Forms of Precipitation

1. Forms of Precipitation • Rain • liquid form • often originates as snow • drizzle • smallest diameter of raindrops • Snow • ice crystals • density much less than rain • 1 inch rainfall = 10 inches snow (avg)

2. Forms of Precipitation, cont. • Sleet • frozen raindrops • rain freezes upon entering a cold, thick surface layer of air • Freezing Rain • ice storm • raindrops freeze upon contact with cold surface

3. Forms of Precipitation, cont. • Hail • lumps of ice • severe thunderstorms (cumulonimbus)

4. The Precipitation Process • Will precipitation occur or not? • gravity vs. air currents • droplet size (diameter) • Average water particle is 10 microns • need about a million to be large enough • coalescence • growth of water particles • three types

5. Ice Crystal Growth • most common form of coalescence • requires water vapor, water droplets (supercooled), and ice crystals • water vapor condenses upon ice crystals • ice crystals intercept water droplets & droplets crystallize • occurs in cold clouds • temperatures below 32 degrees F • resulting precipitation • snow, rain, sleet

6. Water Droplet Collision • not common, but not unusual either • coalescence through collisions • occurs in warm clouds • temperatures above 32 degrees F • resulting precipitation • rain

7. Condensation Nuclei • some air particles are hygroscopic • ex. dust, which attracts water droplets or ice crystals • coalescence may proceed • ice crystal growth or water droplet collision • artificial cloud seeding • may induce precipitation • can enhance precipitation • silver iodide or dry ice (frozen carbon dioxide) serve as nuclei

8. Readings:Text pp. 501-506