On the relative humidity of the atmosphere

Water vapour is highly subsaturated in much of the Earth's atmosphere. This has important consequences for water vapour feedback, and also for general phenomena such as the runaway greenhouse. In this chapter, we discuss the processes that create subsaturation, with reference to both observations, general circulation models, and a range of idealized theoretical models which produce subsaturation. While this chapter focuses on subsaturation of water vapour in Earth's atmosphere, the processes discussed are generic to condensible substances in all planetary atmospheres.