In this chapter we examine the next stage of the hydrologic cycle (Section 6.1). Evaporation was considered in Chapter 4, and the consequent atmospheric humidity in Chapter 6. Then Chapter 7 dealt with atmospheric instability, which is a major cause of uplift, often cooling air to its dewpoint temperature so that it is saturated and cloud forms. Now we will explain how this occurs, and some of the consequences.
Clouds consist of tiny ice particles or water droplets, so small and light in weight that impacts from the air's randomly moving molecules are sufficient to keep the particles and droplets from falling. They control climates in several ways. Clouds are the source of rain, and they obscure and reflect radiation (Chapter 2), so they govern the net radiation which energises photosynthesis (Note 1.B), heating of the ground (Section 3.5) and evaporation (Chapter 4). An increase in low-level cloudiness by just a few per cent would offset any global warming due to more carbon dioxide in the atmosphere.
The ice particles and water droplets of clouds derive from the condensation of water vapour in cooled air (Section 6.2). The cooling may be caused (i) by mixing with colder air, (ii) by local cooling due to either (a) nocturnal radiation loss (Section 2.7), or (b) flow over a cold surface (creating advective cloud), or (iii) by uplift (Section 7.1). Uplift is the most common cause, and the only way to produce rainfall (Chapter 10). The form of uplift cooling determines the kind of cloud (Table 8.1). In more detail, the forms of cooling are as follows.
Table 8. 1 Typical cloud dimensions
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