A variety of human activities result in the release of substantial quantities of aerosol particles and gases which may influence cloud microstructure, precipitation processes, and other weather phenomena. In this section we examine evidence that these particulate and gaseous releases are influencing regional weather and climate. In this chapter, however, we will not focus on urban emissions of particulates and gases; that discussion will be reserved for Chapter 5.

Cloud condensation nuclei and precipitation

In our discussion of purposeful modification of clouds in Part I, we described attempts to enhance precipitation from warm clouds by seeding them with hygroscopic materials. The hygroscopic particles which are called cloud condensation nuclei (CCN) can alter the microstructure of a cloud by changing the concentrations of cloud droplets and the size spectrum of cloud droplets (Matsui et al., 2004, 2006). There are numerous examples of anthropogenic sources of CCN, including automobile emissions (Squires, 1966), certain urban industrial combustion products, and the burning of vegetative matter, especially sugar cane.

Warner and Twomey (1967) observed substantial increases in CCN concentrations beneath the base of cumulus clouds and increases in cloud droplet concentrations above their bases downwind of areas in which the burning of sugar cane fields was taking place. The practice of burning sugar cane fields to remove leaf and trash before harvesting is quite common in most areas where sugar cane is grown.