The Changing Hydrological Cycle

The hydrological cycle fundamentally involves evaporation (E) of moisture from the land and vegetation or ocean surface into the atmosphere, and back again as precipitation (P) (Fig. 10.1). Evaporation produces evaporative cooling at the surface and moistens the air. It includes transpiration from plants in which water enters the atmosphere through the tiny stomata in leaves as photosynthesis occurs. Together these are called evapotranspiration. Precipitation and the relationships to humidity and sea surface temperatures (SSTs) were introduced in Section 5.4. Water vapor is moved around by the atmosphere, and the precipitation occurs elsewhere, often in preferred locations such as mid-latitude storm tracks or tropical monsoons and convergence zones.