The term 'thin high cloud' is a reference to cirrostratus clouds, which are a type of high-level cloud that form at altitudes of 18, 000 to 40, 000 feet. These clouds are composed of ice crystals and are usually thin and wispy. They often cover the entire sky and can produce halos around the sun or moon. Cirrostratus clouds are associated with the approach of a warm front, and can indicate the possibility of rain or snow in the near future.