A view of the Earth from afar reveals swirls of clouds covering the surface. Clouds are a constant presence across much of the world. Exactly how much of the world is covered in clouds? A group of researchers analyzed over a decade of satellite data to calculate just how much of the Earth is covered in clouds at any given time. The study used data collected from the Moderate Resolution Imaging Spectroradiometer (MODIS) over 12 years on the Terra satellite and from 9 years on the Aqua satellite to look at cloud fraction. Cloud fraction is the portion of each pixel on a MODIS image that is covered in clouds. They found that globally, roughly 67% of the Earth is covered in clouds. While they found little diurnal variation in cloud cover, the researchers calculated that slightly more clouds cover the oceans in the afternoon and land has slightly less cloud cover during that time. Seasonally, cloud variation is minimal over the oceans with an average of 72% of oceans experiencing cloud cover. Land experiences greater seasonal variation in cloud cover with an average of 55% of land covered in clouds.
King, M. D., Platnick, S., Menzel, W. P., Ackerman, S. A., & Hubanks, P. A. (2013). Spatial and temporal distribution of clouds observed by MODIS onboard the Terra and Aqua satellites. IEEE Transactions on Geoscience and Remote Sensing, 51(7), 3826-3852.