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

Atmospheric Atmospheric stabilitystability

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

Determining Determining stabilitystability

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

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

Conditionally unstable Conditionally unstable airair

conditional instabilityconditional instability dry and moist adiabatic lapse ratesdry and moist adiabatic lapse rates

Cloud Cloud development and development and

stabilitystability

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

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

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.

Precipitation Precipitation processesprocesses

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.

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!

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?

Precipitation in cloudsPrecipitation in clouds

accretionaccretion ice crystal processice crystal process

Precipitation Precipitation typestypes

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.

SnowSnow

snowsnow fallstreaksfallstreaks dendritedendrite blizzardblizzard

• Snowflake shape depends on both temperature andSnowflake shape depends on both temperature andrelative humidity relative humidity

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

Snow grains and snow Snow grains and snow pelletspellets

snow grainssnow grains snow pelletssnow pellets graupelgraupel

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

Measuring Measuring precipitationprecipitation

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

Doppler radar and Doppler radar and precipitationprecipitation

radarradar Doppler radarDoppler radar