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Download 96Precipitation
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Precipitation General concept • There are several forces acting on a water droplet or ice crystal in a cloud – – – – Winds Atmospheric stability Gravity Drag (friction) • When a droplet reaches a certain critical mass the force of gravity will exceed the other forces and precipitation will fall • Rain drops are 100X larger than cloud droplets Formation of precipitation 1. Condensation and deposition 2. Collision and coalescence 3. The Bergeron process Warm air Cold air Condensation and deposition • As long as the rate of condensation exceeds the rate of evaporation water will be deposited and accumulate on condensation nuclei • Droplets grow very slowly • Rarely produces rain drops Collision and coalescence • As water droplets fall through a cloud, the larger droplets fall faster than the smaller ones, thus they collide • If the droplets coalesce a larger drop is formed • If it gets big enough rain will fall Collision efficiency A drop must be larger than the other drops to be an efficient collider If the drop is too big it will be less efficient, because it creates high pressure that pushes small drops out of the way Cold and cool clouds • Cold < 00C throughout • Cool Part is < 00C Part is > 00C Types of precipitation rain snow sleet graupel hail freezing rain Rain Can originate as liquid drops or ice. Ice melts as it falls through warmer air Shape of rain drops Snow • Ice crystals grow in a cloud. Variety of shapes depending on temperature Graupel • Ice crystals grow by riming • Contain air pockets • Has a spongy texture Sleet • Ice crystals melt as they fall through warn air aloft. Then water drops freeze as they fall through cold air below and hit the surface as ice pellets Freezing Rain • Ice crystals fall through warm air aloft and melt, Then liquid water falls through air at nearly 00C and freezes when it hits the cold (00C)ground Hail • Strong updrafts in a cloud force ice particles upward • As they move up and down in the cloud hail stones accumulate layers of ice • Eventually they get too heavy and fall Distribution of precipitation in the US Why? Distribution of snowfall in the US Why? Any questions? Have a great day! Thanks
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