Clouds contribute to our lives in both direct and indirect ways. Clouds are at once the most visible elements of the sky and the dominant contributors to the weather we experience every day. Less apparent, but perhaps even more important, are the roles clouds play in the global energy and water budgets that determine the climate of Earth. Through their ability to precipitate, clouds provide virtually all of the fresh water on Earth and a crucial link in the hydrologic cycle. Clouds are also the most effective agents cleansing the atmosphere, although some terrestrial and aquatic ecosystems pay the price for anthropogenic emissions of chemicals into the air. With ever-increasing importance being placed on quantifiable predictions, whether to forecast the local weather or to anticipate changes in global climate, we must learn how clouds operate in the real atmosphere, where two-way interactions with natural and anthropogenic pollutants are common.

Clouds have been the subject of observation for centuries, but serious systematic investigations began only a few decades ago. For all practical purposes, the study of clouds can be traced back to Luke Howard, the English pharmacist who began, around 1803, the system of naming cloud types that we still use today (see Appendix A). Speculation about the composition and nature of clouds persisted for many years. Direct observations from balloons and aircraft helped greatly to develop a base of empirical knowledge upon which the research community could later build testable hypotheses.