The key difference between direct radiation and diffuse radiation is that direct radiation is the solar radiation that passes directly through the atmosphere from the sun without any scattering effect, whereas diffuse radiation is the solar radiation that is scattered by the atmosphere and reaches the ground.
Practically, it is important to treat radiation as something that experiences only a forward scattering process along with some unscattered experiences. Therefore, it can be described in two forms as direct and indirect or scattered radiation. This is because radiation coming or not coming from the direction of the sun can undergo these two types. However, direct radiation is usually composed of scattered radiation with sufficient angles (small) of deflection since it is the finite solid angle of the disc of the sun.
CONTENTS
1. Overview and Key Difference
2. What is Direct Radiation
3. What is Diffuse Radiation
4. Direct Radiation vs Diffuse Radiation in Tabular Form
5. Summary – Direct Radiation vs Diffuse Radiation
What is Direct Radiation?
Direct radiation can be described as radiation that is not scattered in the atmosphere; thus, it has a fixed direction since it comes from the disc of the sun. Solar direct radiation usually decreases together with reduced elevation. Solar radiation tends to enter the atmosphere from above, which enables it to travel further to the lower level. The higher the amount of water vapor, aerosol, and mixed gas in this atmosphere, the greater and faster the attenuation of radiation is. When considering the upper part of the troposphere and above, the attenuation of direct solar radiation by the atmosphere is insignificant.
Exposure to very high levels of direct radiation can lead to acute health effects, including skin burns and acute radiation syndrome. It can also cause long-term health effects, including cancer and cardiovascular diseases.
What is Diffuse Radiation?
Diffuse radiation is solar radiation coming to the ground after scattering in the atmosphere. When considering different places as well as different points of time, differences in distribution, and different constitution states, concentrations of water vapor and aerosol that can be found in the atmosphere can vary. This leads to different absorption and scattering of solar radiation.
Generally, diffused radiation from partially clouded skies can be usually substantially greater than diffuse radiation coming from clear skies and from overcast skies that are typical. Therefore, it is an invalid scientific process to attempt the calculation of monthly mean diffuse radiation through the linear interpolation between the estimated overcast sky and estimated clear-sky diffuse radiation values. Typically, the hourly sunshine amount that produces the highest diffuse irradiation conditions can be about 0.3 to 0.5 hours within the hour. However, high global radiation levels can occur under thin high cloud conditions, but the radiation is very diffuse dominated.
What is the Difference Between Direct Radiation and Diffuse Radiation?
The key difference between direct radiation and diffuse radiation is that direct radiation is the solar radiation that passes directly through the atmosphere from the sun without any scattering effect, whereas diffuse radiation is solar radiation that is scattered by the atmosphere and comes to the ground. Moreover, the path of direct radiation is certain and direct while the path of diffuse radiation is uncertain and indirect. Exposure to very high levels of direct radiation can lead to acute health effects, while exposure to diffuse radiation does not lead to severe effects.
The below infographic presents the differences between direct radiation and diffuse radiation in tabular form for side-by-side comparison.
Summary – Direct Radiation vs Diffuse Radiation
Solar radiation can reach the ground in one of two ways: direct and diffuse radiation. The key difference between direct radiation and diffuse radiation is that direct radiation is the solar radiation that passes directly through the atmosphere from the sun without any scattering effect, whereas diffuse radiation is the solar radiation that is scattered by the atmosphere and comes to the ground.
Reference:
1. “Direct Radiation.” An Overview | ScienceDirect Topics.
Image Courtesy:
1. “Solar radiation” By Siyavula Education (CC BY 2.0) via Flickr