The atmosphere always contains water vapor, which is the gaseous form of water, and its presence is measured as humidity. Unsaturated air refers to an air parcel that has the capacity to hold additional water vapor before reaching its saturation point, where condensation would begin. Adding moisture to this unsaturated air is a fundamental driver of the water cycle and weather patterns, including cloud formation and precipitation. This addition occurs through several distinct physical and biological mechanisms that govern the exchange of water between the Earth’s surface and the lower atmosphere. These processes involve the conversion of liquid or solid water into vapor and the subsequent movement of that vapor through the air.
Evaporation and Evapotranspiration
The dominant mechanism for injecting moisture into the atmosphere is the phase change from liquid to gas, which occurs primarily through evaporation. This physical process involves individual water molecules gaining sufficient thermal energy to break the molecular bonds holding them in the liquid state and escape into the air as water vapor. Solar radiation is the main source of the necessary latent heat energy required to power this transformation from large bodies of water, soil moisture, and other surface sources.
Several environmental factors control the rate at which this moisture is added to the air. Higher temperatures mean water molecules possess more energy, making their escape easier and accelerating the evaporation rate. A strong wind sweeps away the newly formed water vapor that accumulates just above the surface, preventing the air from quickly reaching saturation and allowing the process to continue efficiently. Conversely, high humidity slows the rate of evaporation because there is less room for new water molecules to escape into the atmosphere.
Moisture is also added biologically through transpiration, which is the release of water vapor from plant leaves through small pores called stomata. Plants draw up water from the soil and release a significant portion of it into the atmosphere as a regulated part of their life processes. The combined transfer of water vapor from the Earth’s surface, including evaporation from soil and water bodies, and transpiration from plants is often grouped together as evapotranspiration. This combined process is responsible for approximately 90 percent of the water vapor in the Earth’s atmosphere.
Sublimation
A secondary phase change that adds moisture to unsaturated air is sublimation, the direct conversion of solid water into water vapor without first melting into a liquid. This process requires the absorption of energy, specifically the latent heat of sublimation, to allow water molecules to transition directly from the rigid structure of ice crystals into a gas. Sublimation occurs at temperatures below the freezing point of water and is enhanced by low atmospheric pressure and low humidity.
This mechanism is most prominent in cold, dry environments, such as high-altitude regions or polar areas where temperatures remain below freezing. Dry air and wind increase the rate of sublimation by ensuring the newly formed water vapor is quickly carried away, which maintains the concentration gradient necessary for the process to continue.
Advection and Turbulent Mixing
Once moisture is generated through phase changes, its distribution into unsaturated air is governed by atmospheric transport mechanisms, primarily advection and turbulent mixing. Advection is the horizontal transport of existing water vapor by wind, which moves large, moist air masses from one region to another. When a moist air mass originating over an ocean or large forest moves over a drier, unsaturated land area, it adds moisture to the local air parcel.
Turbulent mixing describes the localized, vertical distribution of water vapor within the atmosphere. Air closest to the Earth’s surface is typically the warmest and wettest due to direct evaporation. Atmospheric turbulence, driven by heating and wind shear, creates eddies that lift this moist air upward and mix it with the drier air found higher in the boundary layer. This mixing effectively diffuses the water vapor, increasing the overall humidity of the lower atmosphere.
Human activities can also locally inject moisture into the air, acting as ancillary sources that rely on the same transport principles. Examples include the steam plumes released by industrial cooling towers or the widespread evaporation that results from large-scale agricultural irrigation systems. These localized sources directly introduce water vapor or liquid water that quickly evaporates, which is then dispersed through advection and turbulent mixing into the surrounding unsaturated air.