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Clouds

Clouds. ENVI1400: Lecture 9. Cloud Classification. Four latin terms form the basis for the naming of clouds: Cirrus : fibrous or hair-like Cumulus : a heap or pile Stratus : a horizontal sheet or layer Nimbus : rain-bearing The prefix Alto is used to indicate medium altitude clouds.

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Clouds

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  1. Clouds ENVI1400: Lecture 9

  2. Cloud Classification Four latin terms form the basis for the naming of clouds: • Cirrus : fibrous or hair-like • Cumulus : a heap or pile • Stratus : a horizontal sheet or layer • Nimbus : rain-bearing The prefix Altois used to indicate medium altitude clouds. Terms, and basic classifications first proposed by Luke Howard in 1803. http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/cldtyp/home.rxml - good online guide ENVI 1400 : Meteorology and Forecasting

  3. High Level: • Cloud base above 6000m • Are all forms of cirrus (ice clouds) Medium Level: • Cloud base 2000-6000m Low level: • Cloud base below 2000m (within boundary layer) ENVI 1400 : Meteorology and Forecasting

  4. High-Level Clouds Cirrus (Ci): White, delicate, fibrous in appearance. Forms in patches or narrow bands. May for comma-shaped streaks or “mare’s tails” (cirrus uncinus) Cirrus clouds are formed entirely of ice crystals. These grow and evaporate slowly, leading to soft edges to clouds. ENVI 1400 : Meteorology and Forecasting

  5. Cirrostratus (Cs): Thin, transparent sheet or veil; sun clearly visible & casting shadows at surface. A halo may be seen around the sun (or moon). Sheets of cirrostratus may cover entire sky, and be up to several 1000m deep. ENVI 1400 : Meteorology and Forecasting

  6. Cirrocumulus (Cc): Thin white patch or sheet of cloud; appears dappled or rippled. Dappling results from convective overturning within the cloud, ripples from gravity waves. Aircraft contrails: condensation from aircraft exhaust. May dissipate quickly, or be very long-lived depending on conditions. ENVI 1400 : Meteorology and Forecasting

  7. Medium-Level Clouds Altostratus (As): A greyish sheet of cloud, may be fibrous or uniform in appearance. Thin enough in parts to make out the sun, but no halo. ENVI 1400 : Meteorology and Forecasting

  8. Altocumulus (Ac): white or grey patches arranged in sheets. Shape and texture are variable. There are several distinct sub-classes of altocumulus ENVI 1400 : Meteorology and Forecasting

  9. Altocumulus lenticularis (Ac len): white or grey lenticular (lens shaped) clouds formed by the lifting of air over a topographic barrier. ENVI 1400 : Meteorology and Forecasting

  10. Altocumulus castellanus (Ac cas): white or grey, broken cumulus-like clouds; upper part appearing castle-like. Sometimes arranged in lines. ENVI 1400 : Meteorology and Forecasting

  11. Altocumulus undulatus (Ac und): white or grey patches or sheets of cloud with an undulating or rippled appearance. ENVI 1400 : Meteorology and Forecasting

  12. Low-Level Clouds Cumulus (Cu): Brilliant white to grey, dense detached clouds. Forms clumped or heaped (cauliflower-like) shapes, usually with sharp outlines and flat base. Field of Cu often have bases all at same (lifting condensation) level. ENVI 1400 : Meteorology and Forecasting

  13. Cumulus humilis (Cu hum): small cumulus, of limited vertical extent, may have a flattened appearance. Also called fair-weather cumulus ENVI 1400 : Meteorology and Forecasting

  14. Cumulus mediocris : cumulus, of moderate vertical extent. ENVI 1400 : Meteorology and Forecasting

  15. Cumulus congestus: crowded (congested) field of cumulus or greater vertical extent. May produce rain. ENVI 1400 : Meteorology and Forecasting

  16. Pileus : cap clouds that form above large cumulus as the upward motion of the convective cloud distorts the layer of air above (pileus is latin for skull-cap) ENVI 1400 : Meteorology and Forecasting

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  18. Cumulonimbus (Cb) : huge towering cloud, dark base and white sides. Associated with heavy rain, thunderstorms, and hail. Frequently has an anvil shaped top. ENVI 1400 : Meteorology and Forecasting

  19. mammatus : smooth, rounded shapes sometimes formed on the underside of cumulonimbus; they result from downdrafts within the cloud. ENVI 1400 : Meteorology and Forecasting

  20. Stratocumulus (Sc) : white or grey sheet of cloud, usually formed in mounds or rolls. ENVI 1400 : Meteorology and Forecasting

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  23. Stratocumulus with virga – hair-like strands of falling rain, which evaporate below cloud before reaching the surface. ENVI 1400 : Meteorology and Forecasting

  24. Stratus (St) : grey featureless layer of cloud with a uniform base. Often associated with drizzle or snow. ENVI 1400 : Meteorology and Forecasting

  25. Nimbostratus (Ns) : Dark grey, featureless, thick layer of cloud. Associated with prolonged precipitation. Commonly forms in frontal systems ENVI 1400 : Meteorology and Forecasting

  26. Precipitation • Cloud droplets require a condensation nuclei on which to form; growth then occurs by deposition of water molecules from vapour. • Growth is limited by local supersaturation of the vapour • Growth rate decreases as droplet size increases • Cloud droplets are typically 10 to 30 m in diameter. Growth/evaporation can occur within a few 10s of seconds. • Rain drops are typically 0.5 to 5 mm in diameter, growth from the vapour would take several hours – longer than the lifetime of typical convective clouds. ENVI 1400 : Meteorology and Forecasting

  27. In order to grow into rain drops, cloud droplets must grow by coalescence • Larger droplets fall faster than small ones, and can collide with them • The process by which a sufficient number of large enough droplets is generated remains a topic of active research • Ice crystals provide a more efficient process • Saturation vapour pressure over ice is less than that over water  ice crystals grow at expense of water droplets • If ice crystal touches a droplet, the droplet freezes • Once large enough, ice crystals – or clumps of crystals – fall past droplets and collect them. Rapid growth of soft hail pellet (graupel) by riming. • Graupel falls from cloud, melting before reaching the surface as rain ENVI 1400 : Meteorology and Forecasting

  28. Cloud Radiative Effects • Clouds play an important controlling role in the global radiation budget. • Reflection of incoming solar (short-wave) radiation • Absorption of both solar and thermal infra red (long-wave) radiation (incoming & outgoing) • Emission of infra red radiation (up and down) • The altitude, type, and thickness of cloud, along with that of clouds above & below determines whether the local net effect is to warm or cool the air & surface below. ENVI 1400 : Meteorology and Forecasting

  29. Vertical Transport • Deep convective clouds play a major role in mixing boundary layer air – along with moisture, aerosol particles, and gases (both natural and man-made pollutants) – up into the free-troposphere. ENVI 1400 : Meteorology and Forecasting

  30. Chemistry • Clouds provide an environment within which aqueous phase chemical reactions can take place within the atmosphere • Aerosol particles can be substantially modified within clouds • Aqueous phase reactions with gases dissolved in droplets • Coalescence of droplets brings multiple aerosol particles together • Chemically different aerosol may react • On evaporation of droplet, a single aerosol particle is formed, containing material from all contributing droplets ENVI 1400 : Meteorology and Forecasting

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