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Chapter 5: Cloud Chapter 5: Cloud development and development and
precipitationprecipitation Atmospheric StabilityAtmospheric Stability Determining stabilityDetermining stability Cloud development and stabilityCloud development and stability Precipitation processesPrecipitation processes Precipitation typesPrecipitation types Measuring precipitationMeasuring precipitation
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Atmospheric Atmospheric stabilitystability
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Atmospheric stabilityAtmospheric stability
stable and unstable equilibriastable and unstable equilibria air parcelsair parcels adiabatic processadiabatic process adiabatic lapse ratesadiabatic lapse rates
• Stability does not control whether air will rise or sink.Stability does not control whether air will rise or sink.Rather, it controls whether rising air will Rather, it controls whether rising air will continuecontinue to rise to riseor whether sinking air will or whether sinking air will continuecontinue to sink to sink
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Determining Determining stabilitystability
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A stable atmosphereA stable atmosphere
environmental lapse rateenvironmental lapse rate absolute stabilityabsolute stability stabilizing processesstabilizing processes subsidence inversionssubsidence inversions
• stable air provides excellent conditions for high pollutionstable air provides excellent conditions for high pollutionlevelslevels
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An unstable atmosphereAn unstable atmosphere
absolute instabilityabsolute instability warming of surface airwarming of surface air destabilizing processesdestabilizing processes superadiabatic lapse ratessuperadiabatic lapse rates
• unstable air tends to be well-mixedunstable air tends to be well-mixed
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Conditionally unstable Conditionally unstable airair
conditional instabilityconditional instability dry and moist adiabatic lapse ratesdry and moist adiabatic lapse rates
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Cloud Cloud development and development and
stabilitystability
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Cloud development and Cloud development and stabilitystability
surface heating and free convectionsurface heating and free convection uplift along topographyuplift along topography widespread ascentwidespread ascent lifting along weather frontslifting along weather fronts
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Convection and cloudsConvection and clouds
thermalsthermals fair weather cumulusfair weather cumulus
• fair weather cumulus provide a visual marker of thermalsfair weather cumulus provide a visual marker of thermals
• bases of fair-weather cumulus clouds marks the bases of fair-weather cumulus clouds marks the lifting condensation levellifting condensation level, the level at which rising air, the level at which rising airfirst becomes saturatedfirst becomes saturated
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Topography and cloudsTopography and clouds
orographic upliftorographic uplift rain shadowrain shadow
• The rain shadow works for snow too. Due to frequentThe rain shadow works for snow too. Due to frequentwesterly winds, the western slope of the Rocky Mountainswesterly winds, the western slope of the Rocky Mountainsreceives much more precipitation than the eastern slope.receives much more precipitation than the eastern slope.
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Precipitation Precipitation processesprocesses
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Collision and Collision and coalescence processcoalescence process
terminal velocityterminal velocity coalescencecoalescence warm cloudswarm clouds
• A typical cloud droplet falls at a rate of 1 centimeter per second.A typical cloud droplet falls at a rate of 1 centimeter per second.At this rate it would take 46 hours to fall one mile.At this rate it would take 46 hours to fall one mile.
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Ice crystal processIce crystal process
cold cloudscold clouds supercooled water dropletssupercooled water droplets saturation vapor pressures over saturation vapor pressures over
liquid water and iceliquid water and ice accretionaccretion
• The upper portions of summer thunderstorms are cold clouds!The upper portions of summer thunderstorms are cold clouds!
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Cloud seeding and Cloud seeding and precipitationprecipitation
cloud seedingcloud seeding silver iodidesilver iodide
• It is very difficult to determine whether a cloud seedingIt is very difficult to determine whether a cloud seedingattempt is successful. How would you know whetherattempt is successful. How would you know whetherthe cloud would have resulted in precipitation if it hadn’tthe cloud would have resulted in precipitation if it hadn’tbeen seeded?been seeded?
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Precipitation in cloudsPrecipitation in clouds
accretionaccretion ice crystal processice crystal process
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Precipitation Precipitation typestypes
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RainRain
rainrain drizzledrizzle virgavirga showershower
• Virga is much more commonly observed in the westernVirga is much more commonly observed in the westernUS, because the humid climate of the eastern US US, because the humid climate of the eastern US reduces the visibility.reduces the visibility.
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SnowSnow
snowsnow fallstreaksfallstreaks dendritedendrite blizzardblizzard
• Snowflake shape depends on both temperature andSnowflake shape depends on both temperature andrelative humidity relative humidity
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Sleet and freezing rainSleet and freezing rain
sleetsleet freezing rainfreezing rain rimerime
• Sleet makes a ‘tap tap’ sound when falling on glassSleet makes a ‘tap tap’ sound when falling on glass
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Snow grains and snow Snow grains and snow pelletspellets
snow grainssnow grains snow pelletssnow pellets graupelgraupel
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HailHail
updraft cyclesupdraft cycles accretionaccretion
• a hailstone can be sliced open to reveal accretion rings,a hailstone can be sliced open to reveal accretion rings,one for each updraft cycleone for each updraft cycle
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Measuring Measuring precipitationprecipitation
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InstrumentsInstruments
standard rain gaugestandard rain gauge tipping bucket rain gaugetipping bucket rain gauge
• It is difficult to capture rain in a bucket when theIt is difficult to capture rain in a bucket when thewind is blowing stronglywind is blowing strongly
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Doppler radar and Doppler radar and precipitationprecipitation
radarradar Doppler radarDoppler radar