## 3 Review and Evaluation of Solar Irradiance Forecasting Methods
### Image-Based Methods
#### 3.3.2 All-Sky Images
Very typically, for cloudy images, the pixels in the circumsolar region are incorrectly identified as sky pixels due to this CSL compensation. In order to account for this effect, the Haze Correction Factor (HCF) is iteratively used to correct the images before applying the cloud identification [188; 196]. The FTM method is one of the fixed threshold methods for the RGB values introduced above. It is based on the fact that cloud pixels have higher red intensity (R) values than sky pixels [43]. In general terms, the segmentation is more manageable for single-layer clouds, and preferably for optically thick cumulus clouds, low turbidity, and high solar elevations, due to the high contrast. Instead, optically thin clouds are a considerable challenge for all the segmentation approaches. An illustrative example of results from the correction processing, segmentation, and motion vector generation is presented in Figure 7 by using the results presented [43]. When low solar elevation and a high Linke turbidity factor occur simultaneously, the blue color of the sky becomes low in saturation and appears as a whitish-grey color. Segmentation in these conditions becomes challenging even for a human observer, as only partially illuminated clouds contrast enough with aerosol layers to be distinguishable from each other [40]. Several studies have discussed the reduced boundary between clouds and aerosols in such cases [197; 198; 199]. Cloud classification is a valuable task for researchers due to several reasons. For instance, it can help identify different cloud types, detect vertical growth, and determine if multiple layers are present within a cloud. On one hand, the vertical cloud profiles contain important information for the different cloud types [190,191]. Clouds can be classified into different types based on their height in the atmosphere. The troposphere, the lowest layer of the atmosphere, is typically divided into three levels: lower, middle, and higher. Some clouds, such as nimbostratus (Ns) and deep convective clouds, may extend over multiple layers of the troposphere. The boundary between layers is not static and can vary depending on the latitude of the planet, as well as where a cloud is located. As such, some authors divide the planet into three latitude belts: polar, mid-latitude, and tropical. Each belt has its own associated interlayer boundaries [200]. In [201] the accuracy of the cloud height measurements with two layers, derived from the Atmospheric Infrared Sounder (AIRS), is investigated and measured over a period of five days of observations. Cumulus (Cu), stratus (St), and stratocumulus (Sc) clouds are associated with the lower level of the troposphere. These clouds are typically low-lying and can cover large areas. Altocumulus (Ac) and altostratus (As) clouds are associated with the middle level of the troposphere and are typically found at higher altitudes than lower-level clouds. Cirrus (Ci), circumculus (Cc), and cirrostratus (Cs) clouds are associated with the higher level of the troposphere and are typically found at very high altitudes. These clouds are thin and often appear wispy or feather-like [202]. Conversely, the optical properties of clouds play an essential role in determining their impact on the atmosphere's radiation budget. Clouds have distinct optical properties that can vary depending on factors such as the size and shape of cloud particles, water content, and vertical extent. These distinct properties describe different attenuation degrees, mainly due to the absorption and scattering that occurs in the cloud [203]. The attenuation caused by the clouds is described by the cloud optical thickness (COT), which depends on the micro- and macro-physical properties of a cloud. These properties include particle size distribution, shape, water path (WP), thermodynamic phase, and  
Figure 7: An example of the segmentation and generation of motion vectors. This figure demonstrates the process of segmentation and the generation of motion vectors.