Why does the small amount of water vapour in the atmosphere have an important role in the water cycle?

Despite representing only a tiny fraction of the hydrosphere, water vapour plays a critical role in the water cycle through two primary functions:

• Transporting Water: Water vapour acts as an invisible conductor, carrying fresh water distilled from the oceans over land via wind. This process is essential for making terrestrial life possible, as it facilitates the delivery of precipitation to land-based ecosystems. It serves as the principal phase of the ascending part of the water cycle, where vertical movements lead to condensation and the formation of clouds.12
• Regulating Radiation: Water vapour is a potent greenhouse gas that significantly influences Earth's radiation balance. By absorbing infrared radiation and emitting it back toward the surface, it helps maintain planetary temperatures warmer than they would otherwise be. Furthermore, it acts as a climate feedback, where rising surface temperatures increase evaporation, thereby concentrating more vapour in the atmosphere and further affecting surface heat retention.231