formation of snow and hails. review of last lecture forces acting on a cloud/rain droplet. terminal...
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Formation of snow and hails
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Review of last lecture• Forces acting on a cloud/rain droplet. Terminal velocity. How does it change with cloud drop radius?
• Growth mechanisms for rain and snow (Warm clouds, cool clouds, cold clouds)
• Formation of rain: coalescence process (the collector is larger than the cloud droplets but not too large
• Bergeron process: happens with coexistence of ice and super-cooled water. Key: Saturation vapor pressure of ice < that of super-cooled water at the same temperature.
• Further growth of ice crystals (riming and aggregation)
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SnowSnow
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Monet: Magpie (1869)
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How does human life look alike?It's like wild goose walking on snow and mud.Left by chance is some footprint,But the goose has flown away, not caring to the east or the west.
By Su Shi (1037-1101)人生到处知何似,应似飞鸿踏雪泥。雪上偶然留指爪,鸿飞哪复计东西。[宋]苏轼
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Change of falling ice crystal: dependent on Change of falling ice crystal: dependent on atmospheric temperature and windsatmospheric temperature and winds
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SnowSnow
• Snow is precipitation that forms by the Bergeron process, riming, and aggregation, and reaches the surface without melting
• Crystal form (habit) varies with T and RH
• Large, soggy snowflakes associated with moist air near freezing
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Map of average annual snowfallMap of average annual snowfall
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Lake effectLake effect
• Heat and moisture fluxes from warm lake enhance snowfall in downstream regions
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Melted snowflake Melted snowflake rain rain
• Much of the rain in mid-lats (even in summer) begins as snow!
• The falling snow begins to melt around the freezing level
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A A thunderstorm thunderstorm
in the in the summersummer
0 oC
Water
Mixed ice/water
Ice
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Video: Ice stormVideo: Ice storm
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SleetSleet• Sleet begins as ice crystals which melt into rain as they
fall through the atmosphere. Before reaching the surface they solidify into a frozen state.
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• Freezing Rain forms similarly to sleet, however, the drop does not completely solidify before striking the surface
Freezing rain
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Video: Hail stormVideo: Hail storm
• http://www.youtube.com/watch?v=-eZuqeYlLDo
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• Graupel – ice crystals that undergo extensive riming
• Lose six sided shape and smooth out
• Either falls to the ground or provides a nucleus for hail
GraupelGraupel
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• Hail – concentric layers of ice build around graupel
– Requires very strong updrafts
– graupel carried aloft in updrafts high altitudes freezing temperatures
– water accreting to graupel freezes, forming a layer
– Hail begins to fall, carried aloft again by updrafts, process repeats
– Hailstones are very heavy – high density
– Capable of tremendous amounts of damage
– Great Plains = highest frequency of hail events
HailHail
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Map of hail probabilitiesMap of hail probabilities
• http://www.spc.noaa.gov/new/SVRclimo/climo.php?parm=allHail
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Summary: Different types of precipitationSummary: Different types of precipitationDepends on atmospheric temperature and winds Depends on atmospheric temperature and winds
Snow Rain Sleet Freezing rain Graupel/hail
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Summary
Condensation
Collision-coalescence
Bergeron Process
Warm
clouds
Cool/cold clouds
Rain Snow(can change to rain, sleet, freezing rain, graupel, hail depending on underlying atmosphere
Riming/Aggregation
• Forces acting on a cloud/rain droplet. Terminal velocity. How does it change with cloud drop radius?• Growth mechanisms for rain and snow