File:British Isles North Sea satellite photo 2018.jpg

English: Islands tend to have a lot of cloud cover, thanks to the moisture all around them. Landmasses in middle latitudes also tend to be cloudier than other parts of the planet. And the intersections between different atmospheric circulation patterns can lead to a lot of cloud cover.

The United Kingdom and Ireland fit all of those categories, and they are among the cloudiest places on Earth. Air masses from the Arctic, southern and northern Europe, the Maritimes, and the Gulf Stream all come crashing together in this region. Yet in late June 2018, England, Ireland, Scotland, and Wales all bathed in a rare day of cloud-free skies.

On June 27, 2018, satellites captured the data for the natural-color image above. It is a composite of scenes acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite and by the Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite. While the islands on that day were cloudless, the seas around them were blanketed by lumpy marine stratocumulus clouds. Note, too, the phytoplankton bloom in the North Sea.

According to a 2012 study based on MODIS data, the probability of cloud-free skies on any given day over Great Britain is 21.3 percent, with a maximum probability of 33.3 percent in November and 12.9 probability in March. Overall, about 67 percent of the Earth is bathed in clouds on any given day, with just 10 percent of the ocean being completely cloud-free.

[more and refs at source]