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

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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

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ReadingsText pp. 501-506