precipitation

• Condensation


  Water vapor in the air changes to a liquid
• Saturation


  Air must be saturated for condensation to occur
• Saturation occurs


  1. When air is cooled to its dew point

  2. When water vapor is added to the air
• Condensation nuclei


  • Tiny bits of particulate matter that serve as surfaces for water vapor condensation
  • In their absence, a relative humidity in excess of 100% is needed to produce clouds
• Hygroscopic nuclei


  Particles that absorb water, such as ocean salt
• Clouds


  Visible aggregates of minute droplets of water or tiny crystals of ice
• Clouds


  • Provide a visible indication of what is going on in the atmosphere
  • Classified on the basis of their form and height
• Forms of Clouds


  • Cirrus
  • Cumulus
  • Stratus
• Levels of Cloud Heights

  • High Clouds (above 6000 meters)
  • Middle Clouds (2000 to 6000 meters)
  • Low Clouds (below 2000 meters)
• High Clouds


  • Cirrus
  • Cirrocumulus
  • Cirrostratus
• Middle Clouds


  • Altocumulus
  • Altostratus
• Low Clouds


  • Stratus
  • Stratocumulus
  • Nimbostratus
• Clouds of Vertical Development


  Clouds that do not fit into any one of the three height categories, with bases in the low height range but often extending upward into the middle or high altitudes
• Clouds of Vertical Development

  • Cumulus
  • Cumulonimbus
• Fog


  A cloud with its base at or very near the ground
• Fog formation


  1. By cooling (air is cooled below its saturation point or dew point)

  2. By evaporation and mixing (water vapor is added to the air by evaporation, and the moist air mixes with relatively dry air)
• Precipitation


  Water released from clouds in the form of rain, freezing rain, sleet, snow, or hail
• How Precipitation Forms


  1. Cloud droplets grow in volume by roughly one million times

  2. Bergeron process (precipitation from cold clouds)

  3. Collision-coalescence process (precipitation from warm clouds)
• Forms of Precipitation


  • Rain
  • Snow
  • Sleet
  • Glaze
  • Hail
  • Rime
  • Graupel
• Rain


  Drops of water that fall from a cloud and have a diameter of at least 0.5 mm
• Drizzle


  Fine, uniform drops of water having a diameter less than 0.5 mm
• Snow


  Precipitation in the form of ice crystals (snowflakes) or aggregates of crystals
• Sleet


  Small, spherical to lumpy ice particles that form when raindrops freeze while falling through a layer of sub-freezing air
• Glaze


  Thick coating of ice formed when supercooled raindrops freeze in contact with solid objects
• Hail


  Hard, rounded pellets or irregular lumps of ice produced in large convective, cumulonimbus clouds
• Rime


  Deposit of ice crystals formed by the freezing of supercooled fog or cloud droplets on objects whose surface temperature is below freezing
• Graupel


  Irregular masses of soft ice formed when rime collects on snow crystals
• Rain gauge


  Instrument used to measure rain
• Weather radar


  Instrument that produces images to predict precipitation patterns
• Condensation


  When water vapor changes to a liquid
• Forms of condensation


  • Dew
  • Fog
  • Clouds
• Saturation


  Occurs either when sufficient water vapor is added to the air or when the air is cooled to its dew point
• Radiation cooling


  1. Surface radiates heat away, causing the surface and adjacent air to cool rapidly

  2. Accounts for the formation of dew and some types of fog
• Adiabatic


  (Greek: adiábatos meaning "impassable") - a process where the parcel (refers to an imaginary volume of air) temperature changes due to an expansion or compression, no heat is added or taken away from the parcel
• Adiabatic process


  A process in which no heat transfer takes place. This does not mean that the temperature is constant, but rather that no heat is transferred into or out from the system
• Dry adiabatic rate


  Unsaturated air cools at the constant rate of 10 °C for every 1000 meters of ascent (5.5 °F/1000 feet)
• Wet adiabatic rate


  The slower rate of cooling caused by the addition of latent heat, varies from 5 °C per 1000 meters for air with a high moisture content to 9 °C per 1000 meters for dry air
• Orographic lifting


  1. Elevated terrains, such as mountains, act as barriers to the flow of air, causing air to ascend the mountain slope and adiabatically cool, often generating clouds and precipitation

  2. Air descends the leeward side, warming adiabatically, making condensation and precipitation less likely, resulting in a rain shadow desert
• Frontal wedging


  The cooler, denser air acts as a barrier over which the warmer, less dense air rises
• Convergence


  Whenever air in the lower atmosphere flows together, lifting results