# Prototyping of Environmental Kit for Georeferenced Transient Outdoor Comfort Assessment
## 4 Data Acquisition and Handling
### Solar Radiation  
The radiant (electromagnetic) energy received from the sun is known as solar radiation. There are three main bands on the solar spectrum distributed at different wavelengths, i.e., ultraviolet (100 nm to 400 nm), visible (400 nm to 700 nm), and infrared (700 nm to 1 mm). The visible light is also referred to as photosynthetically active radiation (PAR). The infrared radiation (IR) covers most of the solar spectrum, but the energy of the solar radiation is distributed. The division includes up to 49.4% in IR range, 42.3% in the PAR range, and about 8% in the UV range [25].  
The irradiance of the sun dropping on the earth's atmosphere varies up to 6.6% throughout the year due to changes in the distance between them. The mean distance is about 149,597,890 km (1 AU). The irradiance of the sun on the outer atmosphere at the mean distance is called the solar constant. The accepted values for the solar constant range according to the National Aeronautics and Space Administration (NASA) is 1353 \(\pm\) 21 W/m\({}^{2}\), while the World Meteorological Organization (WMO) recommends 1367 W/m\({}^{2}\). The solar constant is the sum of irradiances at different wavelengths distributed over the solar spectrum.  
The range of the solar spectrum is largely between 200 and 2500 nm in space, and it divides into numerous segments at the earth's surface. The terrestrial spectra consist of direct radiation, diffuse radiation, and reflected radiation (dependent upon a location's albedo), while the sum of direct and diffuse radiation is known as global radiation.  
The radiation spectrum experiences frequent alterations after passing through the atmosphere. Clouds can block most of the direct solar radiation. Similarly, drifts in the ozone layer and seasonal deviations also have significant effects on the ultraviolet segment of the spectrum. These modifications are caused due to absorption and scattering by oxygen, nitrogen, carbon dioxide, and water molecules in the atmosphere. The terrestrial irradiance in clear sky conditions may reduce to ca. 1050 W/m\({}^{2}\) direct radiation and ca. 1120 W/m\({}^{2}\) global radiation from 1367 W/m\({}^{2}\) in space.  
The extraterrestrial spectrum is called \"Air Mass 0\" (AM0) as it does not pass through any air mass. When the sun is at its zenith and direct radiation passes through the atmosphere completely, it is known as \"Air Mass 1 Direct\" (AM1D) radiation. The standards suggest sea level as a reference site. Similarly, \"Air Mass 1 Global\" (AM1G) refers to the global radiation with overhead sun. Three spectra have been published by the American Society for Testing and Materials (ASTM)--AM0, AM1.5 Direct, and AM1.5 Global for tilted surfaces--\"at 37\({}^{\circ}\) angles for the USA\". The collimation of direct solar radiation occurs at circa 0.53\({}^{\circ}\), whereas the diffuse radiation is received from ground reflections, atmospheric scatter, and diffusion through the sky. The global radiation is broadly unvarying. The aerosol causes a high distribution of solar radiation by scattering, causing a ring at the rim of the solar disk called the solar aureole. The direct beam in addition to the solar aureole is known as circumsolar radiation [26].  
Figure 7: Globe temperature, wind speed, and mean radiant temperature curves.