Also, the higher the observer's eyes are from sea level, the farther away the horizon is from the observer. Its distance from the observer varies from day to day due to atmospheric refraction, which is greatly affected by weather conditions. With respect to Earth, the center of the true horizon is below the observer and below sea level. The true horizon surrounds the observer and it is typically assumed to be a circle, drawn on the surface of a perfectly spherical model of the relevant celestial body. On Earth, when looking at a sea from a shore, the part of the sea closest to the horizon is called the offing. The resulting intersection of such obstructions with the sky is called the visible horizon. At many locations, this line is obscured by terrain, and on Earth it can also be obscured by life forms such as trees and/or human constructs such as buildings. The true horizon is a theoretical line, which can only be observed to any degree of accuracy when it lies along a relatively smooth surface such as that of Earth's oceans. This line divides all viewing directions based on whether it intersects the relevant body's surface or not. The horizon is the apparent line that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. Read also our note on Horizon in Meteorological data.A horizon at sunset in High Desert, California, USA They become more significant for very tilted or vertical planes. Nevertheless, we can observe that these contributions (and their errors) are rather low for low plane tilts, since the horizon irradiation has a low cosine factor. The reality is certainly very complex, and requires more experimental investigations to assess these hypotheses on diffuse and albedo contributions. Therefore the user has the opportunity of determining which fraction of calculated albedo he wants to take into account, according to the distance of horizon obstacle. On the other hand, if the "horizon" obstacle is rather near, albedo should be considered as null. We consider the albedo to be linearly decreasing according to the horizon height (up to zero for horizon > 20°). For far horizons, some radiation may be reflected by the ground ahead of the collector plane. This is independent of the sun position, and therefore constant over the year.Īlbedo contribution is more difficult to estimate. We can admit that radiation from the back side of the obstacles is null, and therefore the diffuse attenuation is calculated as an integral of an isotropic radiation over the portion of sphere "seen" by the plane, above the horizon line. The effect on the diffuse component is not so clear. As meteo is recorded in hourly time steps, the program determines the exact time when the sun crosses the horizon line and weights the beam hourly value before performing the transposition. The effect on the beam component is of the "ON/OFF" kind: at a given instant, the sun is or is not visible on the field. NB: A file with PVsyst format is not an ASCII file and cannot be exported to other software. It is stored in the \Shadings\ subdirectory with an extension. You have the possibility of importing horizon profile files from some other tools or Software.Ī horizon profile can be saved to reuse it in another project or meteo calculation. It is not taken into account in the simulation and is not shown on the report. NB: an horizon profile with all heights less than 2° is considered insignificant. Horizon measurements (list of height and azimuth of some significant points) can be obtained on-site with a compass and theodolite (clinometer), a detailed map, panoramic or fish-eye photographies, etc. To delete a point, click on this point with the right button.
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