Water Cycle Processes

This lesson examines the key physical processes that drive the water cycle in detail. While earlier lessons introduced these processes in the context of the global water cycle and drainage basin hydrology, here we explore the underlying science, spatial variations, and the factors that modify each process. A thorough understanding of these processes — and the terminology associated with them — is essential for answering AQA A-Level questions on physical geography.

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Evaporation

Key Definition: Evaporation is the process by which liquid water is converted to water vapour (a gas) at a temperature below its boiling point. It occurs at the surface of water bodies, soil, and wet surfaces.

The Physics of Evaporation

Evaporation requires energy to break the hydrogen bonds between water molecules. This energy is known as the latent heat of vaporisation — approximately 2,260 kJ per kg of water evaporated. When water evaporates, it absorbs this energy from the surrounding environment, producing a cooling effect. This is why evaporation is an important mechanism for heat transfer in the climate system.

Factors Controlling Evaporation Rate