Diffuse sky radiation

The sky looks blue during the day.

Sunlight hits the air and bounces. This light is called sky radiation. It bounces off tiny things in the air. Blue light bounces the most. This is why the sky looks blue.

At sunset, the sky looks orange or red. This happens because the light travels a long way. It hits clouds and makes them bright.

Clouds can also help plants.

On cloudy days, light is soft. It does not make dark shadows. This light reaches leaves deep in the woods. It helps the whole plant make food. This can even help plants grow more.

Sunlight hits our air and bounces in many ways. This bounced light is called diffuse sky radiation.

Most sunlight travels in a straight line. But some light hits tiny parts in the air. This causes scattering, which means the light changes its path.

Blue light scatters more than red or green light. This is why the daytime sky looks blue. At sunrise or sunset, the light travels a longer path. Much of the blue light scatters away. This leaves orange and red colors in the sky.

Clouds can also change how light works. On cloudy days, all light is diffuse sky radiation. This light is soft, like light through frosted glass. This soft light helps plants grow. Direct sunlight makes dark shadows on leaves deep in the woods. But soft, bounced light can reach those leaves. This helps the whole plant make food. After Mount Pinatubo erupted, the air had a thick haze. This haze made more diffuse light. It actually helped many plants grow better for a few years.

Have you ever wondered why the sky changes color? It all comes down to something called diffuse sky radiation. This is sunlight that reaches the ground after bouncing off tiny parts in our air.

Most sunlight travels in a straight line from the sun to Earth. However, molecules and small particles in the atmosphere scatter that light in different directions. This process is what changes the colors we see in the sky every day. It is a very important part of how our world looks.

There are two main ways this light bounces, called Rayleigh scattering and Mie scattering. Rayleigh scattering happens when light hits very tiny things in the air. This type of scattering is special because it treats different colors differently. Blue light has shorter wavelengths, so it scatters much more easily than red or green light. This is why the sky looks blue when you look away from the sun.

At sunrise or sunset, the light has to travel a much longer path through the air. During this long trip, most of the blue light scatters away from your eyes. This leaves behind the beautiful orange and red colors we see in the clouds.

Scientists have studied these light patterns for a long time. In 1871, a scientist named Lord Rayleigh explained why the sky is blue. He used math to show how light waves interact with the air. This was a famous example of using physics to solve a mystery. Today, we measure this light on flat surfaces using a term called diffuse horizontal irradiance. We often measure it in units called watts per square meter.

Nature can change how much light bounces in big ways. For example, a volcano called Mount Pinatubo erupted in the Philippines in June 1991. It sent 17 million metric tons of sulfur dioxide into the sky. This created a huge haze layer around the whole planet for many years. This haze caused a 30% reduction in direct sunlight. Even though it was cloudy, the world did not stop growing. In fact, global temperatures dropped by about 0.5 degrees Celsius.

This extra bounced light actually helps plants in a surprising way. In a forest, direct sunlight creates dark shadows on the leaves at the bottom. These shadows can make it hard for those leaves to make food. But diffuse light acts like light through frosted glass. It spreads out and fills in the shadows under the tree canopy.

After the Mount Pinatubo eruption, many plants actually grew better for three or four years. The extra soft light helped more leaves perform photosynthesis at the same time.

Diffuse sky radiation is the solar radiation that reaches the Earth's surface after scattering from the direct solar beam. This process occurs when sunlight hits molecules or particulates within the atmosphere. It is also known simply as sky radiation. This phenomenon is the main reason why the colors of the sky change throughout the day. Scientists often measure this light on horizontal surfaces. They call this measurement diffuse horizontal irradiance, or DHI. It is usually measured in units called watts per square meter (W/m2).

The mechanism of scattering is a major cause of the attenuation of sunlight. Attenuation is the reduction of the intensity of the light beam as it passes through the air. There are two dominant radiative scattering processes: Rayleigh scattering and Mie scattering. Both of these processes are elastic. This means a photon of light can be deviated from its path without being absorbed. The light also does not change its wavelength during these specific processes. Rayleigh scattering occurs when the ratio of particle diameters to the wavelength is less than about one-tenth. In this state, the scattering coefficient varies inversely with the fourth power of the wavelength.

Different types of scattering depend on the size of the particles in the air. Rayleigh scattering involves diatomic gases and molecules. Because blue light has shorter wavelengths, it is scattered more efficiently than longer wavelengths like red or green. This is why the sky appears blue when you look away from the direct sun. At sunrise or sunset, the solar rays arrive nearly tangentially to the Earth's surface. This means the light travels a much longer path through the atmosphere. During this long journey, much of the blue and green light is scattered away. This leaves the Sun's rays and the clouds looking orange or red.

Mie scattering happens at larger particle ratios. This is common when light hits larger objects like cloud droplets. Under an overcast sky, there is essentially no direct sunlight. Instead, all the light comes from diffuse sky radiation. In these cases, the light flux is not very dependent on the wavelength. This is because cloud droplets are larger than the wavelength of light. They scatter all colors approximately equally. This causes light to pass through translucent clouds much like light through frosted glass. The intensity can range from 10% of direct sunlight in thin clouds to 0.001% under thick storm clouds.

History shows how important these studies are to physics. In 1871, Lord Rayleigh provided a famous explanation for the blue color of the sky. He used dimensional analysis to solve this problem. This remains a classic example of applying physics to natural mysteries. We can also see the impact of atmospheric changes through volcanic events. In June 1991, Mount Pinatubo in the Philippines erupted. It ejected roughly 10 billion cubic meters of magma. It also released 17 million metric tons of sulfur dioxide (SO2) into the atmosphere.

The Pinatubo eruption created a global stratospheric SO2 haze layer. This haze layer lasted for several years. It caused the global average temperature to drop by about 0.5 degrees Celsius. While direct sunlight was reduced by 30%, the impact on agriculture was surprising. For a few months, there was a 5% drop in overall solar irradiation. However, there was no negative impact on global agriculture. In fact, global agricultural productivity and forestry growth increased for three to four years. This happened everywhere except in boreal forest regions.

This increase in growth was linked to the way diffuse light interacts with plants. Under direct sunlight, dark shadows are cast onto the understory leaves. These shadows limit photosynthesis to only the top canopy layer. However, the reduction in direct sunlight also meant an increase in diffuse sunlight. This diffused skylight can illuminate leaves under the canopy. This allows for more efficient total whole-plant photosynthesis. It also increases evaporative cooling from vegetated surfaces. This phenomenon is part of the aerosol direct radiative effect. It can also be caused by other aerosols, such as moderately thick smoke loading from pollution.