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Lecture 12

GEOG 1112 Lecture 12: Precipitation

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GEOG 1112
James Shepherd

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LECTURE 12 | PRECIPITATION Precipitation • Cloud droplets are very small and can easily be kept aloft by weak updrafts • Cloud droplets fall so slowly that it may take days for it to reach the surface o it would most likely evaporate before reaching the surface Cloud Droplets • Typical droplet is between 10 - 50 μm o Fall speed is 1 cm/sec (0.02 mph) to 25 cm/sec (0.5 mph) • A typical rain droplet is 1000 μm o Fall speed is 650 cm/sec (~15 mph) • So, droplets must somehow grow big enough that gravity overcomes the effects of the updrafts... Cloud & Rain Droplets • Highly Curved Drops Require Supersaturation to offset evaporation • “Solute” Effect (material dissolved in water) reduces the amount of supersaturation • Curvature Effect is approximately balanced by “Solute” effect to Enable Nucleation at or near 100% Relative Humidity Nucleation Types • Hygroscopic (water attracting) aerosols enable heterogeneous nucleation closer to 100%, • Homogeneous nucleation (no aerosols) require large relative humidities--not common Growth by Condensation • Initially cloud droplets can grow by condensation of water vapor. • There is limited water vapor and a lot of condensation nuclei competing for the water. • Thus, none will grow very big. • Other processes are needed for further growth... Growth in Warm Clouds • Most clouds in the tropics and some in the middle latitudes are warm clouds - those with temperatures above freezing. • In these clouds, the collision-coalescence process causes precipitation. Collision and Coalescence • Larger drops collect smaller drops & grow • Must collide and coalesce to grow. Growth in Cold and Cool Clouds • Some middle latitude clouds have temperatures below freezing. • Cold and Cool clouds may consist of ice crystals, supercooled droplets, or a combination of the two. • The coexistance of ice and supercooled water droplets is essential for precipitation formation. The process is called the Bergeron Process. Supercooled Water droplets & Ice Nuclei • Supercooled water droplets: water below the melting point but still in liquid form • Just as condensation nuclei are needed for water to condense, ice nuclei (IN) are needed for water to freeze. • IN are less common than CN-must be six-sided structures • A material’s ability to act as an ice nuclei is temperature dependent. • Most are effective at temperatures between -4 oC and
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