On the relative humidity of the atmosphere

TL;DR

This paper explores the causes and implications of water vapor subsaturation in Earth's atmosphere, analyzing observational data, models, and theoretical frameworks to understand its role in climate feedbacks and planetary atmospheres.

Contribution

It provides a comprehensive review of the processes leading to subsaturation of water vapor, integrating observations, models, and theory across planetary atmospheres.

Findings

Water vapor is often highly subsaturated in Earth's atmosphere.

Subsaturation impacts water vapor feedback and climate stability.

Processes causing subsaturation are applicable to other planetary atmospheres.

Abstract

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.