act. We can use the sun to make power. This helps us heat our homes. The sun is a very big part of our life.

The Sun sends power to our world. This is called solar irradiance.

Scientists measure this power in watts per square metre.

At the top of our air, the Sun is very strong. The average amount is about 1361 watts per square metre.

This is a steady amount of power. However, the air acts like a shield. Some light is lost as it travels down. Clouds and moisture can block the light. The time of day also matters.

There are different ways to measure this light. Direct normal irradiance is light that hits a surface straight on.

This light does not include light that has bounced around. We call that scattered light diffuse radiation.

When we add all the light together, we get global irradiance.

We study this to predict the weather. It also helps us make solar power. It can even help us plan space travel. Even the Earth's path around the Sun changes things. These changes are called Milankovitch cycles. They can cause ice ages over a long time.

Solar irradiance is the power we receive from the Sun. It comes to us as electromagnetic radiation. This energy is very important for our world. It affects how plants grow and how animals behave.

Scientists use it to study the weather and the climate. It also helps us plan for space travel. We can even use it to predict how much energy solar power plants will make.

This energy works in different ways as it reaches us. At the top of our atmosphere, we see total solar irradiance, or TSI. This is the power hitting the Earth's upper air.

Some light hits surfaces straight on. This is called direct normal irradiance, or DNI. Other light bounces off things in the air. This scattered light is called diffuse horizontal irradiance, or DHI.

When you add the direct light and the scattered light together, you get global horizontal irradiance, or GHI.

Measuring this energy helps us understand our planet's history. Scientists study how the Sun's power changes over long periods. For example, they look at the solar cycle. During one cycle, the total solar irradiance changed by about 0.1 percent.

Some studies also look at ultraviolet irradiance, which is a type of light. This can change by about 1.5 percent between the high and low points of the solar cycle. These tiny changes help us learn how the Sun works.

There are specific numbers used to describe this power. The average amount of radiation at the top of the atmosphere is about 1361 watts per square metre.

Because the Earth is a sphere, the average amount reaching the whole surface is about 340 watts per square metre. Scientists also use a unit called the langley for solar exposure. One langley is equal to 41,840 joules per square metre. These numbers help experts build better tools for measuring light.

Many things change how much light we feel on the ground. The tilt of a surface and the height of the Sun matter a lot.

Clouds and moisture in the air can also block the light. Even the Earth's path around the Sun causes changes. These are known as Milankovitch cycles.

These cycles change how much energy hits different parts of the Earth. Over very long times, these changes can even cause ice ages.

Solar irradiance is the power received from the Sun in the form of electromagnetic radiation. This energy is measured as power per unit area, often expressed in watts per square metre (W/m2). When scientists integrate this power over a specific time, they call it solar irradiation or insolation. This measurement is vital for many fields. It helps experts predict energy from solar power plants and model the Earth's climate. It also aids in weather forecasting and planning for space travel.

The mechanism of how this energy reaches us involves several stages and interactions. At the very top of the Earth's atmosphere, we measure Total Solar Irradiance (TSI). This is the solar power across all wavelengths hitting the upper atmosphere. The amount of energy at this level depends on the Sun's distance from Earth and the solar cycle. As the radiation travels toward the surface, the atmosphere acts as a filter. It absorbs and scatters some of the energy through moisture and cloud cover. This process reduces the amount of light that actually reaches the ground.

Scientists categorize solar irradiance into several distinct types based on how the light is measured. Direct Normal Irradiance (DNI), also called beam radiation, is measured perpendicularly to the Sun's direction. It excludes any light that has been scattered by the atmosphere. Diffuse Horizontal Irradiance (DHI), or diffuse sky radiation, comes from all points in the sky. This is the light that has been scattered by atmospheric components. When you combine the direct component and the diffuse component on a horizontal surface, you get Global Horizontal Irradiance (GHI).

There are even more specific ways to measure this energy for certain technologies. Global Tilted Irradiance (GTI) is the total radiation received on a surface with a specific tilt and direction. This is a key reference for photovoltaic power plants, where solar modules are often mounted on fixed or tracking structures. Another measurement is Global Normal Irradiance (GNI), which is the total radiation on a surface perpendicular to the Sun at a specific location. For detailed studies, scientists use spectral versions of these measurements. These look at how irradiance varies across different wavelengths or frequencies.

Historically, understanding these measurements has changed how we view Earth's climate. The study of solar radiation helps explain long-term changes like ice ages. These are linked to Milankovitch cycles, which are variations in Earth's orbit. These orbital changes affect how solar energy is distributed across different latitudes. For example, a past axial tilt reached 24 degrees during the Holocene climatic optimum. Such shifts in the Earth's position change the amount of energy hitting specific regions. This helps scientists reconstruct the history of our planet's temperature and ice cover.

The scale of solar energy is quite massive. At the top of the atmosphere, the average annual solar radiation is about 1361 W/m2. This value represents the power hitting the circular disc of the Earth as viewed from the Sun. Because the Earth is a sphere, the total area is larger than this disc. When you average the radiation over the entire spherical surface, the value is about 340 W/m2. This average is a crucial number for understanding radiative forcing. Scientists also use the unit "langley" for solar exposure, where one langley equals 41,840 J/m2.

Solar energy levels are not perfectly constant and undergo various fluctuations. Total solar irradiance (TSI) changes slowly over decades. During solar cycle 21, the variation was approximately 0.1 percent from peak to peak. Ultraviolet (EUV) irradiance shows more significant changes, varying by about 1.5 percent between solar maxima and minima. These changes can be tracked using specialized tools like pyranometers and pyrheliometers.

By measuring these tiny shifts, researchers can better understand the relationship between solar activity and the Earth's atmospheric systems.