Lecture 9

• Condensation


  Energy releasing process of transformation from vapor to liquid
• Clouds


  • Visible manifestation of condensation in the atmosphere
  • Aggregate of tiny water droplets or ice crystals suspended in the air
  • Precursor to rain or snow, thunder or lightning, rainbows or halos
• High clouds


  • Cirrus
  • Cirrocumulus
  • Cirrostratus
• Low clouds


  • Stratus
  • Stratocumulus
  • Cumulonimbus
• Middle clouds


  • Nimbostratus
  • Altostratus
  • Altocumulus
• Collision-Coalescence Theory


  1. Cloud droplets are too small to fall as rain

  2. They form on cloud condensation nuclei

  3. Hygroscopic nuclei are salts, dusts, aerosols

  4. They allow condensation at RH less than 100%

  5. Cloud droplets, in above-freezing air, can grow larger by collision and coalescence

  6. Faster-falling, bigger droplets collide and coalesce with smaller and slower droplets in their path
• Precipitation


  • Rain - liquid precipitation with water droplets > 0.5 mm in diameter; most common form in the tropics
  • Drizzle - numerous small drops with < 0.5 mm in diameter
  • Snow (sleet, hail, glaze, etc.) - solid precipitation of branched hexagonal crystals; occurs in middle to high latitudes
• Precipitation Characteristics


  • Amount - expressed in depth (mm, cm, in)
  • Intensity - in depth per unit time (mm/day, mm/s, cm/hr, in/hr)
  • Duration - length of time precipitation occurred
• Precipitation Types


  • Convective - coming from an upward movement of warm air masses due to differential heating or air masses near the ground surface; may be in the form of light showers, cloud bursts or thunderstorms; usually localized and of short duration
  • Cyclonic - coming from the movement of air masses due to difference in pressure; Non-frontal - caused by the presence of low pressure into which air flow converges; Frontal - results when two air masses of different temp and RH characteristics meet
  • Orographic - results when moist air masses moving over mountain barriers are cooled, resulting in condensation and subsequent precipitation
• Doppler radar


  • Measures the position and radial velocity of objects
  • Uses the "Doppler shift" or the apparent shift in the frequency and wavelength of a wave perceived by an observer moving relative to the source of the wave