## 4 Cloud Parameter Databases and Retrieval Methods  
The retrieval of cloud parameters is a topic that has always interested the scientific community working in the field of Earth science, and numerous projects have been dedicated to it since the 1980s. Among the first in chronological order, there is certainly the International Satellite Cloud Climatology Project (ISCCP) described by [121]. This project aimed to provide some data of interest for cloud (optical thickness and top pressure) from 1983 to 2009. PATMOS-x also deals with operational processed cloud retrieval by providing a complete suite of atmospheric and cloud products based on observations from the AVHRR sensor flying on polar-orbiting weather satellites since the late 1970s. PATMOS-x was developed at NOAA/NESDIS/STAR in collaboration with the University of Wisconsin and Madison Cooperative Institute for Meteorological Satellite Studies (CIMSS) and is described by [122]. AVHRR data recorded on board the PM satellites of the NOAA Polar Operational Environmental Satellite (POES) missions are collected in the 35-year (1982-2016) climatological Cloud_CCI Advanced Very High Resolution Radiometer post meridiem (AVHRR-PM) dataset (version 3), containing global cloud and radiation properties on a grid with a latitude-longitude resolution of 0.05\({}^{\circ}\) (details in [123]). The CM SAF cLoud, Albedo and surface RAdiation dataset from AVHRR data (CLARA) dataset is a global dataset containing surface radiation products and data on surface albedo, as well as cloud properties, generated via the EUMETSAT CM SAF. Its first version (CLARA-A1) is based on AVHRR measurements recorded in polar orbit onboard the NOAA and MetOp satellites. The data cover a 28-year period (1982-2009) and are arranged on a regular global grid with a resolution of 0.25 degrees (latitude-longitude). More details can be found in [124]. The CLARA-A1 dataset has been updated to CLARA-A2, the second edition, described in [125]. The reference time is extended to 34 years, reaching up to 2015. As regards clouds, the averaged products are available with two temporal samples (daily and monthly), while, as global products, a daily resampling is available for individual satellites. The dataset contains the cloud mask, cloud-top temperature/pressure/height, cloud thermodynamic phase, and cloud optical thickness, particle effective radius, and cloud water path. Spatially, the monthly and daily averages are available on a global 0.25\({}^{\circ}\)\(\times\) 0.25\({}^{\circ}\) grid. Furthermore, for the polar regions, cloud products are also provided in two equal 25 km \(\times\) 25 km grids. The v.4 retrieval algorithms for CERES-MODIS products were implemented from 2012 until MODIS data were available at a resolution of 1\({}^{\circ}\)\(\times\) 1\({}^{\circ}\) (latitude by longitude). CERES aims to study Earth's climate system to analyze the role of cloud and radiation feedback [126]. During this study, the following cloud product retrievals were obtained: ice water path, liquid-water path, cloud particle phase, cloud base pressure, cloud-top pressure, ice particle effective diameter, water particle radius, and cloud visible optical depth [127]. As mentioned above, the MOD06 and MYD06 databases for Terra and Aqua, based on level 2 (L2) MODIS products, provide pixel-level retrievals of optical properties of clouds (optical thickness, effective particle radius, CLWP, CIWP, and thermodynamic phases--daytime only) and cloud-top properties (temperature, pressure, and height--both day and night). An overview of the major updates contained in the latest reprocessing (MOD06 C6 of 2000 and MYD06 of 2002, respectively) is outlined in [79]. Although the methodologies underlying the cloud product dataset and ancillary data may vary from case to case, clear sky reference data and cloud radiance simulations are typically produced using radiative transfer models.