Supplementing the efforts to collect and analyze global data are more detailed and intensive field experiments conducted from airplanes and balloons and on the surface. For example, radars and lidars (light radars) give key information about cloud structures. Major experiments have been undertaken in the United States, Europe, Japan and the U.S.S.R.; more are planned there as well as in Australia, Brazil and China.
A thorough study of all these data will take many years and will lead, of course, to new experiments, but the investigations have already provided fresh insights into how clouds might change with the climate. For example, the study of low stratus clouds over the ocean off the western coast of the U.S. has suggested that a change in air pollution could alter the physical makeup of clouds. As the number of particles suspended in the air increases, so does the number of droplets in clouds, because each particle acts as a nucleus around which a droplet can form. The increase in droplets could in turn change the way the clouds reflect sunlight.
Intensive study has also focused on high cirrus clouds becuase of the prominent role they are thought to play in the greenhouse effect. Measurements of cirrus clouds in the central U.S. and in north-western Europe show that such clouds may be far more abundant than thought: many of them turn out not to be visible from the ground. The U.S. clouds also contain a surprisingly large number of minute ice crystals, which reflect more sunlight than the larger particles once thought typical. Thus, at least for these cirrus clouds, the cooling effects of reflection seem to be much stronger than they are thought to be for cirrus clouds in general, and they might more nearly offset the trapping of thermal radiation from the surface.