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GEO 203 Notes – 10/19/10 (Lecture #10, part 2)CLOUD DEVELOPMENTActive:Occurs in unstable airBuoyancy force stimulates and sustains rising motionAir cools to saturation, cloud formsCondensation formsLarge vertical dimensionPassive:In stable air that suppresses vertical developmentSaturation through:Large scale lifting (fronts and lows)Forced lifting (topography)Radiative coolingStratiform (layered) cloudsMechanisms for cloud formation:Convection due to surface heatingForced lifting over terrainWidespread ascent due to convergence of surface air toward a surface low pressure systemUplift along weather frontsH = 125 (T-Td) =125 m x Dew Point DepressionCloud base determined by difference between surface temperature and dew point temperatureLarger difference = higher cloud baseGEO 203 Notes – 10/19/10 (Lecture #11: Precipitation)Precipitation: Any liquid or solid water particles that fall from the atmosphere and reach the groundCan be long-lasting and steady or brief and intenseRemove water vapor from the atmosphere, the most important sink for water vapor in the atmosphere (important source: oceans)Rain, snow, sleet, freezing rain, hail: forms of precipitationCloud droplet: average diameter is 20 micrometersRaindrop: average diameter is 2000 micrometers; 100x larger than average cloud dropletsFor precipitation to form, cloud droplets must grow in volume by roughly one million timesCURVATURE EFFECTWater molecules are less strongly attached to curved water surface—they evaporate more easilyIn order to keep tiny cloud droplets in equilibrium, the air must be super saturated, i.e. over 100%Smaller diameter = higher relative humidity necessaryCondensation causes cloud droplets to formDoes further condensation of water vapor onto cloud droplets cause cloud droplets to grow into raindrops? No! Condensation provides an inefficient means of raindrop formation.Bottom of cloud: Warm cloud section, temperature above freezing, liquid water onlyIn warm clouds, cloud droplets grow into raindrops by process called collision and coalescenceCollision: small cloud droplets join together by random collisionsCoalescence: large and faster-moving cloud droplets overtake smaller and slower drops in their path to form larger dropletsSizes of raindrops in warm clouds affected by:Cloud liquid water content: more water = larger raindropsRange of cloud droplet sizes: larger spread = larger raindropsCloud thickness: thicker clouds = larger raindropsUpdraft strengths: stronger updrafts = larger raindropsDroplets’ electrical charges: opposite charges = larger raindropsMiddle of cloud: Mixed cloud section, temperature below freezing, both ice and waterIn mixed clouds, more water pressure needs to surround liquid than ice; difference in vapor pressure causes water vapor molecules to move from water droplets to ice in process called ice crystal process or Bergeron-Wagener ProcessIn a cloud that has both ice crystals and water droplets, the ice crystal will grow larger atthe expense of surrounding water dropletsTop of cloud: Cold cloud section, extremely cold temperature, ice onlyIn cold clouds, cloud droplets grow into raindrops by process called aggregation and accretionAggregation: ice crystals collide and form single larger ice particle (snowflakes)Accretion: falling ice crystals sweep up supercooled water


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MSU GEO 203 - Lecture #10, part 2

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