Author
 |
Hanson, Clayton |
|
Submitted to: Book Chapter
Publication Type: Book / Chapter Publication Acceptance Date: 7/24/2001 Publication Date: N/A Citation: N/A Interpretive Summary: Precipitation is the primary component of the hydrologic cycle. Precipitation includes all water particles, whether liquid or solid, that fall from clouds and reach the ground. Precipitation includes both liquid (drizzle, rain), freezing (freezing drizzle, freezing rain) and frozen (snow, ice crystals, hail) water. For precipitation to occur, air must be cooled sufficiently to cause condensation and droplet growth. The mechanism that causes precipitation is adiabatic-expansion cooling as air is lifted in the atmosphere. When cooling is sufficient, vapor condenses on nuclei which are generally small particles of dust or salt, and combustion products that are always present in the atmosphere to form either ice crystals and supercooled liquid cloud droplets, or only liquid cloud droplets. Air is generally lifted by four means; (a) cyclonic convergence (frontal convergence), (b) orographic lifting, (c) thermal convection, or (d) tropical cyclones (hurricanes). Knowledge of how and where precipitation occurs is basic to all hydrologic and water resource investigations. Technical Abstract: Precipitation includes all water particles, whether liquid or solid, that fall from clouds and reach the ground. Precipitation includes both liquid (drizzle, rain), freezing (freezing drizzle, freezing rain) and frozen (snow, ice crystals, hail) water. For precipitation to occur, air must be cooled sufficiently to cause condensation and droplet growth. The mechanism that causes precipitation is adiabatic-expansion cooling as air is lifted in the atmosphere. When cooling is sufficient, vapor condenses on nuclei which are generally small particles of dust or salt, and combustion products that are always present in the atmosphere to form either ice crystals and supercooled liquid cloud droplets, or only liquid cloud droplets. Clouds that extend above the 0oC level are cold clouds and those that do not extend above the 0oC level are warm clouds. Ice particles grow to sufficient mass to fall as precipitation in cold clouds by three processes; vapor condensation, collisions with supercooled droplets and aggregation with other ice particles. In warm clouds, droplets grow large enough to fall as precipitation through a coalescence process where larger particles which fall faster than small particles collide and coalesce. Air is generally lifted by four means; (a) cyclonic convergence (frontal convergence), (b) orographic lifting, (c) thermal convection, or (d) tropical cyclones (hurricanes). |
