In addition to these gases, water vapour (H2O) is a vital atmospheric constituent, averaging about 1% by volume, with significant variations across spatial and temporal scales. Its presence in the atmosphere forms part of the global hydrological cycle. Water vapour, being the most important natural greenhouse gas on account of its abundance, plays a crucial role in the regulation of the atmosphere’s energy budget. Despite this, the total volume of water in the atmosphere is relatively small, and, if precipitated completely and evenly over the whole Earth, would yield only about 25mm or 1 inch of rainfall (Kemp, 1994). In reality, of course, rainfall distribution is highly uneven, due to the internal dynamic processes within the global climate system.
In addition to the gases in Table 1.1, there are other reactive gas species produced by cycles of sulphur (S), nitrogen (N2) and chlorine (Cl) halogens. For a further discussion of these species, Wayne (1991) offers a useful text.