Recent maps of the cloudiness over the globe in winter and summer (1, 2) were based on work by Berliand & Strokina (1980) and London et al. (1989), respectively. The areas of more than 80% cloud in June-August lie at around 60° S over the southern ocean, around 60° N over the Pacific and north of 45° N over the Atlantic. The southern ocean band of cloudiness widens to 45 - 70° S during the warmer months of December - February, whilst the northern patches of more than 80% cloudiness are slightly smaller.
Less than 30% cloud at year end occurs over much of Australia, part of China, north and west Africa, Arabia and India, California and a high-latitude part of western South America. In mid year such clear skies occur over the Mediterranean and the Near East, north Africa and South Africa, most of Australia, northeast Brazil and central South America, and coastal northwest USA. Obviously, there is much more cloud over the seas than inland.
Recently, efforts have been made to improve the global climatology of cloudiness by means of satellite data, mainly through the International Satellite Cloud Climatology Project.
(1) Peixoto, J.P. & A.H. Oort 1992. Physics of Climate. (Amer. Inst. Physics) p.173.
(2) Barry, R. & R. Chorley 1994. Atmosphere, Weather & Climate (Routledge) p.24.