The atmospheric branch of the hydrologic cycle comprises moisture in three forms: (1) water vapor, (2) condensed (liquid) and sublimated (ice) water in clouds, and (3) precipitation (primarily rain and snow). Although radiation and energy budgets are the fundamental basis of physical [Page 144]geographic processes, by themselves they are insufficient to generate weather and climate: Moisture is essential. Latent heat absorbed when water evaporates at Earth's surface, and released in condensation when clouds form, links the energy and moisture budgets. Moreover, atmospheric moisture modulates the incoming solar (i.e., shortwave, SW) radiation and Earth-emitted long-wave (LW) radiation streams, significantly affecting the surface net radiation (SW + LW) and energy budget. Much like CO2 (carbon dioxide) and methane, water vapor is a greenhouse gas (GHG): More of ...