Global cloud cover

There are difficulties in determining cloud cover and layer structure from both satellite and ground observations. Surface-based estimates of cloud amounts are some 10 per cent greater than those derived from satellites, mainly because of the problem of estimating gaps near the horizon. The greatest discrepancies occur in summer in the subtropics and in polar regions. Total cloud amounts show characteristic geographical, latitudinal and seasonal distributions (see Figures 3.8 and 5.11). During the northern summer there are high percentages over West Africa, northwestern South America and Southeast Asia, with minima over the southern hemisphere continents, southern Europe, North Africa and the Near East. During austral summer there are high percentages over tropical land areas in the southern hemisphere, due partly to convection along the Intertropical Convergence Zone, and in subpolar ocean areas due to moist air advection. Minimal cloud cover is associated with the subtropical high-pressure regions throughout the year, whereas persistent maximum cloud cover occurs over the Southern Ocean storm belt at 50-70°S and over much of the ocean area north of 45 °N.

Cloud acts both as an important sink for radiative energy in the earth-atmosphere system, through absorption, as well as a source due to reflection and re-radiation (see Chapter 3B, C). The mean annual net forcing effect of clouds is negative (~ -20 W m-2) because the albedo effect on incoming solar radiation outweighs the infrared absorption (Figure 5.12). However, cloud forcing is complex; for example, more total cloud implies more absorption of outgoing terrestrial radiation (positive forcing, leading to warming) whereas more high cloud produces increased reflection of incoming solar radiation (negative forcing, leading to cooling).

There is evidence that cloud amounts increased during the twentieth century. Observations, for example, show a striking increase in cloud cover over the United States (especially between 1940 and 1950). This may be associated with higher atmospheric sulphate concentrations due to increased coal burning. The relationship with temperature is unclear.

Continue reading here: Formation Of Precipitation

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