## Data and Methods
### Isccp-H  
The second satellite dataset used is ISCCP-H, processed by the National Centers for Environmental Information (NCEI) of NOAA. Similarly to the ISCCP-D series products, the primary instrumentsthat serve as inputs to the ISCCP-H series are the imaging radiometers on operational weather satellites. These include the AVHRR on the polar-orbiting satellites and a variety of imagers that fly onboard geostationary meteorological satellites [8; 9]. Many improvements have been made in the ISCCP H-series, compared to its predecessors C- and D-series [30], such as higher resolution input satellite data and expanded period of record. Monthly-mean ISCCP-H TCC was averaged to the same 2.5\({}^{\circ}\)\(\times\) 2.5\({}^{\circ}\) latitude-longitude grid as CLARA-A2. The ISCCP-H data cover the 29-year period 1984-2012, which is the common period of both satellite datasets.  
In contrast to CLARA-A2, the ISCCP-H products incorporate only one visible (VIS \(\approx\) 0.65 \(\pm\) 0.05-0.20 \(\upmu\)m) and one infrared (IR \(\approx\) 10.5 \(\pm\) 0.5-0.75 \(\upmu\)m) \"window\" channel. At latitudes between 55\({}^{\circ}\)S and 55\({}^{\circ}\)N these measurements are provided by the geostationary satellites if available, while the second and third options are the afternoon and morning AVHRR satellite instruments, respectively. At higher latitudes, ISCCP uses both morning and afternoon polar orbiting satellites with AVHRR simultaneously for the gridded data products. However, if there are more than 2 afternoon orbiters ISCCP chooses only 1, while it does the same for morning orbiters. In the ISCCP analysis the orbiter with the equator overpass time closest to the intended orbit is selected. Both the geostationary and the AVHRR GAC measurements are resampled to a resolution of \(\sim\)10 km. The radiances from imagers onboard the geostationary satellites are normalized to the AVHRR radiances from the afternoon polar orbiter satellite series. In this approach, NOAA-9 acts as the absolute reference through 2009 [31]. The temporal resolution of the ISCCP-H dataset is 3-hourly where geostationary data is used, and varies with latitude where the polar orbiters are used.  
The ISCCP-H cloud masking algorithm can be described by four steps. First, tests of the space and time variations of the observed radiances on several scales are used to estimate cloud-free radiances. Results of the space-time tests are used in conjunction with the ancillary products to obtain a global composite of clear-sky radiances for each image pixel location and time. Second, cloudy conditions are diagnosed when IR- or VIS-observed satellite radiances sufficiently deviate from estimated values using various combinations of VIS and IR thresholds [32; 33]. Then, the composite clear-sky radiances are revised based on the prior detection threshold results and application of revised threshold tests of each image's pixels against the revised composite clear-sky radiance values using the ancillary products. Finally, cloud properties are retrieved producing the HXS product (H-series pixel level single satellite product) [8; 30; 34] which are then used to produce the HGM products (H-series Gridded Monthly products) that provide the monthly averages that are used here [8].