Earth Cloud Climatology

Clouds cover approximately 2/3 of the Earth’s surface and are often organized into coherent systems by large-scale atmospheric flows. The figure below shows that clouds occur most frequently over the tropical precipitation belts and the middle-latitude oceanic storm track regions  with the continental desert and central subtropical oceans being relatively cloud free. Clouds are composed of liquid at temperatures above zero, ice below about -38C, and either or both phases at intermediate temperature. Since clouds extend higher in the tropics, ice  and mixed-phase  clouds are important throughout all latitudes . At any given time, most clouds are not precipitating. But precipitation can be crucial in regulating clouds and associated moist turbulence processes.

Figure 1 from the IPCC AR5 report (Chapter 7, Fig.7.5(a) annual mean cloud fractional occurrence. (b) annual zonal mean liquid and ice water path. (c-d) latitude-height distributions of annual zonal mean cloud and precipitation occurrence (magnitude of precipitation occurrence is doubled to make use a common color scale).