chapter_07 - Chapter 7 Precipitation I Precipitation...

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Chapter 7 Precipitation
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Precipitation : any form of water that falls from a cloud and reaches the ground How do cloud droplets grow larger? I. Precipitation Processes
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As air parcel rises and cools, it becomes saturated; with further rising condensation occurs. Excess water vapor condenses on nuclei or aerosols to form tiny cloud droplets that grow Growth rate is inversely proportional to droplet size, meaning that as droplets grows bigger, its growth slows down. Surface to volume ratio decreases while growing Eventually the growth effectively stops before reaching raindrop size. Formation of Droplets - Condensation
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Recall saturation vapor pressure is the maximum amount of water vapor necessary to keep moist air in equilibrium with a surface of pure water or ice If a cloud droplet is in equilibrium with its surroundings it will not grow Consider a Flat vs Curved Surface
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Saturation vapor pressure (SVP) around a droplet is greater than a flat surface RH > 100% to maintain droplet Curvature Effect
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Solute Effect Solute Effect – dissolved particles can reduce the RH required for condensation to begin Salt particles are good condensation nuclei Condensation can occur at 78% RH Ionic solutions bind water molecules and prevent evaporation Equilibrium vapor pressure is lowered
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Collision & Coalescence Process Warm clouds Larger drops fall faster than small droplets. Rain drops are formed by collection small cloud droplets
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Trajectory of Rain Drops A rain drop needs to capture millions of droplets Thicker, wetter clouds produce larger drops
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Significant Factors in Raindrop Formation Liquid water content Range of droplet sizes Cloud thickness Updrafts Electric charge of droplets and electric field in the cloud
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Ice Crystal (Bergeron) Process Important in the middle and high latitudes Need clouds composed of ice crystals and supercooled water droplets
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Ice Nuclei Provide a surface for ice to form on Rare, geometry resembles an ice crystal Clay, bacteria, or another ice crystal
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http://www.iac.ethz.ch/groups/lohmann/research/lab/in Types of Ice Forming Nuclei
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Tiny SVP Difference between Water & Ice Solid ice holds molecules better than liquid water Water molecules escape or evaporate easier from droplets than ice crystals
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Ice Crystals Grow at the Expense of Droplets Saturation vapor pressure of ice crystals < water droplets Difference in vapor pressure causes vapor to move or “diffuse” towards the ice crystal The result is that the ice crystal grows at the expense of the water droplets
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Ice Crystal Habit (Shape)
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Click here to animate this figure Click here to animate this figure Growing Your Own Crystals Some of the most intense snows are made up of dendrites Why does the dendrite grow faster than other habits?
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Further Growth of Ice Crystals… Ice crystals need to grow large enough to overcome the updraft to fall out of the cloud Accretion – growth by collision with supercooled water droplets Aggregation – ice crystals collide and stick together
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